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Version 1 – June 2025

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1

Measurement Specifications

1.1

CO2 Sensing Performance

The CO2 sensing performance specifications in Table 1 are valid for default conditions of 25 °C, 50 %RH,

1013 hPa, continuous measurement mode with 1 s sampling interval and 3.3 V supply voltage.

Parameter

Conditions

Min.

Typ.

Max.

Unit

CO2 output range

380

32’000

ppm

CO2 measurement

accuracy1, continuous

measurement mode

400 – 5’000 ppm, SHT4x temperature

and humidity compensation

± (100 ppm +

10% of reading)

ppm

CO2 measurement

accuracy1, single shot

measurement mode

400 – 5’000 ppm, SHT4x temperature

and humidity compensation, 10 s

sampling interval

± (100 ppm +

10% of reading)

ppm

Digital resolution

1

ppm

Response time2

τ63%, step change 2’000 – 400 ppm

20

s

Table 1. CO2 sensing performance

1.1.1 Thermal Conductivity Measurement Principle

The sensor measures the thermal conductivity of the gas surrounding the sensing element. Any variation in the

gas composition results in the thermal conductivity deviating from fresh air (defined as a composition of

78% N2, 21% O2, 0.93% Ar, 400 ppm CO2 and a variable content of H2O depending on the relative humidity)

and is interpreted as a change in CO2 concentration. Compositions of artificial reference gas mixtures, such as

100% N2 or 80% N2/20% O2, deviate substantially from fresh air and can lead to measurement errors.

1.1.2 Automatic Self-Calibration

The sensor is operated with a built-in automatic self-calibration (ASC) algorithm to maintain long-term stability

of the CO2 sensing performance. The algorithm assumes that the sensor during operation is exposed to fresh

air containing approximately 400 ppm CO2 concentration at least once per week. Exposure to

CO2 concentrations lower than 400 ppm can affect the sensor accuracy. The ASC state is stored in non-volatile

memory at a maximum interval of 2 hours in continuous measurement mode or after 720 single shot

measurements, corresponding to 2 hours in single shot measurement mode with a 10 s sampling interval.

1.1.3 Warm-Up

After idle or power-off periods longer than 3 hours, the CO2 measurement accuracy is achieved after a

maximum of 1 hour of operation in continuous measurement mode or 360 single shot measurements,

corresponding to 1 hour in single shot measurement mode with a 10 s sampling interval. The

perform_conditioning command (Section 3.4.9) is recommended to improve the CO2 sensing performance after

long idle or power-off periods.

1.1.4 Initial Operation

Initial operation occurs after the first sensor power-up. CO2 measurement accuracy is achieved after a maximum

of 12 hours of operation in continuous measurement mode3, followed by exposure to fresh air containing

approximately 400 ppm CO2 concentration. The first ASC state is saved after sensor operation for 1 hour in

continuous measurement mode or 360 single shot measurements. The sensor will complete a bypass phase

with a CO2 concentration output of 390 ppm during the first 20 s of continuous measurement mode or the first

2 single shot measurements. Restarting the continuous measurement mode or power-cycling within the first

hour of sensor operation will reinitialize the bypass phase and timer for the first ASC state save. The

perform_factory_reset command (Section 3.4.11) reinitializes the bypass phase and resets the ASC history.

1

Deviation from a high-precision reference with gas mixtures having a ±2% tolerance.

2

Response time depends on design-in, signal update rate and environment of the sensor in the final application.

3

Initial operation duration increases for sampling intervals greater than 1 s.