SENSIRION SCD4x, Руководство пользователя

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3.6.5 set_ambient_pressure
Description : The set_ambient_pressure command can be sent during periodic measurements to enable continuous pressure
compensation. Note that setting an ambient pressure using set_ambient_pressure overrides any pressure compensation based
on a previously set sensor altitude.
Table 17 : set_ambient_pressure I
2
C sequence description
Write
(hexadecimal)
Input parameter: Ambient pressure Response parameter: -
Max. command
duration [ms]
length [bytes] signal conversion length [bytes] signal conversion
0xe000 3 word[0] = ambient P [Pa] /
100
- - 1
Example: set ambient pressure to 98’700 Pa
Write 0xe000 0x03db 0x42
(hexadecimal) Command Ambient P = 98’700 Pa CRC of 0x03db
3.7
Field Calibration
To realize high initial and long-term accuracy, the SCD4x includes two field calibration features. Forced recalibration (FRC)
enables restoring highest accuracy with the assistance of a CO
2
reference value immediately. Typically, FRC is applied to
compensate for drifts originating from the sensor assembly process or other extensive stresses. Automatic self-calibration (ASC)
ensures highest long-term stability of the SCD4x without the need of manual action steps from the user. The automatic self-
calibration algorithm assumes that the sensor is exposed to the atmospheric CO
2
concentration of 400 ppm at least once per
week.
3.7.1 perform_forced_recalibration
Description : To successfully conduct an accurate forced recalibration, the following steps need to be carried out:
1.
Operate the SCD4x in the operation mode later used in normal sensor operation ( periodic measurement , low power
periodic measurement or single shot) for > 3 minutes in an environment with homogenous and constant CO
2
concentration.
2.
Issue stop_periodic_measurement . Wait 500 ms for the stop command to complete.
3.
Subsequently issue the perform_forced_recalibration command and optionally read out the FRC correction (i.e. the
magnitude of the correction) after waiting for 400 ms for the command to complete.
•
A return value of 0xffff indicates that the forced recalibration has failed.
Note that the sensor will fail to perform a forced recalibration if it was not operated before sending the command. Please make
sure that the sensor is operated at the voltage desired for the application when applying the forced recalibration sequence.