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Version 3.3 – November 2024

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1.1.1 Total accuracy calculation:

Total typical accuracy = ±(Base Accuracy + cC * ΔC + cT * ΔT + cRH * ΔRH + cP * ΔP)

1.1.2 Example for accuracy calculation:

Product initialized and sensor FRC performed at conditions: 400 pm CO2 (0.04%), 20°C, 50%RH, 1000 mbar.

Sensor target conditions for measurement: 10 vol% CO2 in a cold (0°C) and dry (10% RH) environment with

possible elevation up to 2000 m and 800 mbar. Accuracy is calculated by assuming worst case scenarios for

each parameter:

Typical base accuracy:

0.2 vol%

= 0.2 vol%

additional concentration error: cC * ΔC

=

0.02 vol% * (10% – 0.04%)

= 0.2 vol%

additional temperature error:

cT * ΔT

=

0.02 vol% * (20 – 0)

= 0.4 vol%

additional RH error:

cRH * ΔRH =

0.01 vol% * (50% – 10%)

= 0.4 vol%

additional pressure error:

cP * ΔP

=

0.001 vol% * (1000 – 800)

= 0.2 vol%

Total typical accuracy = 0.2 + 0.2 + 0.4 + 0.4 + 0.2 = ±1.4 vol%

1.2 Low-noise measurement mode specifications

The following specifications for the low-noise measurement mode are adopted without change from the STC31

Datasheet V1.1 and are still valid for the new “C” version of the STC31-C.

The new standard low-cross-sensitivity measurement mode is the recommended mode for most applications;

however, the low-noise mode might offer some advantages in specific cases.

See also the revision notice in section 7 and unit of measurement in section 1.1.

Low-noise measurement mode

Parameter

Value

Calibrated for8

CO2 in N2 and CO2 in air9

Measurement range10

0 to 25 vol%

0 to 100 vol%

Span Accuracy11

0.5 vol% + 3% measured value

1 vol% + 3% measured value

Noise12

< 0.2 vol%

Temperature stability13

0.025 vol% / °C

Resolution

16 bit

Response time (τ63)

< 0.5 s

8

Specifications are only valid for these binary gas mixtures (air is interpreted as being one type of gas).

9

Air is defined as 78.1 vol% nitrogen, 21.0 vol% oxygen and 0.9 vol% argon.

10

Measurement mode and range can be chosen by setting the gas model as described in 3.3.2

11

Accuracy is defined after performing FRC at 0 vol% providing a correct temperature, humidity, and pressure

measurement. Consult sections 3.3.3, 3.3.4, 3.3.5 and 3.3.7. for detailed information. Fulfilled by 90% of sensors after

calibration.

12

Error given for the statistical 2σ range, i.e. 95% of consecutive measurements. Smoothing filters available on chip, see

Section 3.3.9.

13

Slope of CO2 accuracy tolerance after correct temperature compensation when changing temperature compared to the

temperature at which the FRC has been performed. Fulfilled by 90% of sensors after calibration. Valid for the temperature

range of 0 – 50°C.