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USER'S MANUAL

121GW

MULTIMETER

Last Revised

08 February 2019

08 August 2019

CONTENTS

Contact .............................................................................................................. 5

Glossary of Terms .............................................................................................. 6

Change log ......................................................................................................... 7

Firmware Version .............................................................................................. 8

Firmware Version .............................................................................................. 9

General .............................................................................................................. 9

General ............................................................................................................ 11

FCC/IC Information ........................................................................................ 9

FCC/IC Information ...................................................................................... 11

Information to the User ................................................................................ 9

Information to the User .............................................................................. 11

Introduction .................................................................................................... 10

Introduction .................................................................................................... 12

Features .......................................................................................................... 11

Features .......................................................................................................... 13

Specifications .................................................................................................. 13

Specifications .................................................................................................. 15

General ........................................................................................................ 13

General ........................................................................................................ 15

Voltage Measurement ................................................................................. 15

Voltage Measurement ................................................................................. 17

Current Measurement ................................................................................. 16

Current Measurement ................................................................................. 18

Resistance & Capacitance Measurement .................................................... 17

Resistance & Capacitance Measurement .................................................... 19

Diode Measurement.................................................................................... 18

Diode Measurement.................................................................................... 20

Frequency & Duty Cycle Measurement ...................................................... 19

Frequency & Duty Cycle Measurement ...................................................... 21

VA Measurement ........................................................................................ 20

VA Measurement ........................................................................................ 22

Temperature & Continuity .......................................................................... 21

Temperature & Continuity .......................................................................... 23

Bluetooth Specifications .............................................................................. 22

Bluetooth Specifications .............................................................................. 24

Standards ........................................................................................................ 23

Standards ........................................................................................................ 25

Compliance Symbols ................................................................................... 23

Compliance Symbols ................................................................................... 25

IEC Standards ............................................................................................... 23

IEC Standards ............................................................................................... 25

ANSI Standards ............................................................................................ 23

ANSI Standards ............................................................................................ 25

Safety .............................................................................................................. 23

Safety .............................................................................................................. 25

Warning ....................................................................................................... 24

Warning ....................................................................................................... 26

Caution ........................................................................................................ 27

Caution ........................................................................................................ 29

Operation ........................................................................................................ 28

Operation ........................................................................................................ 30

Measuring ....................................................................................................... 33

Measuring ....................................................................................................... 35

Voltage .............................................................................................. 33

Voltage .............................................................................................. 35

dBm .......................................................................................................... 35

dBm .......................................................................................................... 37

Voltage ................................................................................................ 36

Voltage ................................................................................................ 38

Temperature ......................................................................................... 37

Temperature ......................................................................................... 39

Current ......................................................................................... 38

Current ......................................................................................... 40

Burden Voltage ........................................................................................ 40

Burden Voltage ........................................................................................ 42

Resistance ............................................................................................. 42

Resistance ............................................................................................. 44

Continuity Testing ................................................................................ 43

Continuity Testing ................................................................................ 45

Cable Break Testing ............................................................................. 44

Cable Break Testing ............................................................................. 46

Capacitance .......................................................................................... 45

Capacitance .......................................................................................... 47

Frequency ........................................................................................... 46

Frequency ........................................................................................... 48

Duty Cycle ........................................................................................... 47

Duty Cycle ........................................................................................... 49

Period .................................................................................................. 48

Period .................................................................................................. 50

Diode Measurement ............................................................................ 49

Diode Measurement ............................................................................ 51

Power (VA) ................................................................... 51

Power (VA) ................................................................... 53

Data Logging ................................................................................................ 52

Data Logging ................................................................................................ 54

Backlight ...................................................................................................... 53

Backlight ...................................................................................................... 55

Bluetooth ..................................................................................................... 53

Bluetooth ..................................................................................................... 55

Measurement HOLD .................................................................................... 54

Measurement HOLD .................................................................................... 56

Relative Measurement (REL) ....................................................................... 55

Relative Measurement (REL) ....................................................................... 57

MIN/MAX/AVG ............................................................................................ 56

MIN/MAX/AVG ............................................................................................ 58

Setup Menu ................................................................................................. 57

Setup Menu ................................................................................................. 59

Temperature ............................................................................................ 58

Temperature ............................................................................................ 60

Battery Voltage ........................................................................................ 58

Battery Voltage ........................................................................................ 60

Auto Power Off (APO) .............................................................................. 58

Auto Power Off (APO) .............................................................................. 60

Buzzer On/Off .......................................................................................... 59

Buzzer On/Off .......................................................................................... 61

LCD Contrast ............................................................................................ 59

LCD Contrast ............................................................................................ 61

Date.......................................................................................................... 60

Date.......................................................................................................... 62

Multimeter ID .......................................................................................... 61

Multimeter ID .......................................................................................... 63

Logging Interval ....................................................................................... 61

Logging Interval ....................................................................................... 63

Maintenance ................................................................................................... 62

Maintenance ................................................................................................... 64

Debugging .................................................................................................... 62

Debugging .................................................................................................... 64

Cleaning and Storage ................................................................................... 62

Cleaning and Storage ................................................................................... 64

Fuse Replacement ....................................................................................... 62

Fuse Replacement ....................................................................................... 64

Calibration ....................................................................................................... 64

Calibration ....................................................................................................... 66

Calibration Mode ......................................................................................... 65

Calibration Mode ......................................................................................... 67

Saving Calibration Data ............................................................................... 66

Saving Calibration Data ............................................................................... 68

Loading Calibration Data ............................................................................. 66

Loading Calibration Data ............................................................................. 68

Zero Offset Calibration ................................................................................ 67

Zero Offset Calibration ................................................................................ 69

Full Scale GAIN Calibration .......................................................................... 68

Full Scale GAIN Calibration .......................................................................... 70

Calibration Reference Table ............................................................................ 69

Calibration Reference Table ............................................................................ 71

Firmware Update ............................................................................................ 70

Firmware Update ............................................................................................ 72

Warranty ......................................................................................................... 71

Warranty ......................................................................................................... 73

CONTACT

If you have a warranty claim, you can contact us at the following email ad-

dress:

sales@eevblog.com

This multimeter and accessories can be purchased from:

www.eevblog.com

Community Support for the meter is provided on the EEVblog Forum:

www.eevblog.com/forum

Issues should be reported in the Report an issue section of the multimeters

official page:

www.eevblog.com/product/121gw/

Phone:

Ph: +61(2)8626 8743

GLOSSARY OF TERMS

IEC

(International Electrotechnical Commission)

An organisation that publishes various international standards relating to

electrical, electronics and related technology.

ANSI

(American National Standards Institute).

Americas non-profit standards organisation that oversee the development

and use of standards in various domains.

ETL

(ETL Testing Laboratories)

The compliance certification mark of Intertek Group in Europe.

(Overload Protection)

The voltage that a function of the Multimeter than withstand.

Accuracy

The closeness of a measured value to a known standard.

Low BurdenTM

Provides low burden voltage when measuring current compared with typical

multimeters, minimising impact on your in-circuit current measurements.

CHANGE LOG

The following represents the changes made to the manual:

Date

Changes

15/01/17

Added Serial Number section

Added Hold requirement to save setup items

Fixed various grammar errors

Added Bluetooth symbol to frontpage

31/01/18

Changed incorrect fuse rating to

400 mA / 600 V HRC Fuse

Added Low Z mode description

04/03/18

Added Multimeter ID to list on page 53.

11/04/18

Minor grammar and a spelling change.

Added VA specifications

Specification format changes

24/05/18

Added information about new firmware version 1.15 and

1.17.

09/07/18

Added information about new firmware version 1.21 and

1.22.

Added more information about use of Low Z mode.

26/07/18

Modified formatting of VAC table.

Described Relative mode.

Improved burden voltage section steps.

Fixed error in offset calibration table.

Changed steps for all setup items to allow for units that

have buzzer turned off.

Some grammar errors resolved.

27/07/18

Updated IAP section, removed incorrect step.

24/08/18

Updated DC mode section description for clarity.

30/08/18

Updated sections on battery type.

Added warning about lithium batteries.

11/09/18

Added information about 500VA range.

04/12/18

Moved case into optional accessory.

29/01/19

Added comment about modes being preserved between

resets.

08/08/2019

Added battery voltage check to the firmware update pro-

cedure.

Text moved with changes from lines 1581-1583 (66.7% similarity)

FIRMWARE VERSION

This manual is written for version 1.07 of the 121GW firmware. The following

is a list of changes for each version:

Version

Changes

1.00

Added support for exporting and importing calibration

data to/from SD card.

Displays version number on boot.

1.01

Internal version.

1.02

Internal version.

1.03

Fixed possible import export calibration bug.

1.04

Resistance auto-ranging speed improved.

Can now turn beeper off.

Negative VA values now readable.

1.05

Further improved auto-ranging speed.

Improved Low-Z measurement up to 20KHz.

1.06

Internal version.

1.07

Improved VA mode over-estimation error.

1.15

Improved display to show OFL in appropriate sections,

fixed lookup table bug with various range.

1.17

Improved accuracy of voltage measurement in AC and DC

VA modes.

1.21

Added support for .C and .F in the Bluetooth packet.

1.22

Added 2 more bits of data for the main data to

accommodate values over 65535.

1.25

Auto-ranging loop in M Ohms range resolved.

Burden Voltage glitch resolved.

1.26

Improved resistance range noise when logging.

1.51

208ms data logging interval -> 200ms.

File date and time in file meta info corrected.

DC+AC auto ranging loop issue resolved.

Amp range sensitivity improved.

AC mode frequency persistence issue resolved.

VAC + LPF bargraph issue resolved.

2.00

Improved the Auto-ranging reading speed in both ca-

pacitance and resistance ranges.

GENERAL

FCC/IC INFORMATION

This device complies with Part 15 of the FCC Rules and CAN ICES-3(A). Opera-

tion is subject to the following two conditions: (1) this device may not cause

harmful interference, and (2) this device must accept any interference re-

ceived, including interference that may cause undesired operations.

INFORMATION TO THE USER

This equipment has been tested and found to comply with the limits for a

Class B digital device, pursuant to part 15 of the FCC Rules. These limits are

designed to provide reasonable protection against harmful interference in a

residential installation.

This equipment generates, uses and can radiate radio frequency energy and,

if not installed and used in accordance with the instructions, may cause

harmful interference to radio communications. However, there is no guaran-

tee that interference will not occur in an installation.

If this equipment does cause harmful interference to radio or television re-

ception, which can be determined by turning the equipment off and on, the

user is encouraged to try to correct the interference by one or more of the

following measures:

• Reorient or relocate the receiving antenna.

• Increase the separation between the equipment and receiver.

• Connect the equipment into an outlet on a circuit different from that

to which the receiver is connected.

• Consult the dealer or an experienced radio/TV technician for help.

WARNING

Any changes or modifications not expressly approved by the manufacturer

could void the user’s authority to operate the equipment.

INTRODUCTION

Congratulations on purchasing the EEVblog 121GW multimeter. It's one of

the most feature packed small form factor meters on the market, designed to

provide maximum functionality in as little bench space as possible. It pro-

vides many unique features not found on other multimeters, like low burden

voltage measurement and display, simultaneous voltage and current (VA

power) measurement, 15 V diode test voltage, Bluetooth built-in, and Micro

SD card data logging.

You've made a wise choice buying this instead of some questionable quality

cheapie, it should return the favour with many years of safe and accurate ser-

vice.

That ETL logo you see stamped on the back of the meter ensures that the me-

ter design has been independently tested to the latest IEC/UL/EN61010-1 3rd

edition safety standard by Intertek.

With a genuine 600 V CAT III rating and proper 1000V HRC fuses and

MOV+PTC protection you can feel confident and safe that you can use this

meter on most high energy mains rated equipment.

And remember, don't turn it on, take it apart!

This meter is hackable, go for it.

Have fun.

Dave Jones

www.eevblog.com

FEATURES

Summary

• True RMS sensing up to crest factor 10 (AD8436 chipset)

• 600 V CAT III ETL Listed Rating

• 1000 V HRC fuses

• High voltage 15V diode test mode

• Dual 50,000 count display

• Relative Mode

• 1ms Peak Capture

• MIN, MAX or AVG on all functions

• Low BurdenTM current measurement feature

• OPEN and CLOSED continuity testing (for wire breaks)

• Temperature, Capacitance, Frequency and Duty Cycle

• Unique burden voltage measurement & display in current mode

• Simultaneous voltage and current measurement and VA calculation

• Low Z input mode eliminates ghost voltages (not for under 12V)

• Automatic touch measurement data hold

• Audible and visual incorrect input jack warning

• Closed case calibration of individual ranges

Logging Capabilities

• Bluetooth LE (Low Energy) Communication

• Data logging onto a Micro SD card (CSV format)

Mechanical

• Compact size

• Backlit display

Software

• Android, iPhone, and Windows 10 support, [Mac]1

• Updatable Firmware (via micro SD Card)

1 Under development

Batteries

• RTC Battery (CR1220)

• Main Battery (4 x AA)

• Long battery life of >500 hrs

SPECIFICATIONS

The following specifications are given in SI units with their accuracy notated

as ± (% of reading digits) + (number of digits) unless otherwise indicated.

GENERAL

The following table represents general specifications (at 23°C ± 5°C):

Item

Specification

Display

4 . digits 50,000 counts, dual display

Update Rate

5 times per second nominal

Operating Temperature

0°C to 50°C (32°F to 122°F)

Relative Humidity

< 75%

Altitude

Operating below 2000 m

Storage Temperature

-20°C to 50°C (-44°F to 122°F)

Temperature

Coefficient

Temperature coefficient (Nominal):

....=0.1 . (.................. ................) / °..

RMS Sensing

True RMS sensing (AD8436)

Ingress Protection

IP41

Pollution Degree

Safety

cETLus 3rd Edition, CAT III 600 V, CE Conformity,

IP41, RoHS Compliant, IEC/EN61010-1: 2010 3rd

Edition

Drop Protection

1 m

Transient Protection

6 kV 2 ., Pulsed Source

E.M.C.

Commercial limits compliance with EN61326-1

Bluetooth

BLE112 Class 2

Chipset

Hycon HY3131

ST ARM STM32L152D

Overload Protection

mA and .A

400 mA / 600 V DC/AC

IR 10kA HRC FAST + DIODE

A and 500mA

11 A / 1000 V DC/AC

IR 20 kA HRC FAST + DIODE

V & Auto V

600 V DC/AC RMS PTC & MOV Protection

mV, Ohm & others

600 V DC/AC RMS PTC & MOV Protection

Low Battery Voltage

Below approx. 4.2 V

Battery Type

4 x 1.5 V AA alkaline battery2

(ANSI/NEDA 15 A or IEC LR6)

Current Consumption

5 mA Typical

Battery Life

500 hours typical on Alkaline

Dimension

84 mm x 165 mm x 57 mm

(3.31" x 6.50" x 2.24")

Weight

485 g (17.1 oz.) with batteries

Accessories

Rubber Boot, Temperature probe, Test leads

Optional

Purchase Accessories

Magnetic hanger

Carry Case

2 Use of lithium batteries may causes an issue due to their higher voltage.

VOLTAGE MEASUREMENT

The following section provides the specifications

Range

Resolution

Accuracy

Test

DC V

50 mV3

1 .V

±0.1% + 10

N/A

500 mV

10 .V

±0.1% + 10

N/A

5 V

0.1

±0.05% + 5

N/A

50 V

1 mV

±0.05% + 5

N/A

500 V

10 mV

±0.05% + 5

N/A

600 V

0.1 V

±0.1% + 10

N/A

AC V

50 mV2

1 .V

±1.2% + 15

45 to 400 Hz

True RMS

500 mV

10 .V

±0.8% + 10

45 to 400 Hz

5 V

0.1 mV

±0.3% + 10

45 to 400 Hz

Crest-Factor

50 V

1 mV

±0.3% + 10

45 to 400 Hz

<10

500 V

10 mV

±0.3% + 10

45 to 400 Hz

600 V

0.1 V

±0.5% + 10

45 to 400 Hz

50 mV

1 .V

±2.0% + 20

0.4 to 5 kHz

500 mV

10 .V

±1.5% + 10

0.4 to 5 kHz

5 V

0.1 mV

±1.5% + 10

0.4 to 5 kHz

50 V

1 mV

±1.5% + 10

0.4 to 5 kHz

500 V

10 mV

±1.5% + 10

0.4 to 5 kHz

600 V

0.1 V

±1.5% + 10

0.4 to 5 kHz

AC V+DC V

5 V

0.1 mV

±1.0% + 10

45 to 1000 Hz

True RMS

50 V

1 mV

±1.0% + 10

45 to 1000 Hz

500 V

10 mV

±1.0% + 10

45 to 1000 Hz

600 V

0.1 V

±1.0% + 10

45 to 1000 Hz

3 Using Relative Mode, zero with shorted test leads.

This channel has overload protection for up to 600 V.

The AC Bandwidth extends beyond 5 kHz, but the meter is only specified and

calibrated at 5 kHz. The AD8436 used specifies an additional 1% error up to

65 kHz. Higher frequency is possible with additional error above that again.

CURRENT MEASUREMENT

The following section refers to the current measurement ranges and their re-

spective performances. The current measurement module is protected by a

400 mA / 600 V HRC Fuse and an 11 A / 1000 V HRC Fuse.

Range

Resolu-

tion

Accuracy

Tested @

Burden Volt-

age4

DC A

50 .A5

1 nA

±1.5%+15

N/A

100 .V/.A

500 .A

10 nA

±1.5%+15

N/A

100 .V/.A

5 mA

0.1 .A

±0.25%+5

N/A

2 mV/mA

50 mA

1 .A

±0.25%+5

N/A

2 mV/mA

0.5 A

10 .A

0.75%+15

N/A

0.03 V/A6

5 A

100 .A

0.75%+15

N/A

0.03 V/A6

10 A

1000 .A

0.75%+15

N/A

0.03 V/A6

AC A

50 .A

1 nA

±2.0%+20

45 to 1000 Hz

100 .V/.A

500 .A

10 nA

±2.0%+20

45 to 1000 Hz

100 .V/.A

5 mA

0.1 .A

±1.0%+5

45 to 1000 Hz

2 mV/mA

50 mA

1 .A

±1.0%+5

45 to 1000 Hz

2 mV/mA

0.5 A

10 .A

±1.5%+15

45 to 1000 Hz

0.03 V/A6

5 A

100 .A

±1.5%+15

45 to 1000 Hz

0.03 V/A5

10 A

1000 .A

±1.5%+15

45 to 1000 Hz

0.03 V/A5

4 The measured burden voltage is displayed on the secondary display.

5 This mode will use the x10 amplified and may have additional offset error

that should be REL’d out before measurement.

6 The burden voltage can change with the type of fuse fitted.

RESISTANCE & CAPACITANCE MEASUREMENT

The following section outlines the specifications for the impedance ranges

Range

Resolution

Diff salvati

Testo originale

Apri file

USER'S MANUAL

121GW 
MULTIMETER

Last Revised 
08 February 2019

CONTENTS 
Contact .............................................................................................................. 5 
Glossary of Terms .............................................................................................. 6 
Change log ......................................................................................................... 7 
Firmware Version .............................................................................................. 8 
General .............................................................................................................. 9 
FCC/IC Information ........................................................................................ 9 
Information to the User ................................................................................ 9 
Introduction .................................................................................................... 10 
Features .......................................................................................................... 11 
Specifications .................................................................................................. 13 
General ........................................................................................................ 13 
Voltage Measurement ................................................................................. 15 
Current Measurement ................................................................................. 16 
Resistance & Capacitance Measurement .................................................... 17 
Diode Measurement.................................................................................... 18 
Frequency & Duty Cycle Measurement ...................................................... 19 
VA Measurement ........................................................................................ 20 
Temperature & Continuity .......................................................................... 21 
Bluetooth Specifications .............................................................................. 22 
Standards ........................................................................................................ 23 
Compliance Symbols ................................................................................... 23 
IEC Standards ............................................................................................... 23 
ANSI Standards ............................................................................................ 23 
Safety .............................................................................................................. 23 
Warning ....................................................................................................... 24 
Caution ........................................................................................................ 27 
Operation ........................................................................................................ 28 
Measuring ....................................................................................................... 33 
Voltage .............................................................................................. 33 
dBm .......................................................................................................... 35 
Voltage ................................................................................................ 36 
Temperature ......................................................................................... 37 
Current ......................................................................................... 38 
Burden Voltage ........................................................................................ 40 
Resistance ............................................................................................. 42 
Continuity Testing ................................................................................ 43 
Cable Break Testing ............................................................................. 44 
Capacitance .......................................................................................... 45 
Frequency ........................................................................................... 46 
Duty Cycle ........................................................................................... 47 
Period .................................................................................................. 48 
Diode Measurement ............................................................................ 49 
Power (VA) ................................................................... 51 
Data Logging ................................................................................................ 52 
Backlight ...................................................................................................... 53 
Bluetooth ..................................................................................................... 53 
Measurement HOLD .................................................................................... 54 
Relative Measurement (REL) ....................................................................... 55 
MIN/MAX/AVG ............................................................................................ 56 
Setup Menu ................................................................................................. 57 
Temperature ............................................................................................ 58 
Battery Voltage ........................................................................................ 58 
Auto Power Off (APO) .............................................................................. 58 
Buzzer On/Off .......................................................................................... 59 
LCD Contrast ............................................................................................ 59 
Date.......................................................................................................... 60 
Multimeter ID .......................................................................................... 61 
Logging Interval ....................................................................................... 61 
Maintenance ................................................................................................... 62 
Debugging .................................................................................................... 62 
Cleaning and Storage ................................................................................... 62 
Fuse Replacement ....................................................................................... 62 
Calibration ....................................................................................................... 64 
Calibration Mode ......................................................................................... 65 
Saving Calibration Data ............................................................................... 66 
Loading Calibration Data ............................................................................. 66 
Zero Offset Calibration ................................................................................ 67 
Full Scale GAIN Calibration .......................................................................... 68 
Calibration Reference Table ............................................................................ 69 
Firmware Update ............................................................................................ 70 
Warranty ......................................................................................................... 71

CONTACT

If you have a warranty claim, you can contact us at the following email ad-
dress: 
sales@eevblog.com

This multimeter and accessories can be purchased from: 
www.eevblog.com

Community Support for the meter is provided on the EEVblog Forum: 
www.eevblog.com/forum

Issues should be reported in the Report an issue section of the multimeters 
official page: 
www.eevblog.com/product/121gw/

Phone: 
Ph: +61(2)8626 8743

GLOSSARY OF TERMS

IEC

(International Electrotechnical Commission) 
An organisation that publishes various international standards relating to 
electrical, electronics and related technology.

ANSI

(American National Standards Institute). 
Americas non-profit standards organisation that oversee the development 
and use of standards in various domains.

ETL

(ETL Testing Laboratories) 
The compliance certification mark of Intertek Group in Europe.

(Overload Protection) 
The voltage that a function of the Multimeter than withstand.

Accuracy 
The closeness of a measured value to a known standard.

Low BurdenTM 
Provides low burden voltage when measuring current compared with typical 
multimeters, minimising impact on your in-circuit current measurements.

CHANGE LOG

The following represents the changes made to the manual:

Date

Changes

15/01/17

Added Serial Number section

Added Hold requirement to save setup items

Fixed various grammar errors

Added Bluetooth symbol to frontpage

31/01/18

Changed incorrect fuse rating to 
400 mA / 600 V HRC Fuse

Added Low Z mode description

04/03/18

Added Multimeter ID to list on page 53.

11/04/18

Minor grammar and a spelling change.

Added VA specifications 
Specification format changes

24/05/18

Added information about new firmware version 1.15 and 
1.17.

09/07/18

Added information about new firmware version 1.21 and 
1.22. 
Added more information about use of Low Z mode.

26/07/18

Modified formatting of VAC table. 
Described Relative mode. 
Improved burden voltage section steps. 
Fixed error in offset calibration table.

Changed steps for all setup items to allow for units that 
have buzzer turned off. 
Some grammar errors resolved.

27/07/18

Updated IAP section, removed incorrect step.

24/08/18

Updated DC mode section description for clarity.

30/08/18

Updated sections on battery type.

Added warning about lithium batteries.

11/09/18

Added information about 500VA range.

04/12/18

Moved case into optional accessory.

29/01/19

Added comment about modes being preserved between 
resets.

FIRMWARE VERSION

This manual is written for version 1.07 of the 121GW firmware. The following 
is a list of changes for each version:

Version

Changes

1.00

Added support for exporting and importing calibration 
data to/from SD card. 
Displays version number on boot.

1.01

Internal version.

1.02

Internal version.

1.03

Fixed possible import export calibration bug.

1.04

Resistance auto-ranging speed improved.

Can now turn beeper off.

Negative VA values now readable.

1.05

Further improved auto-ranging speed.

Improved Low-Z measurement up to 20KHz.

1.06

Internal version.

1.07

Improved VA mode over-estimation error.

1.15

Improved display to show OFL in appropriate sections, 
fixed lookup table bug with various range.

1.17

Improved accuracy of voltage measurement in AC and DC 
VA modes.

1.21

Added support for .C and .F in the Bluetooth packet.

1.22

Added 2 more bits of data for the main data to 
accommodate values over 65535.

1.25

Auto-ranging loop in M Ohms range resolved. 
Burden Voltage glitch resolved.

1.26

Improved resistance range noise when logging.

1.51

208ms data logging interval -> 200ms. 
File date and time in file meta info corrected. 
DC+AC auto ranging loop issue resolved.

Amp range sensitivity improved. 
AC mode frequency persistence issue resolved.

VAC + LPF bargraph issue resolved.

GENERAL

FCC/IC INFORMATION

This device complies with Part 15 of the FCC Rules and CAN ICES-3(A). Opera-
tion is subject to the following two conditions: (1) this device may not cause 
harmful interference, and (2) this device must accept any interference re-
ceived, including interference that may cause undesired operations.

INFORMATION TO THE USER

This equipment has been tested and found to comply with the limits for a 
Class B digital device, pursuant to part 15 of the FCC Rules. These limits are 
designed to provide reasonable protection against harmful interference in a 
residential installation.

This equipment generates, uses and can radiate radio frequency energy and, 
if not installed and used in accordance with the instructions, may cause 
harmful interference to radio communications. However, there is no guaran-
tee that interference will not occur in an installation.

If this equipment does cause harmful interference to radio or television re-
ception, which can be determined by turning the equipment off and on, the 
user is encouraged to try to correct the interference by one or more of the 
following measures:

• Reorient or relocate the receiving antenna. 
• Increase the separation between the equipment and receiver. 
• Connect the equipment into an outlet on a circuit different from that 
to which the receiver is connected. 
• Consult the dealer or an experienced radio/TV technician for help.

WARNING

Any changes or modifications not expressly approved by the manufacturer 
could void the user’s authority to operate the equipment.

INTRODUCTION

Congratulations on purchasing the EEVblog 121GW multimeter. It's one of 
the most feature packed small form factor meters on the market, designed to 
provide maximum functionality in as little bench space as possible. It pro-
vides many unique features not found on other multimeters, like low burden 
voltage measurement and display, simultaneous voltage and current (VA 
power) measurement, 15 V diode test voltage, Bluetooth built-in, and Micro 
SD card data logging.

You've made a wise choice buying this instead of some questionable quality 
cheapie, it should return the favour with many years of safe and accurate ser-
vice.

That ETL logo you see stamped on the back of the meter ensures that the me-
ter design has been independently tested to the latest IEC/UL/EN61010-1 3rd 
edition safety standard by Intertek.

With a genuine 600 V CAT III rating and proper 1000V HRC fuses and 
MOV+PTC protection you can feel confident and safe that you can use this 
meter on most high energy mains rated equipment.

And remember, don't turn it on, take it apart! 
This meter is hackable, go for it.

Have fun. 
Dave Jones 
www.eevblog.com

FEATURES

Summary

• True RMS sensing up to crest factor 10 (AD8436 chipset) 
• 600 V CAT III ETL Listed Rating 
• 1000 V HRC fuses 
• High voltage 15V diode test mode 
• Dual 50,000 count display 
• Relative Mode 
• 1ms Peak Capture 
• MIN, MAX or AVG on all functions 
• Low BurdenTM current measurement feature 
• OPEN and CLOSED continuity testing (for wire breaks) 
• Temperature, Capacitance, Frequency and Duty Cycle 
• Unique burden voltage measurement & display in current mode 
• Simultaneous voltage and current measurement and VA calculation 
• Low Z input mode eliminates ghost voltages (not for under 12V) 
• Automatic touch measurement data hold 
• Audible and visual incorrect input jack warning 
• Closed case calibration of individual ranges

Logging Capabilities

• Bluetooth LE (Low Energy) Communication 
• Data logging onto a Micro SD card (CSV format)

Mechanical

• Compact size 
• Backlit display

Software

• Android, iPhone, and Windows 10 support, [Mac]1 
• Updatable Firmware (via micro SD Card)

1 Under development

Batteries

• RTC Battery (CR1220) 
• Main Battery (4 x AA) 
• Long battery life of >500 hrs

SPECIFICATIONS

The following specifications are given in SI units with their accuracy notated 
as ± (% of reading digits) + (number of digits) unless otherwise indicated.

GENERAL

The following table represents general specifications (at 23°C ± 5°C):

Item

Specification

Display

4 . digits 50,000 counts, dual display

Update Rate

5 times per second nominal

Operating Temperature

0°C to 50°C (32°F to 122°F)

Relative Humidity

< 75%

Altitude

Operating below 2000 m

Storage Temperature

-20°C to 50°C (-44°F to 122°F)

Temperature 
Coefficient

Temperature coefficient (Nominal): 
....=0.1 . (.................. ................) / °..

RMS Sensing

True RMS sensing (AD8436)

Ingress Protection

IP41

Pollution Degree

Safety

cETLus 3rd Edition, CAT III 600 V, CE Conformity, 
IP41, RoHS Compliant, IEC/EN61010-1: 2010 3rd 
Edition

Drop Protection

1 m

Transient Protection

6 kV 2 ., Pulsed Source

E.M.C.

Commercial limits compliance with EN61326-1

Bluetooth

BLE112 Class 2

Chipset

Hycon HY3131 
ST ARM STM32L152D

Overload Protection

mA and .A

400 mA / 600 V DC/AC 
IR 10kA HRC FAST + DIODE

A and 500mA

11 A / 1000 V DC/AC 
IR 20 kA HRC FAST + DIODE

V & Auto V

600 V DC/AC RMS PTC & MOV Protection

mV, Ohm & others

600 V DC/AC RMS PTC & MOV Protection

Low Battery Voltage

Below approx. 4.2 V

Battery Type

4 x 1.5 V AA alkaline battery2 
(ANSI/NEDA 15 A or IEC LR6)

Current Consumption

5 mA Typical

Battery Life

500 hours typical on Alkaline

Dimension

84 mm x 165 mm x 57 mm 
(3.31" x 6.50" x 2.24")

Weight

485 g (17.1 oz.) with batteries

Accessories

Rubber Boot, Temperature probe, Test leads

Optional 
Purchase Accessories

Magnetic hanger 
Carry Case

2 Use of lithium batteries may causes an issue due to their higher voltage.

VOLTAGE MEASUREMENT

The following section provides the specifications

Range

Resolution

Accuracy

Test

DC V

50 mV3

1 .V

±0.1% + 10

N/A

500 mV

10 .V

±0.1% + 10

N/A

5 V

0.1

±0.05% + 5

N/A

50 V

1 mV

±0.05% + 5

N/A

500 V

10 mV

±0.05% + 5

N/A

600 V

0.1 V

±0.1% + 10

N/A

AC V

50 mV2

1 .V

±1.2% + 15

45 to 400 Hz

True RMS

500 mV

10 .V

±0.8% + 10

45 to 400 Hz

5 V

0.1 mV

±0.3% + 10

45 to 400 Hz

Crest-Factor

50 V

1 mV

±0.3% + 10

45 to 400 Hz

<10

500 V

10 mV

±0.3% + 10

45 to 400 Hz

600 V

0.1 V

±0.5% + 10

45 to 400 Hz

50 mV

1 .V

±2.0% + 20

0.4 to 5 kHz

500 mV

10 .V

±1.5% + 10

0.4 to 5 kHz

5 V

0.1 mV

±1.5% + 10

0.4 to 5 kHz

50 V

1 mV

±1.5% + 10

0.4 to 5 kHz

500 V

10 mV

±1.5% + 10

0.4 to 5 kHz

600 V

0.1 V

±1.5% + 10

0.4 to 5 kHz

AC V+DC V

5 V

0.1 mV

±1.0% + 10

45 to 1000 Hz

True RMS

50 V

1 mV

±1.0% + 10

45 to 1000 Hz

500 V

10 mV

±1.0% + 10

45 to 1000 Hz

600 V

0.1 V

±1.0% + 10

45 to 1000 Hz

3 Using Relative Mode, zero with shorted test leads.

This channel has overload protection for up to 600 V.

The AC Bandwidth extends beyond 5 kHz, but the meter is only specified and 
calibrated at 5 kHz. The AD8436 used specifies an additional 1% error up to 
65 kHz. Higher frequency is possible with additional error above that again.

CURRENT MEASUREMENT

The following section refers to the current measurement ranges and their re-
spective performances. The current measurement module is protected by a 
400 mA / 600 V HRC Fuse and an 11 A / 1000 V HRC Fuse.

Range

Resolu-
tion

Accuracy

Tested @

Burden Volt-
age4

DC A

50 .A5

1 nA

±1.5%+15

N/A

100 .V/.A

500 .A

10 nA

±1.5%+15

N/A

100 .V/.A

5 mA

0.1 .A

±0.25%+5

N/A

2 mV/mA

50 mA

1 .A

±0.25%+5

N/A

2 mV/mA

0.5 A

10 .A

0.75%+15

N/A

0.03 V/A6

5 A

100 .A

0.75%+15

N/A

0.03 V/A6

10 A

1000 .A

0.75%+15

N/A

0.03 V/A6

AC A

50 .A

1 nA

±2.0%+20

45 to 1000 Hz

100 .V/.A

500 .A

10 nA

±2.0%+20

45 to 1000 Hz

100 .V/.A

5 mA

0.1 .A

±1.0%+5

45 to 1000 Hz

2 mV/mA

50 mA

1 .A

±1.0%+5

45 to 1000 Hz

2 mV/mA

0.5 A

10 .A

±1.5%+15

45 to 1000 Hz

0.03 V/A6

5 A

100 .A

±1.5%+15

45 to 1000 Hz

0.03 V/A5

10 A

1000 .A

±1.5%+15

45 to 1000 Hz

0.03 V/A5

4 The measured burden voltage is displayed on the secondary display.

5 This mode will use the x10 amplified and may have additional offset error 
that should be REL’d out before measurement.

6 The burden voltage can change with the type of fuse fitted.

RESISTANCE & CAPACITANCE MEASUREMENT

The following section outlines the specifications for the impedance ranges

Range

Resolution

Accuracy

R .

50 .7

0.001 .

±0.5%+20

500 .

0.01 .

±0.3%+5

5 k.

0.1 .

±0.2%+5

50 k.

1 .

±0.2%+5

500 k.

10 .

±0.2%+5

5 M.

0.1 k.

±0.3%+5

50 M.

1 k.

±1.2%+20

C .F

10 nF

0.01 nF

±2.5%+5

100 nF

0.1 nF

±2.5%+5

1 .F

1 nF

±2.5%+5

10 .F

10 nF

±2.5%+5

100 .F

100 nF

±2.5%+5

9999 uF

1 .F

±3.0%+58

7 Using Relative Mode, zero with shorted test leads.

8 Accuracies depend on capacitor type.

This channel has overload protection for up to 600 V.

DIODE MEASUREMENT

Range

Resolution

Test Current

Accuracy

Diode V

3 V

0.001 V

1.4 mA

Not specified

15 V

0.001 V

7 mA

Not specified

This channel has overload protection for up to 600 V.

A nominal 2.2 K resistor (includes PTC) is in series with the voltage source and 
is what limits the test current.

FREQUENCY & DUTY CYCLE MEASUREMENT

Range

Resolution

Accuracy

F Hz9

99.999 Hz

0.001 Hz

±0.1%+3

999.99 Hz

0.01 Hz

±0.1%+3

9.9999 KHz

0.1 Hz

±0.1%+3

99.999 KHz

1 Hz

±0.1%+3

999.99 KHz

10 Hz

±0.1%+3

Duty 
Cycle 
%10

99 %

0.1 %

±0.2% + 5 + 0.2/kHz

9 Sensitivity > 2.5 V RMS

10 0.01 to 200 kHz, Pulse Width: >5 us, Sensitivity: > 2.5 V RMS

This channel has overload protection for up to 600 V. 
This range has an apparent 99,999 counts.

VA MEASUREMENT

Function

Range

Resolution

Accuracy

DC VA

250.00 uVA

0.01 uVA

±1.0% + 5 digits

2500.0 uVA

0.1 uVA

±0.5% + 5 digits

25000 uVA

1 uVA

±0.5% + 5 digits

25.000 mVA

0.001 mVA

±0.5% + 5 digits

250.00 mVA

0.01 mVA

±0.5% + 5 digits

2500.0 mVA

0.1 mVA

±0.5% + 5 digits

25000 mVA

1 mVA

±0.5% + 5 digits

50.000 VA

0.001 VA

±1.0% + 10 digits

500.00 VA11

0.01 VA

±1.0% + 10 digits

AC VA

250.00 uVA

0.01 uVA

±2.0% + 10 digits

2500.0 uVA

0.1 uVA

±2.0% + 10 digits

25000 uVA

1 uVA

±1.5% + 10 digits

25.000 mVA

0.001 mVA

±1.5% + 10 digits

250.00 mVA

0.01 mVA

±1.5% + 10 digits

2500.0 mVA

0.1 mVA

±1.5% + 10 digits

25000 mVA

1 mVA

±1.5% + 10 digits

50.000 VA

0.001 VA

±2.0% + 20 digits

500.00 VA11

0.01 VA

±2.0% + 20 digits

11 500 VA, with 50 V and 10 A.

The TRMS bandwidth of the AC VA range is from 45 to 1 kHz. 
This channel has overload protection for up to 600V. 
Protection includes 440mA/1000V HRC Fuse 11A/1000V HRC Fuse.

TEMPERATURE & CONTINUITY

Range

Resolution

Accuracy

Temp 
°C

-200 to 1350°C 
K-Type

0.1°C

±0.5%+3°C

Continuity

30 . or 300 . 
User Selectable 
OPEN or CLOSED

10 m.

Not Specified

This channel has overload protection for up to 600 V.

External temperature measurement uses a K-Type thermocouple.

Internal temperature measurement is inside a sealed case, so will have some 
thermal lag.

The continuity tester can be set for beep on CLOSED or beep on OPEN (for 
wire breaks) with a user adjustable threshold level.

BLUETOOTH SPECIFICATIONS

• The 121GW uses a BLE112-A Class 2 certified Low Energy Bluetooth 
module. 
• This device complies with Part 15 of the FCC Rules. 
• This device complies with Industry Canada licence-exempt RSS stand-
ard(s). 
• This device complies with Japan MIC certification ARIB STD-T66.

Contains Transmitter Module FCC ID: QOQBLE112 
Contains Transmitter Module IC: 5123A-BGTBLE112

The Bluetooth module is in conformity with the essential requirements and 
other relevant requirements of the CE RED(2014/53/EU).

The product is in conformity with the following standards and/or normative 
documents.

Essential Requirement

Applicable Standards / Specifications

Safety Requirements

EN 60950-
1:2006+A11:2009+A1:2010+A12:2011+A2:2013

EMC Requirements

EN 301 489-1 V2.1.1 
EN 301 489-17 V3.1.1

Radio Spectrum

EN 300 328 V2.1.1

The BLE112 Bluetooth module is KCC certified with following certification 
numbers:

• BLE112-A: KCC-CRM-BGT-BLE112-A 
• BLE112-E: KCC-CRM-BGT-BLE112-E 
• BLE112-N: KCC-CRM-BGT-BLE112-N

STANDARDS

This Multimeter complies to the following standards.

COMPLIANCE SYMBOLS

The device is marked with the following symbols, these have the following 
meanings:

Symbol

Definition

RCM, this is the Regulatory Compliance Mark. It indicates that 
the product is compliant to required Australian Standards.

CE, this is the Conformite Europeenne mark. It indicates that the 
product complies with required European standards.

ETL, the listed CUS Mark. It Indicates that the product is the 
mark that indicates testing by Intertek.

IEC STANDARDS

The product is Extra-Low voltage system, as per IEC 60038.

The product VAC does not supply 50 or 120 VDC.

The product is classified as IP41 as per IEC 60529 standards. Tested for IEC 
60950-1 and IEC/EN61010-1: 2010 3rd Edition.

ANSI STANDARDS

Safety requirements tested as per ANSI 61010-1 and testing for hand-held 
equipment as per ANSI 61010-2.

CAT III rating as per ANSI. 61010-1.

SAFETY

This manual contains information and warnings that must be followed when 
operating the device. Following these instructions will protect you from dam-
aging the device or risking your well-being. The device can only protect you 
up to a point and it should not be used in situations that do not comply with 
the devices ratings and the recommendations in this section.

The following symbols are used on the device and through this manual to in-
dicate whether something is a warning or a precaution.

Symbol

Definition

Caution: Possibility of electric shock

Caution: See explanation in section

WARNING

A condition or action that could result in serious injury or death to the user.

1. Using the device outdoors risks electric shock or fire. The device is 
not designed to operate in rain or moisture. 
2. The product is rated IP41 and should be treated accordingly. This 
means that the product should not be exposed to liquids or dust. 
3. Inspect leads and ensure that no defects or damage exists on the 
leads, connectors or probes. Defective parts must not be used and 
need to be replaced before use. 
4. Only use UL Listed Probe Assemblies with ratings that are greater or 
equal to the meter rating. 
5. CAT III & CAT IV test probe tips must be 4 mm 
(As per IEC 61010-031). 
6. All accessories must possess a greater than or equal to CAT rating to 
that of the Multimeter.

French:

Securite

Ce manuel contient des informations et des avertissements qui doivent etre 
suivis lors de l'utilisation de l'appareil. En suivant ces instructions, vous prote-
gerez d'endommager l'appareil ou de risquer votre bien-etre. L'appareil ne 
peut que vous proteger jusqu'a un certain point et ne doit pas etre utilise 
dans des situations qui ne sont pas conformes a la note des appareils et aux 
recommandations de cette section.

Les symboles suivants sont utilises sur le peripherique et par le biais de ce 
manuel pour indiquer si quelque chose est un avertissement ou une precau-
tion.

Symbole

Definition

Attention: Possibilite de choc electrique

Attention: voir l'explication dans la section

Attention

Une condition ou une action qui pourrait entrainer des blessures graves ou la 
mort de l'utilisateur.

1. L'utilisation de l'appareil a l'exterieur risque d'etre electrique ou 
d'incendie. L'appareil n'est pas concu pour fonctionner sous la pluie 
ou l'humidite. 
2. Le produit est classe IP41 et devrait etre traite en consequence. Cela 
signifie que le produit ne doit pas etre expose aux liquides ou a la 
poussiere. 
3. Inspectez les cables et assurez-vous qu'aucun defaut ou dommage 
n'existe sur les conducteurs, les connecteurs ou les sondes. Les

pieces defectueuses ne doivent pas etre utilisees et doivent etre 
remplacees avant utilisation. 
4. N'utilisez que des ensembles de sonde homologues UL avec des 
valeurs nominales superieures ou egales a celles du compteur. 
5. Les pointes de la sonde de test CAT III et VAT IV doivent etre de 4 mm 
(selon CEI 61010-031). 
6. Tous les accessoires doivent posseder une valeur nominale 
superieure ou egale a celle du multimetre.

CAUTION

A condition or action that could result in damage or malfunctions in the in-
strument.

1. Before changing functions, remove test leads from any measurement 
point. 
2. Before any measurement, ensure test leads are connected to appro-
priate connector on the device and in an appropriate manner to the 
test points.

French:

Une condition ou une action pouvant entrainer des degats ou des dysfonc-
tionnements dans l'instrument.

1. Avant de modifier les fonctions, retirez les fils de test de n'importe 
quel point de mesure. 
2. Avant toute mesure, assurez-vous que les fils de test sont connectes 
au connecteur approprie sur l'appareil et de maniere appropriee aux 
points de test.

OPERATION

The following section will outline how to operate the device. This section uti-
lises some symbols which are defined in the table below:

Digital Display: Readings are displayed on dual 4 . -digit 50,000 count LCD 
display. One Primary and one smaller secondary function display. The sec-
ondary function display is only used when a function that requires it is se-
lected. A fast updating bargraph is below the main display.

Rotary Switch: Used to turn the power off and on and select the test func-
tion. There are two OFF positions for convenience, both work identically.

RANGE Button: Used to swap between ranges of a function. 
This could be moving the decimal point for the voltage range, 
swapping between 3 V or 15 V diode mode etc...

HOLD Button: Used to select the Auto HOLD function

REL Button: Used to select the relative measurement mode. 
Secondary use as the UP arrow key for setup selection. Holding 
down this button in AC mode selects the 1 kHz low pass filter.

1ms PEAK Button: Used to select the 1 ms peak capture mode 
(VAC only). Secondary use as the DOWN arrow key for setup se-
lection.

Holding down this button turns on and off the Bluetooth data 
transfer functionality.

MODE Button: Used to select and cycle through the different 
function modes displayed in RED on the mode dial. Holding 
down this button will activate and deactivate the display back-
light.

MIN/MAX Button: Cycles through and selects the MIN, MAX, 
and AVG measurement modes.

MEM Button: Holding down this button starts the SD card data 
logging. Pressing once allows viewing of the stored data.

SETUP Button: Displays the setup menu and cycles through the 
options. Press and hold the button when on the required option 
to edit that option.

A / 500mA Jack: This input jack is used for the 10 A, 5 A and 
500 mA current ranges. It is 11 A HRC fused. An input warning 
alert functions for this jack.

mA / .A Jack: This input jack is used for the 50 mA, 500 .A and 
50 .A current ranges. It is 400 mA HRC fused. An input warning 
alert functions for this jack.

COM Jack: The negative common jack for all ranges and functions.

V. Jack: The positive input jack used for voltage, re-
sistance, capacitance, diode, continuity, and temperature 
functions. This input jack is PTC and MOV protected.

MEASURING

VOLTAGE

When measuring AC voltages, you need to select the AC mode using the 
mode button. You will see the AC icon appear on the screen when you are in 
the correct mode.

In VAC mode, you can use the 1ms PEAK function, this will determine the 
maximum peak voltage, instead of the RMS voltage.

To measure DC voltages, you must select the DC mode. 
This is done by pressing the mode button. The DC icon will be shown on the 
screen when you are in the correct mode.

Modes

Pressing the MODE button will cycle through the following modes: 
The following modes are available on the voltage range:

1. DC 
2. AC 
3. DC+AC

The following modes are available on the millivolts range:

1. DC 
2. AC 
3. Temperature

DBM

In AC mode the multimeter supports standard dBm measurement to view 
this measurement press SETUP until the screen shows dBm in the top right.

The value of dBm is defined as follows, ........ is by convention:

..=10log10(
..
..0)

..=
........
2
........

........=600 .

..0=1 ....

.......=10log10(
........
26000.001)=10log10(
53
........
2)

VOLTAGE

The Low Z mode of the multimeter is vastly different to the VAC mode. The 
mode provides a means of draining ghost-voltages. The impedance of this 
range is particularly low when compared to other ranges. 
The range connected to ground through 1K resistor and a 1.2K PTC.

This range is used in the same manner to the VAC mode.

The range is not designed to be operated at low voltages (below 12V), it is 
also not designed for high frequencies. This range is typically used between 
50Hz and 400Hz.

TEMPERATURE

When measuring temperature be sure to have the leads connected as above 
and in the correct range.

To enter the temperature mode, press the mode button twice after selecting 
mV on the dial.

CURRENT

When measuring current be sure to have the leads connected as above and 
in the correct range, for higher currents ensure that the red lead is connected 
to the A 500 mA socket.

When measuring AC current, you must press the mode button until the AC 
icon appears on screen.

The factory default mode (new settings will be preserved between resets) 
will be DC measurement. If the DC icon is not shown on the screen press the 
mode button until it is.

Modes

Pressing the MODE button will cycle through the following modes: 
The following modes are available on the amp/milliamp range :

1. DC 
2. AC

The following modes are available on the microamp range :

1. DC 
2. AC

1. Press and hold , until bd.XXX flashes. 
2. Press UP or DOWN to toggle on or off the burden voltage. 
3. Press and hold , until bd.XXX no longer flashes. 
4. Press again to display the burden voltage on the secondary 
display.

BURDEN VOLTAGE

When measuring current on the multimeter there is a voltage drop across the 
resistor used to measure the current. This voltage is called the “burden volt-
age”. 
To enable or disable the burden voltage measurement, perform the following 
steps:

To measure the burden voltage an additional lead is required, this must be 
connected between the active current terminal and the voltage input, as 
shown below:

RESISTANCE

When measuring resistance select the range as shown above with the leads 
connected as shown above.

Resistance is measured between the two leads.

Modes

Pressing the MODE button will cycle through the following modes: 
The following modes are available on the resistance range :

1. Resistance 
2. Continuity tester 
3. Diode tester 
4. Capacitance measurement

CONTINUITY TESTING

When taking measurements, you must connect the leads as shown above.

To check if a short (Selectable <30 . or <300 .) exists you can use the conti-
nuity tester. To use this range, you need to press the MODE button once. To 
change the continuity threshold level between >30 . or >300 ., select the 
Continuity mode and:

1. Press SETUP until “UP xxx” or “dN xxx” appears on the small display. 
2. Hold SETUP to change the mode. 
3. Press the UP and DOWN buttons to toggle between continuity testing 
mode and cable break testing mode. Continuity mode is the “dN” mode, 
and you can select either dN 30 or dN 300 for 30 . or 300 . respectively. 
4. Hold SETUP to store the required mode (Until it beeps).

CABLE BREAK TESTING

When taking measurements, you must connect the leads as shown above.

To check if a break exists (Selectable > 30 . or > 300 .) you can use the cable 
break tester. To use this range, you need to select the Continuity mode and:

1. Press SETUP until “UP xxx” or “dN xxx” appears on the small display. 
2. Hold SETUP to change the mode. 
3. Press the UP and DOWN buttons to toggle between continuity testing 
mode and cable break testing mode. Cable break mode is the “UP” 
mode, and you can select either UP 30 or UP 300 for 30 . or 300 . 
respectively. 
4. Hold SETUP to store the mode (Until it beeps).

CAPACITANCE

When taking measurements, you must connect the leads as shown above.

To enter the capacitance mode, press the MODE button three times.

You can zero out the residual lead and meter capacitance with the REL func-
tion.

Ensure that the capacitor is discharged before measurement.

FREQUENCY

When taking measurements, you must connect the leads as shown above.

Frequency measurement is the factory default mode (new settings will be 
preserved between resets) for the dial position shown above.

Modes

Pressing MODE will result in a change in the measurement mode 
The following modes are available on the frequency range :

1. Frequency 
2. Pulse width 
3. Duty Cycle

DUTY CYCLE

When taking measurements, you must connect the leads as shown above.

To enter the duty cycle mode press mode twice after selecting the Hz dial po-
sition.

PERIOD

When taking measurements, you must connect the leads as shown above.

To enter the period measurement mode press MODE once after selecting the 
Hz dial position. The period in milliseconds will be displayed.

DIODE MEASUREMENT

When taking measurements, you must connect the leads as shown above.

The diode measurement range also supports a high voltage mode (up to 15 
V) that enables the measurement of Zener voltages, LED string arrays and 
other similar potentials.

The output current is limited by the nominal 2.2 k. total PTC + Resistor pro-
tection devices, and the voltage mode selected. It is not a constant current 
generator, so take this into account when measuring devices.

Voltage mode is toggled between 3 V and 15 V by pressing the RANGE button 
after selecting the diode measurement mode, the secondary display will then 
show the maximum diode voltage measurable.

The voltage range is displayed on the secondary display as follows:

POWER (VA)

Power measurements require measurements of both voltage and current, be 
careful to connect the system in a manner that the common terminal is com-
mon for both the voltage measurement and the current measurement.

An example is shown below:

DATA LOGGING

Data logging is started by holding the MEM button until a beep is heard and 
the MEM icon flashes on screen.

Logged data is stored to an internal Micro SD card which is accessible behind 
the battery cover. Data is by default recorded once per second.

The logging data is stored on the SD card in the following format as a CSV:

Main

Secondary

Sample

Function

Value

Units

Function

Value

Units

DCV

0.0000

etc.

The data that is recorded is taken from the currently displayed function, if 
you are measuring voltage then voltage is recorded. The number of accumu-
lated logged data points is displayed on the secondary display.

To stop data logging press and hold the MEM button.

BACKLIGHT

To enable the backlight, hold the MODE button.

BLUETOOTH

To enable Bluetooth communications, hold the “1ms PEAK” button until BT is 
displayed on the LCD.

Ensure that Bluetooth is enabled on the host device before running the appli-
cation on the remote computing device.

The multimeter will continue to functional as normal in Bluetooth mode, but 
the display data will be transmitted via the Bluetooth connection.

To disable Bluetooth mode press and hold the 1ms Peak button until BT is 
not displayed.

MEASUREMENT HOLD

There are two measurement HOLD functions.

1. Manual Hold

To pause the measurement on the main display, press the HOLD button. To 
resume measurement, you need to press the button again until A- and HOLD 
are no longer visible.

The bargraph display will continue to update in “live” mode while the cap-
tured value is held on the screen.

2. Automatic Hold

Press the HOLD button twice until A-HOLD is displayed. 
This mode will wait for a stable reading and automatically lock that value on 
the display. A beep will be heard to indicate this capture. 
It will continue to hold this value until the measurement changes significantly 
and a new stable reading is found. This mode is great for when you must 
keep your hands on the probes.

RELATIVE MEASUREMENT (REL)

If you are measuring something relative to another measurement or need to 
calibrate any non-zero error out you can press the REL button to zero, the 
currently displayed value. 
All subsequent measurements will have this stored relative value subtracted 
from the result. The displayed measurement is the difference between the 
actual measured value and the stored relative value.

If is useful for example to cancel out the resistance of test probes in the ohms 
range.

MIN/MAX/AVG

To find the minimum and maximum value found during sampling you can use 
the MIN/MAX button.

This button also provides an average function AVG. This can be entered by 
pressing the button until the AVG symbol appears.

To capture min, max and average results press the MIN/MAX button until all 
three icons are shown on the LCD.

To exit out of these modes hold the button until a beep is heard, and the 
icons are cleared from the screen.

Pressing the MIN/MAX button will cycle through the displayed results of each 
measurement.

SETUP MENU

To configure and read settings for the multimeter you can press the SETUP 
button to view the settings. If you press the setup button multiple times you 
will cycle through the menu, the order of the menu is as follows:

1. Temperature 
2. Battery Voltage 
3. Auto Power Off (APo) 
4. Buzzer On/Off (b-Off) 
5. LCD Contrast 
6. Year 
7. Month-Day 
8. Hour-Minute 
9. Multimeter ID 
10. Logging Interval (ln 0) 
11. Additional settings dependent upon the function selected.

TEMPERATURE

The multimeter has an internal temperature which can be used for rough am-
bient temperature measurement. Note that there will be thermal lag due to 
the sealed enclosure. It is displayed on the secondary display.

The displayed temperature can be changed from Celsius to Fahrenheit using 
the SETUP menu. To do this:

1. Navigate to the temperature item using SETUP. 
2. Press and hold SETUP, until display flashes. 
3. Use the UP/DOWN arrow keys to toggle between “c” and “F”. 
4. To save the setting press and hold SETUP, until display no longer 
flashes.

BATTERY VOLTAGE

To check the status of the batteries in the multimeter press the SETUP button 
until “bAt” is displayed. The Low Battery icon will activate when the battery 
voltage falls below approximately 4.2 V

It is recommended that you change the battery when the battery icon is dis-
played, as the unit’s specifications are no longer guaranteed.

Lithium batteries have a higher voltage and may cause the device to malfunc-
tion. Alkaline batteries are recommended.

AUTO POWER OFF (APO)

With some measurements you may not want the multimeter to automatically 
power off (After 30 minutes). To disable automatic power off:

1. Navigate to the APo menu item using SETUP. 
2. Press and hold SETUP, until display flashes. 
3. Press UP/DOWN to toggle between “APO.oN” and “APO.oF”. 
4. To save the setting press and hold SETUP, until display no longer 
flashes.

APO is disabled in data logging mode.

BUZZER ON/OFF

If you wish to disable/enable the buzzer to perform the following steps:

1. Navigate to the b-OFF or b-ON menu item using the SETUP button. 
2. Press and hold SETUP, until display flashes. 
3. Press UP/DOWN to toggle between “b-OFF” and “b-ON”. 
4. To save the setting press and hold SETUP, until display no longer 
flashes.

LCD CONTRAST

If the multimeter has ghosted characters you may want to turn the contrast 
down, if the screen is not dark enough you may need to turn up the contrast.

This may need to be changed due to variations in ambient operating temper-
ature.

To change the contrast:

1. Navigate to the menu item that says LCD X using the SETUP button. 
2. Press and hold SETUP, until display flashes. 
3. Press UP and DOWN to configure the screen contrast, change until 
the screen contrast is as desired. 
4. To save the setting press and hold SETUP, until display no longer 
flashes.

DATE

There are three menu items that can be used to read and set the currently 
configured date. These are the following menu items:

1. Year “YYYY” 
2. Month-Day “MM-DD” 
3. Hour-Minute “HH-mm”

To change the date items:

1. Navigate to the menu item. 
2. Press and hold SETUP, until display flashes. 
3. Press UP and DOWN to configure the date/time item. 
4. To save the setting press and hold SETUP, until display no longer 
flashes.

MULTIMETER ID

This is a user configurable ID which is used to differentiate between multiple 
121GW units. This value is what appears in the App and can be changed by 
doing the following:

1. Navigate to the menu item “XXXXX” using SETUP (Where X are num-
bers) 
2. Press and hold SETUP, until display flashes. 
3. Press UP and DOWN keys to configure the value. 
4. Hold SETUP until a beep is heard to save the value.

LOGGING INTERVAL

The logging interval can range from 200 ms to 999 seconds. If you need to 
change the interval, then you should:

5. Navigate to the menu item “In x” using SETUP. 
6. Press and hold SETUP, until display flashes. 
7. Press UP and DOWN keys to configure the time interval in seconds. 
If “x” is set to zero, the sample rate will be set to the ADC’s maximum 
rate (approximately 200 ms). 
8. Hold SETUP until a beep is heard to save the value.

MAINTENANCE

The 121GW maintenance is outlined in this section.

Before opening case for maintenance, disconnect test leads from the 
meter and turn off the meter.

DEBUGGING

1. If a low battery alert appears on the device as indicated in the opera-
tion section, the back of the device must be opened to replace the 
AA batteries. 
2. If the measurements fail to respond to changes in measured devices 
check the fuse, leads and test probe connections.

CLEANING AND STORAGE

The device should be kept in a clean condition, this requires periodic clean-
ing. The device should be cleaned slightly damp cloth with mild detergent. Do 
not use solvents or abrasives as they could damage the device.

When the device is placed into long term storage (>50 days) the batteries 
should be removed to reduce the risk of leakage or unnecessary drain.

FUSE REPLACEMENT

To check whether you need to replace a fuse perform the following steps:

1. Switch to the voltage range and connect a lead as shown in the below 
diagram. 
2. If the multimeter beeps the fuse is functioning, if it fails to beep it 
indicates that the fuse is blown.

If it fails to beep when connected to the “A” jack, then it is likely that the 
larger A fuse is blown.

If it fails to beep when connected to the “mA” jack, then it is likely that the 
smaller mA fuse is blown.

• mA/.A current input fuse: 400 mA / 600 V DC/AC, IR 10 kA HRC FAST 
• Designed to UL 248-1 
• Dimension: 6mm x 32 mm 
• Recommended: ASTM HV610.0.4 400 mA (600V) or HV620.0.4 
(1000V) 
• A/500 mA current input fuse: 11 A/1000 V DC/AC, IR 20 kA HRC FAST 
• Dimension: 10mm x 38 mm 5AG 
• Recommended: ASTM HV110.11A, Bussmann DMM-B-11A, or Little-
fuse FLU011

CALIBRATION

WARNING:

Entering the calibration mode can render your unit non-operational or out of 
specification. Factory calibration will be VOID if this function is used. This 
mode is only intended for those with the experienced knowledge and equip-
ment necessary to calibrate their own equipment.

The calibration procedure can be performed for only one, or all desired 
ranges and functions. When complete, re-power the meter to exit calibration 
mode.

It is possible to recalibrate a single range and function if desired, there is no 
need to calibrate all ranges once started.

The calibration values are stored in EEPROM and are not affected by firm-
ware updates.

CALIBRATION MODE

To enable calibration mode the following procedure must be followed:

1. Turn off the meter 
2. Press and hold the MODE button. 
3. Turn on device. 
4. If the screen says, “CAL 1”. Press REL button, enter calibration mode. 
5. The secondary screen will display “CAL” when the device is in calibra-
tion mode. 
6. Switch to the function mode you want to calibrate as per normal me-
ter operation. (voltage, current, resistance etc) 
7. Press the RANGE button to enter the range you need to calibrate. 
To double check the current range notice the SI units displayed and 
the position of the decimal point. Calibrating the incorrect range 
could ruin your day. 
8. After selecting the range and mode you can perform calibration, see 
next section.

NOTE: Calibration should be performed in this order:

1. Offset calibration 
2. Scale calibration

SAVING CALIBRATION DATA

To backup calibration data you can perform the following procedure:

1. Ensure an SD card is in the multimeter. 
2. Hold MEM. 
3. Turn on the meter. 
4. Release MEM. 
5. Press the UP button. 
6. Press the Mode button. 
7. The calibration data will now be saved to the SD card.

LOADING CALIBRATION DATA

To load calibration data from SD card you can perform the following proce-
dure:

8. Ensure an SD card is in the multimeter. 
9. Hold MEM. 
10. Turn on the meter. 
11. Release MEM. 
12. Press the DOWN button. 
13. Press the Mode button. 
14. The calibration data will now be loaded from the SD card.

ZERO OFFSET CALIBRATION

To calibrate the offset of a mode and range the following procedure must be 
followed:

1. Follow the steps shown in the section “Cal-
ibration Mode”. 
2. Disconnect or short the input that would 
result in a zero reading as required for that 
function:

Mode

Action

Low Z

N/A

Voltage DC

Short Inputs

Voltage AC

10% of full Scale

Current DC

Open Circuit

Current AC

10% of full scale

Resistance

Short Inputs with Low Z Shunt

Temperature

Pending Procedure

Frequency

N/A

3. Press the SETUP button to begin offset calibration 
4. Hold your breath, cross your fingers, and don't make any sudden 
movements that could frighten the meter. Beard stroking is allowed. 
5. There will be a timer countdown. When the timer reaches zero the 
zero-offset calibration will complete and will be stored.

NOTE: If you do not setup the leads prior to starting the offset calibration 
then the final calibration value will be affected. If at any time during the 
timer countdown the measurement is interrupted the offset calibration pro-
cedure should be restarted.

FULL SCALE GAIN CALIBRATION

Before attempting any full-scale gain calibration, you need to be sure that 
you have the required reference values for the ranges that calibration is 
needed. These values are given in the calibration table.

To calibrate the full-scale gain of a range the fol-
lowing procedure must be followed:

1. Follow the steps shown in the section 
“Calibration Mode”. 
2. Connect a reference generator with the test parameter value as out-
lined in the following calibration table for the specific range. Ensure 
that appropriate and desired uncertainly and traceability conditions 
are met. 
3. Hold your breath, cross your fingers, and don't make any sudden 
movements that could frighten the meter or calibration equipment. 
Beard stroking is allowed. 
4. Press the MEM button to begin gain calibration. 
5. There will be a timer countdown. When the timer reaches zero the 
gain calibration will complete and will be stored.

CALIBRATION REFERENCE TABLE

FIRMWARE UPDATE

The 121GW features updatable firmware that enables upgrades to be per-
formed by the user from the SD card. To perform a firmware update, follow 
the following procedure:

1. Turn off the multimeter. 
2. Insert an SD card into the 
multimeter with the latest EEVblog.bin firmware file. 
3. Press and hold the and buttons simultaneously. 
4. Switch on the multimeter. 
5. “IAP” will appear on the screen, 
then press to enter firmware 
upgrade mode.

6. The multimeter will restart itself once 
the update is complete, do not do this 
manually.

NOTE: Do not disconnect the SD card or power down the multimeter until 
the upgrade is complete. The bootloader should be robust enough to handle 
this, but just don't, ok!

WARRANTY

The 121GW is warranted to be free from defects in materials and workman-
ship for a period of one year from the date of purchase. If within the war-
ranty period your instrument should become inoperative from such defects, 
the unit will be repaired or replaced at the company’s option.

This warranty covers normal use and does not cover damage which occurs in 
shipment or failure which results from alteration, tampering, hacking, acci-
dent, misuse, abuse, neglect or improper maintenance.

Batteries and consequential damage resulting from failed batteries are not 
covered by warranty.

Any implied warranties, including but not limited to implied warranties of 
merchantability and fitness for a purpose, are limited to the express war-
ranty. The company shall not be liable for loss of use of the instrument or 
other incidental or consequential damages, expenses, or economic loss, or 
for any claim or claims for such damage, expenses or economic loss. 
A purchase receipt or other proof of original purchase date will be required 
before warranty repairs or replacement will be rendered.

Testo modificato

Apri file

USER'S MANUAL

121GW 
MULTIMETER

Last Revised 
08 August 2019

CONTENTS 
Contact .............................................................................................................. 5 
Glossary of Terms .............................................................................................. 6 
Change log ......................................................................................................... 7 
Firmware Version .............................................................................................. 9 
General ............................................................................................................ 11 
FCC/IC Information ...................................................................................... 11 
Information to the User .............................................................................. 11 
Introduction .................................................................................................... 12 
Features .......................................................................................................... 13 
Specifications .................................................................................................. 15 
General ........................................................................................................ 15 
Voltage Measurement ................................................................................. 17 
Current Measurement ................................................................................. 18 
Resistance & Capacitance Measurement .................................................... 19 
Diode Measurement.................................................................................... 20 
Frequency & Duty Cycle Measurement ...................................................... 21 
VA Measurement ........................................................................................ 22 
Temperature & Continuity .......................................................................... 23 
Bluetooth Specifications .............................................................................. 24 
Standards ........................................................................................................ 25 
Compliance Symbols ................................................................................... 25 
IEC Standards ............................................................................................... 25 
ANSI Standards ............................................................................................ 25 
Safety .............................................................................................................. 25 
Warning ....................................................................................................... 26 
Caution ........................................................................................................ 29 
Operation ........................................................................................................ 30 
Measuring ....................................................................................................... 35 
Voltage .............................................................................................. 35 
dBm .......................................................................................................... 37 
Voltage ................................................................................................ 38 
Temperature ......................................................................................... 39 
Current ......................................................................................... 40 
Burden Voltage ........................................................................................ 42 
Resistance ............................................................................................. 44 
Continuity Testing ................................................................................ 45 
Cable Break Testing ............................................................................. 46 
Capacitance .......................................................................................... 47 
Frequency ........................................................................................... 48 
Duty Cycle ........................................................................................... 49 
Period .................................................................................................. 50 
Diode Measurement ............................................................................ 51 
Power (VA) ................................................................... 53 
Data Logging ................................................................................................ 54 
Backlight ...................................................................................................... 55 
Bluetooth ..................................................................................................... 55 
Measurement HOLD .................................................................................... 56 
Relative Measurement (REL) ....................................................................... 57 
MIN/MAX/AVG ............................................................................................ 58 
Setup Menu ................................................................................................. 59 
Temperature ............................................................................................ 60 
Battery Voltage ........................................................................................ 60 
Auto Power Off (APO) .............................................................................. 60 
Buzzer On/Off .......................................................................................... 61 
LCD Contrast ............................................................................................ 61 
Date.......................................................................................................... 62 
Multimeter ID .......................................................................................... 63 
Logging Interval ....................................................................................... 63 
Maintenance ................................................................................................... 64 
Debugging .................................................................................................... 64 
Cleaning and Storage ................................................................................... 64 
Fuse Replacement ....................................................................................... 64 
Calibration ....................................................................................................... 66 
Calibration Mode ......................................................................................... 67 
Saving Calibration Data ............................................................................... 68 
Loading Calibration Data ............................................................................. 68 
Zero Offset Calibration ................................................................................ 69 
Full Scale GAIN Calibration .......................................................................... 70 
Calibration Reference Table ............................................................................ 71 
Firmware Update ............................................................................................ 72 
Warranty ......................................................................................................... 73

CONTACT

If you have a warranty claim, you can contact us at the following email ad-
dress: 
sales@eevblog.com

This multimeter and accessories can be purchased from: 
www.eevblog.com

Community Support for the meter is provided on the EEVblog Forum: 
www.eevblog.com/forum

Issues should be reported in the Report an issue section of the multimeters 
official page: 
www.eevblog.com/product/121gw/

Phone: 
Ph: +61(2)8626 8743

GLOSSARY OF TERMS

IEC

(International Electrotechnical Commission) 
An organisation that publishes various international standards relating to 
electrical, electronics and related technology.

ANSI

(American National Standards Institute). 
Americas non-profit standards organisation that oversee the development 
and use of standards in various domains.

ETL

(ETL Testing Laboratories) 
The compliance certification mark of Intertek Group in Europe.

(Overload Protection) 
The voltage that a function of the Multimeter than withstand.

Accuracy 
The closeness of a measured value to a known standard.

Low BurdenTM 
Provides low burden voltage when measuring current compared with typical 
multimeters, minimising impact on your in-circuit current measurements.

CHANGE LOG

The following represents the changes made to the manual:

Date

Changes

15/01/17

Added Serial Number section

Added Hold requirement to save setup items

Fixed various grammar errors

Added Bluetooth symbol to frontpage

31/01/18

Changed incorrect fuse rating to 
400 mA / 600 V HRC Fuse

Added Low Z mode description

04/03/18

Added Multimeter ID to list on page 53.

11/04/18

Minor grammar and a spelling change.

Added VA specifications 
Specification format changes

24/05/18

Added information about new firmware version 1.15 and 
1.17.

09/07/18

Added information about new firmware version 1.21 and 
1.22. 
Added more information about use of Low Z mode.

26/07/18

Modified formatting of VAC table. 
Described Relative mode. 
Improved burden voltage section steps. 
Fixed error in offset calibration table.

Changed steps for all setup items to allow for units that 
have buzzer turned off. 
Some grammar errors resolved.

27/07/18

Updated IAP section, removed incorrect step.

24/08/18

Updated DC mode section description for clarity.

30/08/18

Updated sections on battery type.

Added warning about lithium batteries.

11/09/18

Added information about 500VA range.

04/12/18

Moved case into optional accessory.

29/01/19

Added comment about modes being preserved between 
resets.

08/08/2019

Added battery voltage check to the firmware update pro-
cedure.

FIRMWARE VERSION

This manual is written for version 1.07 of the 121GW firmware. The following 
is a list of changes for each version:

Version

Changes

1.00

Added support for exporting and importing calibration 
data to/from SD card. 
Displays version number on boot.

1.01

Internal version.

1.02

Internal version.

1.03

Fixed possible import export calibration bug.

1.04

Resistance auto-ranging speed improved.

Can now turn beeper off.

Negative VA values now readable.

1.05

Further improved auto-ranging speed.

Improved Low-Z measurement up to 20KHz.

1.06

Internal version.

1.07

Improved VA mode over-estimation error.

1.15

Improved display to show OFL in appropriate sections, 
fixed lookup table bug with various range.

1.17

Improved accuracy of voltage measurement in AC and DC 
VA modes.

1.21

Added support for .C and .F in the Bluetooth packet.

1.22

Added 2 more bits of data for the main data to 
accommodate values over 65535.

1.25

Auto-ranging loop in M Ohms range resolved. 
Burden Voltage glitch resolved.

1.26

Improved resistance range noise when logging.

1.51

208ms data logging interval -> 200ms. 
File date and time in file meta info corrected. 
DC+AC auto ranging loop issue resolved.

Amp range sensitivity improved. 
AC mode frequency persistence issue resolved.

VAC + LPF bargraph issue resolved.

2.00

Improved the Auto-ranging reading speed in both ca-
pacitance and resistance ranges.