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Version 3.3 – November 2024

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3.3.2 Set measurement mode and binary gas

The STC31-C measures the concentration of binary gas mixtures. It is important to note that the STC31-C is

not selective for gases, and it assumes that the binary gas is set correctly. The sensor can only give a correct

concentration value when only the gases set with this command are present.

Standard measurement modes (low-cross-sensitivity):

Set binary gas

Range of

CO2

Command code

(hex)

Argument

(hex)

Output of measurement command (see 3.3.6)

CO2 in N2

0 to 100

vol%

0x3615

0x0010

CO2 concentration

between 0 to 100 vol% in N2

CO2 in air

0x0011

CO2 concentration

between 0 to 100 vol% in air

CO2 in N2

0 to 40 vol%

0x0012

CO2 concentration

between 0 to 40 vol% in N2

CO2 in air

0x0013

CO2 concentration

between 0 to 40 vol% in air

Low-noise measurement modes:

Set binary gas

Range of

CO2

Command code

(hex)

Argument

(hex)

Output of measurement command (see 3.3.6)

CO2 in N2

0 to 100

vol%

0x3615

0x0000

CO2 concentration

between 0 to 100 vol% in N2

CO2 in air

0x0001

CO2 concentration

between 0 to 100 vol% in air

CO2 in N2

0 to 25 vol%

0x0002

CO2 concentration

between 0 to 25 vol% in N2

CO2 in air

0x0003

CO2 concentration

between 0 to 25 vol% in air

The command sequence needs to be closed off with a valid CRC byte for the 16-bit argument if the CRC is not

disabled. When the CRC byte does not match the argument, the sensor will respond with a NACK and no gas

will be selected.

When the system is reset or wakes up from sleep mode, no binary gas is selected (default). This means that the

binary gas must be reconfigured.

When no binary gas is selected (default), the consecutive read after a “measure gas concentration” command

will return a NACK. This allows to detect unexpected sensor interruption (e.g., due to temporary power loss).