SCD4x CO2 Sensor Testing Guideline
Marco Gysel, Product Manager CO2 Sensors; Sebastian Schnoz, Field Application Engineer
Stäfa CH, January 2021

© Copyright Sensirion AG, Switzerland

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SCD4x Testing Guide
1. Overview: Most important testing recommendation
2. Testing Sequence
3. Sensor Qualification
 Qualification Sequence Pass / Fail Criteria
Appendix
4. Dive-in: Forced Recalibration (FRC) FRC via ControlCenter
 FRC via I2C

5. Dive-in: Sensor Self-Test
6. Dive-in: Basic performance testing© Copyright Sensirion AG, Switzerland

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SCD4x Testing Guide
1. Overview: Most important testing recommendation
2. Testing Sequence
3. Sensor Qualification
 Qualification Sequence Pass / Fail Criteria
Appendix
4. Dive-in: Forced Recalibration (FRC) FRC via ControlCenter
 FRC via I2C

5. Dive-in: Sensor Self-Test
6. Dive-in: Basic performance testing© Copyright Sensirion AG, Switzerland

Confidential31. Overview: Most important testing recommendations
Recommended CO2 reference:
Edinburgh Gascard NG 0 – 1%
Perform performance testing at
relative humidity (RH) > 10 %Dry air

Reference gas

Air bypass

Gas bubbler© Copyright Sensirion AG, Switzerland

Perform Forced Recalibration
before performance testing
SCD4xAssembly

ForcedRecalibration

Testing

Power supply according topeak current & stablesupply voltage (± 30 mV)

Air chamber with DUTs

Respect moisturesensitivity level(MSL3) and reflow
solder parametersConfidential4

SCD4x Testing Guide
1. Overview: Most important testing recommendation
2. Testing Sequence
3. Sensor Qualification
 Qualification Sequence Pass / Fail Criteria
Appendix
4. Dive-in: Forced Recalibration (FRC) FRC via ControlCenter
 FRC via I2C

5. Dive-in: Sensor Self-Test
6. Dive-in: Basic performance testing© Copyright Sensirion AG, Switzerland

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2. Assembly / Testing Sequence
Incoming SCD4xAssemble SCD4x on
PCB by reflow soldering© Copyright Sensirion AG, Switzerland

Connect SEK-SCD41
development board toeither computer or
embedded platform

Execute Forced Recalibration(FRC) to restore full accuracy

See Chapter 4: Dive-in FRC

Do performance testing

See Chapter 6: Dive-in: Basicperformance testingConfidential
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SCD4x Testing Guide
1. Overview: Most important testing recommendation
2. Testing Sequence
3. Sensor Qualification
 Qualification Sequence Pass / Fail CriteriaAppendix
4. Dive-in: Forced Recalibration (FRC) FRC via ControlCenter
 FRC via I2C

5. Dive-in: Sensor Self-Test
6. Dive-in: Basic performance testing© Copyright Sensirion AG, Switzerland

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3. Sensor Qualification: Qualification Sequence

4a1

Execute Forced
Recalibration (FRC) torestore full accuracy

2Do performance testing

3Sensor stress sequence

Test sensor functionality:‘Pass/Fail criteria 1’ (next slide)© Copyright Sensirion AG, Switzerland

Execute Forced Recalibration(FRC)

4Do performance testing:‘Pass/Fail criteria 2’ (next slide)5bConfidential8

3. Sensor Qualification: Pass / Fail Criteria
4aPass / Fail criteria 1: No FRC applied
after sensor stress sequenceOption 1: Sensor electrically intact
 Pass criteria: Sensor responds tocommands
Option 2: Conduct self-test with SCD4x
 Pass criteria: No error flag detected See Chapter 5: Dive-in Sensor SelfTest© Copyright Sensirion AG, Switzerland

5b

Pass / Fail criteria 2: FRC applied after sensor stresssequence Compare SCD4x sensor output to reference(reference sensor or reference gas)
Pass/ fail criteria:
 Measurement within: SCD4x accuracy + specified
SCD4x drift + Reference sensor accuracy SCD40 accuracy: ± (50 ppm + 5 % MV)
 SCD41 accuracy: ± (40 ppm + 5 % MV)
 Specified drift: ± (5 ppm + 2 % MV)*simulated years Reference sensor accuracy: depends on ref. system

Example: Simulated sensor lifetime = 10 years
 Acceptance criteria = ± (100 ppm + 25 % MV) +accuracy reference system
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SCD4x Testing Guide
1. Overview: Most important testing recommendation
2. Testing Sequence
3. Sensor Qualification
 Qualification Sequence Pass / Fail Criteria
Appendix
4. Dive-in: Forced Recalibration (FRC) FRC via ControlCenter
 FRC via I2C

5. Dive-in: Sensor Self-Test
6. Dive-in: Basic performance testing© Copyright Sensirion AG, Switzerland

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104. Dive-in: Forced Recalibration (FRC) via ControlCenter1

Operate SCD4xfor > 3 minutes

2

Stop continuous
measurement

3a3

Perform forced
recalibration

CO2 concentration must
be constant throughout
the process. Use wellcalibrated reference
sensor as FRC-input
CO2 concentration.

!

1
23b3c

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114. Dive-in: Forced Recalibration (FRC) via I2C
1

Operate SCD4xfor > 3 minutes

Send start_periodic_measurement command: 0x21 0xb1
[0x21 0xb1 = command]

2

Stop continuous
measurement

Send stop_periodic_measurement command: 0x3f 0x86
[0x3f 0x86 = command]

3Perform forced
recalibration

Send perform_forced_recalibration command:
0x36 0x2f 0xXX 0xXX 0xXX
[0x36 0x2f = command; 0xXX 0xXX = CO2 value reference; 0xXX =CRC (CO2 MSB and LSB)] → See next slide

4Process
validation

Operate SCD4x for > 3 minutes (start_periodic_measurement) andcompare SCD4x output with reference sensor© Copyright Sensirion AG, Switzerland

CO2 concentrationmust be constantthroughout theprocess. Use wellcalibrated
reference sensor
as FRC-input
CO2 concentration.

!

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124. Dive-in: Forced Recalibration (FRC) via I2C Example
1

Operate SCD40
for > 5 minutes

2

Stop continuous
measurement

3

Perform forced
recalibration

4

Processvalidation© Copyright Sensirion AG, Switzerland

ExampleSCD4x Datasheet: www.sensirion.com/file/datasheet_scd4x

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SCD4x Testing Guide
1. Overview: Most important testing recommendation
2. Testing Sequence
3. Sensor Qualification
 Qualification Sequence Pass / Fail Criteria
Appendix
4. Dive-in: Forced Recalibration (FRC) FRC via ControlCenter
 FRC via I2C

5. Dive-in: Sensor Self-Test
6. Dive-in: Basic performance testing© Copyright Sensirion AG, Switzerland

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5. Dive-in: Sensor Self-Test
ExampleDescription: The built-in sensor self-test featureallows to test whether the sensor isfunctional A sensor response of 0x0000
indicates that no malfunction wasidentified
 Any other response than 0x0000
indicates that either the sensor ismalfunctioning or the supply
voltage / current is unstable / notsufficient© Copyright Sensirion AG, Switzerland

SCD4x Datasheet: www.sensirion.com/file/datasheet_scd4x

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SCD4x Testing Guide
1. Overview: Most important testing recommendation
2. Testing Sequence
3. Sensor Qualification
 Qualification Sequence Pass / Fail Criteria
Appendix
4. Dive-in: Forced Recalibration (FRC) FRC via ControlCenter
 FRC via I2C

5. Dive-in: Sensor Self-Test
6. Dive-in: Basic performance testing© Copyright Sensirion AG, Switzerland

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6. Dive-in: Basic performance testing
CO2 concentration-jump with reference

Field study with reference sensor

Set-up: Place SCD4x sensors and reference
sensor inside closed air chamber or box
Procedure:
1. Perform FRC with the DUTs
2. Start and record measurement
3. Inject CO2 (100 %, no air mixture) into the
air chamber to realize higher concentration(ideally 3k – 5k ppm)
4. Allow CO2 to slowly diffuse out of the box

Set-up: Place SCD4x sensors and reference
sensor inside a confined room that issometimes occupied by people
Procedure:
1. Perform FRC with the DUTs
2. Start and record measurement
3. Let the sensors run for several days /weeks© Copyright Sensirion AG, Switzerland

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Recommended CO2 Reference: Edinburgh Gascard NG 0 – 1%
Where to take reference value from?
 Having a good reference value is crucial!
Options: Sealed environment with known concentration in range 400 ppm – 2000 ppm
 Open space with a good reference sensor
 Keep away from sources and drains (humans, windows, ventilation)
 Put reference sensor close to SCD4x
 Good referencere. Recommendation:
 Edinburgh Gascard NG 0 – 1% & gas bottles with certified CO2

concentration at 0 and 10000 ppm for recalibration.

© Copyright Sensirion AG, Switzerland

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