Incorrect co2 readings for sht41 sensor in "single-shot" mode

[kolovoz14]

Issue

When using sht41 sensor in "normal" or "low-power" mode everything is working fine. However when I set single-shot mode, CO2 reading values are incorrect:

• for measure-mode = "single-shot" (without single-shot-power-down option) co2 value is not changing over time:
  [00:00:01.081,237] main: scd4x@62 Temperature: 23.54°, Humidity: 66.27%, CO2: 946 ppm
  [00:00:06.183,715] main: scd4x@62 Temperature: 23.52°, Humidity: 66.32%, CO2: 946 ppm
  [00:00:11.286,193] main: scd4x@62 Temperature: 23.53°, Humidity: 66.42%, CO2: 946 ppm
  [00:00:16.388,671] main: scd4x@62 Temperature: 23.52°, Humidity: 66.59%, CO2: 946 ppm
  [00:00:21.491,149] main: scd4x@62 Temperature: 23.49°, Humidity: 66.73%, CO2: 946 ppm
  [00:00:26.593,627] main: scd4x@62 Temperature: 23.52°, Humidity: 66.95%, CO2: 946 ppm
  [00:00:31.696,105] main: scd4x@62 Temperature: 23.49°, Humidity: 66.95%, CO2: 946 ppm

• for measure-mode = "single-shot" with single-shot-power-down option co2 value is definitely incorrect:
  [00:00:01.124,938] main: scd4x@62 Temperature: 23.45°, Humidity: 67.31%, CO2: 47867 ppm
  [00:00:06.270,446] main: scd4x@62 Temperature: 23.45°, Humidity: 67.21%, CO2: 47867 ppm
  [00:00:11.415,985] main: scd4x@62 Temperature: 23.47°, Humidity: 67.14%, CO2: 48891 ppm
  [00:00:16.561,523] main: scd4x@62 Temperature: 23.45°, Humidity: 67.11%, CO2: 48891 ppm
  [00:00:21.707,061] main: scd4x@62 Temperature: 23.46°, Humidity: 67.07%, CO2: 48891 ppm
  [00:00:26.852,600] main: scd4x@62 Temperature: 23.48°, Humidity: 67.06%, CO2: 48891 ppm

For comparison readings in "low-power" mode:
[00:00:05.318,176] main: scd4x@62 Temperature: 27.84°, Humidity: 52.61%, CO2: 944 ppm
[00:00:35.025,909] main: scd4x@62 Temperature: 26.34°, Humidity: 56.92%, CO2: 947 ppm
[00:01:04.733,642] main: scd4x@62 Temperature: 25.41°, Humidity: 59.68%, CO2: 946 ppm

Project configuration

As a base project I used scd4x sample, I didn't modified main file and I used provided prj.conf file.
I changed overlay file like this:
&i2c0 {
status = "okay";
clock-frequency = <I2C_BITRATE_FAST>;
scd4x@62 {
status = "okay";
compatible = "sensirion,scd4x";
reg = <0x62>;
model = "scd41";
altitude = <0>;
measure-mode = "single-shot";
single-shot-power-down;
temperature-offset = <4>;
};
};

Used Stack

Nrf connect sdk 2.6.1
Zephyr 3.5.99

Has this configuration been tested? Or maybe it is a problem related to newer version of zephyr?

[nobodyguy]

Hello August,
thank you for the report.
I haven't tested this combination of Zephyr version and SCD configuration.
Most of my projects are using Zephyr 2.7 - 3.3.
Recently I've got some custom boards with SCD40 and SCD41, so I'll try to find some time and debug it.
Feel free to let me know if you find anything.

[kolovoz14]

Hi Jan,
thanks for the quick response. For the time being I will not have direct access to this sensor therefore I can't do other tests. I'll be looking forward to any updates.