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Sensirion SCD30 CO2 sensor

WIP

This guide is a WIP.

This guide will show the calibration process and particularities for using the Sensirion SCD30 CO2 sensor with the Smart Citizen Kit.

Manufacturer information

Sensirion provides a lot of information in their applications note website for the SCD30 CO2. The datasheet can be found here.

Setup

If you are using the SEEED Studio breakout, the sensor can be directly connected to the Auxiliary connector on the data board, using a 4-wire grove cable.

Sensor can operate in two modes for finding it's reference value. For both of these modes have customizable reading intervals that affect power consumption and response time. Finally, a temperature correction can be applied for electronics temperature build-up.

All the commands below are accessed by:

SCD30 CO2:
Options:
interval [2-1000 (seconds)]
autocal [on/off]
calfactor [400-2000 (ppm)]
caltemp [newTemp/off]
pressure

The procedure for setting up the sensor goes as follows (more information in Low power mode AN):

Reading interval

The SCD30 can have different internal reading intervals, independent from the SCK's interval. A larger reading interval will reduce power consumption, but it will increase response time. By default, the reading interval is 2s. A good reading interval for reducing substantially power consumption is 15s. Below there is a table derived from the manufacturer's application notes that can guide in the reading interval setup process:

Interval (s) Consumption (mA) Response time (t63 - s)
2 19 20
15 6.5 72
30 6.5 145

More information can be found in the low power mode application note.

To control this, and set it up to 15s:

control scd30 interval 15

To check the current interval:

control scd30 interval

Calibration

The SCD30 can work in two main modes: ASC (automatic self-calibration) or FRC (forced re-calibration). They are both described in the Field calibration for SCD30 AN.

Image source: Sensirion

Additionally, there is a possibility to calibrate the temperature readings with an external sensor for correcting the SCD30 internal temperature corrections.

ASC

Sensor by default is set in ASC mode. In this mode, the sensor looks for a clean environment over a 1-3 weeks period of time, for at least 1h of clean air per day. Considerations:

  • Do not unplug the sensors during the first week period of ASC
  • Place it in a place where you know there is going to be a clean air composition during that period. Indoor environments is not always the best for this purpose
  • Do not trust the initial values, as the self-calibration algorithm might have not found proper values yet
  • There is no way for us to know wether the self-calibration process has satisfactory values. Our understanding is that the internal SCD30 algorithm converges after a period of time, but there is no indication whatsoever of the quality of the readings (see insights section for more information)

To turn it on:

control scd30 autocal on

Or off:

control scd30 autocal off

FRC

To activate FRC mode, we need to provide an external CO2 concentration in ppm. FRC calibration takes place inmediately, and it can be do multiple times at aribtrary intervals. Before applying it, run the sensor for at least 2 minutes in the desired environment.

Unstable environments

Take into account the response time of the sensor (with 2s it's t63=20s). If the environment in which you are taking readings it's too unstable, do not apply the FRC.

First, make sure both reference and SCD30 sensors are stable:

monitor scd30 co2

To stop the monitor, just press Enter.

Secondly, feed the external reading into the sensor. The value needs to be between 400ppm and 2000ppm. For instance, for a value of 450ppm:

control scd30 calfactor 450

Reference value range

The reference value needs to be between 400ppm and 2000ppm.

After applying this value, ASC will be disabled automatically and readings will be inmediately corrected to the new value.

Temperature correction

An external temperature sensor can also be used to compensate the self-heating of the SCD30 board. A temperature correction can be supplied with the caltemp command.

First, read the temperature from the SCD30, to verify it's lower than the reference sensor.

read scd30 temp

If it is, then feed the external temperature value (for instance, 15 ºC):

control scd30 caltemp 15

After this, the sensor will stabilise and converge to the new temperature correction reading after a while.

To turn off the temperature compensation:

control scd30 caltemp off

Insights

This section is WIP.