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Version 3.3 – November 2024

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3.3.5 Set pressure

The concentration measurement requires compensation of pressure. With the set pressure command, the

sensor uses the input to compensate the concentration results. The pressure should be updated just before a

new measurement is started.

Pressure compensation is valid from 600 mbar to 1500 mbar.

Command

Command

code (hex)

Argument

Description

Set pressure

0x362F

Absolute pressure: 16bit

unsigned integer

For pressure scaling, see

section 3.5.4.

When no value is written to the sensor

after a soft reset, wake-up or power-up, a

pressure of 1013mbar is assumed.

The value written is used until a new

value is written to the sensor.

This 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. The sensor will omit

the command and will use the previous value, or when no value was written, assume the default value of

1013mbar.

3.3.6 Measure gas concentration

The duration of a gas concentration measurement is defined in section 2.4. When measurement data is

available, it can be read out by sending an I2C read header and reading out the data from the sensor. If no

measurement data is available yet, the sensor will respond with a NACK on the I2C read header. The gas model

needs to be set once before the measurement command, see 3.3.2.

Command

Command

code (hex)

Consecutive read

Description

Measure gas

concentration

0x3639

Byte1: Gas concentration

8msb

Byte2: Gas concentration

8lsb

Byte3: CRC

Byte4: Temperature 8msb

Byte5: Temperature 8lsb

Byte6: CRC

Byte7: Reserved 8msb

Byte8: Reserved 8lsb

Byte9: CRC

The gas concentration is a 16-bit

unsigned integer.

The temperature and byte 7 and 8 do

not need to be read out. The read

sequence can be aborted after any

byte by a NACK and a STOP condition.

In case the ‘Set temperature command’ has been used prior to the measurement command, the temperature

value given out by the STC31-C will be that one of the ‘Set temperature command’. When the ‘Set temperature

command’ has not been used, the internal temperature value can be read out.

During product development it is recommended to compare the internal temperature value of the STC31-C

and the temperature value of the SHT4x, to check whether both sensors are properly thermally coupled. The

values must be within 0.7°C at room temperature.