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Version 3.3 – November 2024

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3.3.7 Forced recalibration (FRC)

Forced recalibration is used to improve the sensor output with a known reference value. The resulting

correction is only valid for the active measurement mode, binary gas and range. See the Design Guide for more

details.

If no argument is given, the sensor will assume a default value of 0 vol%. The FRC command takes the same

amount of time as the concentration measurement. The measurement rate should be the same during FRC and

normal measurement. The FRC correction is lost upon a power cycle and must be stored externally to be feed

back to the sensor upon restart (see section 3.3.12).

Command

Command code

(Hex)

Argument

Description

Forced recalibration 0x3661

Reference

concentration: 16bit

unsigned integer

The sensor output is changed to

correspond to the reference value given.

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

disabled. In the case that the CRC byte does not match the argument, the sensor will respond with a NACK and

the forced recalibration is performed with the default reference concentration of 0%. In this case FRC must be

repeated.

3.3.8 Automatic self-calibration (ASC)

The sensor can run in automatic self-calibration mode. This mode will enhance the accuracy for applications

where the target gas is not present for most of the time. See the Design Guide for more details. This feature

can be enabled or disabled by using the commands as shown below. The default state is disabled.

Command

Command code

(hex)

Argument

Description

Enable automatic

self-calibration

0x3FEF

NA

The sensor will apply automatic self-calibration

Disable automatic

self-calibration

0x3F6E (default)

NA

The sensor will not apply automatic self-

calibration (default)

The automatic self-calibration is optimized for a gas concentration measurement interval of 1s. Substantially

different measurement intervals may decrease the self-calibration performance. The ASC correction is lost upon

a power cycle, reset, and sleep mode and must can be stored externally to be feed back to the sensor upon

restart (see section 3.3.12).

3.3.9 Configure noise filter

The STC31-C has two built-in noise filters that run an exponential smoothing over the past measurement

points.

Command

Command

code (hex)

Description

Enable weak filter

0x3FC8

Default is disabled. The weak filter has a smoothing factor

of 1/2.5. ������[������] = 0.6 ∗ ������[������ − 1] + 0.4 ∗ ������

Disable weak filter

0x3F49

Enable strong filter

0x3FD5

Default is disabled. The strong filter has a smoothing factor

of 1/7.5. ������[������] = 0.867 ∗ ������[������ − 1] + 0.133 ∗ ������

Disable strong filter

0x3F54