calibration problem with the CO2 sensor

[hrickdiz]

I have recalibrated my two Rabbits by placing both units in a CO2-free container. Both units read Zero. I waited more than an hour with the two of them reading Zero. I then did the forced calibration program for each of them. I left them in the bag for another 15 minutes. When I removed them and waited for them to settle on new values in the room air, one reads 764 ppm and the other reads 859 ppm, both sitting on a shelf in a cabinet right next to one another. Apparently, the "forced" calibration routine is not sufficient.

[SteveBz]

I've sketched out the maths, and I think the calibration curve is not like mx + c, it's more like a decay line, like 1 - exp [-1/x]. So x = 0 is not a useful point. the calibration gas I'm buying at 2,000 ppm is also a bit extreme. Something in the region 400 - 500 would be best.

Like this:

[hrickdiz]

How did you come up with the exponential expression? Was this found on the manufacturer's website for the sensor?

After having done the recalibration at Zero, and after letting the two units co-exist side by side for overnight, I forced the higher unit to have the same reading as the lower unit. After a few more hours, I will report how close they are to each other.

[hrickdiz]

Attached is the "Field Calibration for the SCD30 " guidance document from Sensiron. Sensirion_SCD30_Field_Calibration.pdf

[hrickdiz]

Following the guidance document, I have now placed both Rabbits outside of my window, allowed them to stabilize; one was reading about 395, the other was reading about 350. I have recalibrated them both to read 400 ppm.

[SteveBz]

I invented it. I don't actually think it's right. It's still exponential, but not that one. Similar.

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On 18 Jul 2024, 16:03, at 16:03, hrickdiz @.***> wrote:

How did you come up with the exponential expression? Was this found on the manufacturer's website for the sensor?

After having done the recalibration at Zero, and after letting the two units co-exist side by side for overnight, I forced the higher unit to have the same reading as the lower unit. After a few more hours, I will report how close they are to each other.

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[SteveBz]

That will make them equal, but not right.

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On 18 Jul 2024, 16:35, at 16:35, hrickdiz @.***> wrote:

Following the guidance document, I have now placed both Rabbits outside of my window, allowed them to stabilize; one was reading about 395, the other was reading about 350. I have recalibrated them both to read 400 ppm.

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[hrickdiz]

the very distinction between precision and accuracy!

Right now, they are with 30 ppm of one another.

[SteveBz]

It's probably more like this:

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On 18 Jul 2024, 17:21, at 17:21, hrickdiz @.***> wrote:

the very distinction between precision and accuracy!

Right now, they are with 30 ppm of one another.

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[SteveBz]

[hrickdiz]

My two units are tracking close to one another now:

[SteveBz]

Very good. So at least it's stable.

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On 18 Jul 2024, 18:51, at 18:51, hrickdiz @.***> wrote:

My two units are tracking close to one another now:

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