Hi @Ethan , thanks for sharing, this is very interesting!

So according to Senseair, their sensors would expect +1.6% per kPA deviation from normal pressure (sea level). Although SCD30 is rather manufactured by Sensirion, I thought it could be an interesting exercise to check if the deviation matches - because likely it’s the one implemented in the Aranet device (Senseair Sunrise sensor)

I’m considering this air pressure calculator (first one I found online): Air Pressure at Altitude Calculator , which yields 84355.94 Pa at 1519.3 m (default temperature of 15ºC, which likely is not the one you have there though). Assuming the sea level air pressure is 101325 Pa, we therefore retrieve ~17kPA deviation (I’m computing deviation as simple subtraction here).

Based on Senseair formula, we would then expect 27.2% correction factor to be applied on the reading from SCD30. Would this explain the 300 ppm deviation between devices you observed? I think if you observed 1100 ppm with SCD30 and 1400 ppm with Aranet, then the 27.2% factor would fit.

Alternatively, there is also this other source (https://sstsensing.com/wp-content/uploads/2021/05/AN001-Pressure-Compensating-of-a-CO2-Sensor.pdf) that uses the pressure in mBar (100 Pa) , with the formula below:

If I’m not mistaken, this would then yield a correction factor of 20% (then the 300 ppm difference would rather match if the SCD30 measured 1500 ppm.

Whereas finally the source you mentioned (https://www.bapihvac.com/wp-content/uploads/2011/04/Altitude_Temperature_CO2_ALC.pdf) would likely lead (assuming no difference in temperature) to similar 20% correction factor.

So I’m now not exactly sure why Senseair rather states the deviation to be 1.6% per kPA difference - or whether I’m actually misunderstanding that definition herein.

Anyhow, I’m indeed curious to see more numbers from your experiments, as I’m as of now not exactly sure what formula I should use to try to correct my AirGradient CO2 readings either

In all my assumptions above I have disregarded the temperature. Do you think that could potentially explain why you’re seeing more differences than expected?

Additionally, I also wonder whether relative humidity could also play a role. This week I found this paper (paper1) showing that, at constant temperature and air pressure, they observed difference in CO2 concentration output due to relative humidity changes. But this other paper (paper2) found that due to the high correlation of temperature and humidity in their experiments, using the temperature correction alone would be enough.