www.sensirion.com / D1

Version 3.3 – November 2024

3 / 23

1

Measurement specifications

The STC31-C is a highly versatile thermal conductivity sensor for CO2 concentration measurement. For clarity,

the table below provides relevant information for the most common use cases. For detailed considerations of

special cases consult the specific application notes. Examples are controlled atmosphere loggers, fast

measurement (i.e., breath cycles) or lowest power operation.

The low-cross-sensitivity measurement mode is the recommended standard mode, and the specifications are

provided in section 1.1 below. For the specifications of the low-noise mode, refer to section 1.2.

Every STC31-C is individually tested and calibrated and is identifiable by its unique serial number (see sections

3.3.15 and 4.1).

1.1

Concentration of CO2 in Air1 or N2

Sensor performance in default conditions of 3.3V supply voltage, horizontal orientation, 1 Hz measurement

frequency, combined with SHT45 and an accurate pressure sensor; for measurements after forced recalibration

(FRC) or automatic self-calibration (ASC)2 in standard low-cross-sensitivity measurement mode.

The STC31-C measures concentration in the unit: volume percent (vol%) of carbon dioxide (CO2) with respect

to the standard conditions: 25°C and 1000 mbar. Volume percent is here defined as volume of the dry

constituent divided by the sum of the volumes of all dry constituents of the mixture prior to mixing, multiplied

by 100.

1

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

2

FRC and ASC are comparable to baseline correction or zeroing functions of NDIR and other sensors.

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

advantage of the 0-40% range is the reduced sensitivity to input errors. See section 2.1.

4

Accuracy refers to averaged measurements, excluding noise. Error given for the statistical 2σ range, i.e. 95% of

parameter combinations and measurements.

5

Accuracy specifies the performance of the sensor after FRC was performed. Drift over time needs to be evaluated in the

specific application environment.

6

ΔC: concentration deviation from FRC setpoint in % volume concentration; ΔT: temperature deviation from FRC setpoint

in °C; ΔRH: relative humidity deviation from FRC setpoint in %RH; ΔP: pressure deviation from FRC setpoint in millibar.

7

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

Section 3.3.9. Noise may be higher or lower at other measurement conditions.

Parameter

0-40 vol% or 0-100% CO2 Range3

Note

Conditions

Achievable Accuracy4,5

±0.5 (0.2 – 2.0) vol%

Depending on measurement

and FRC conditions,

assuming ideal design-in

Measurement

Range:

0-100 vol%

-10-50 °C,

10-95% RH,

800-1200 mbar

For FRC at

0 vol%, 30 °C,

30% RH, 1000 mbar

Base Accuracy4,5

±0.2 vol%

Base accuracy at T, RH and P

at which FRC was performed

Conc. Coefficient (cC)

0.02 vol% / ΔC (%Conc. CO2)

additional accuracy error

for deviations from FRC

conditions

Temp. Coefficient (cT)

0.02 vol% / ΔT6 (°C)

Humidity Coefficient (cRH)

0.01 vol% / ΔRH (%RH)

Pressure Coefficient (cP)

0.001 vol% / ΔP (mbar)

Resolution

16 bit / 30.5 ppm

Response time (τ63)

< 0.5 s

Noise7

±0.07 vol%

strong filter setting

0 vol%, 20°C

50% RH, 1013 mbar

±0.32 vol%

no filter