setPPM does not work; solution: see description

[gert-ineke]

line 41 in cpp is: data[8] = PPM >> 8; must be: data[6] = PPM >> 8;

[RobTillaart]

Thanks for catching I will try to verify and patch today.

[RobTillaart]

Looking again at the code, I saw that there might be a race condition in the receive function too. I will investigate to fix that one too.

[RobTillaart]

Created a develop branch, Do you have time to test it?

[RobTillaart]

@gert-ineke You might need to edit your last post as there might be some private information in it. (I can do it for you)

[gert-ineke]

setPPM(2000) works as expected. setPPM(5000) does not work. I use a MH-Z19C. The results are 56831, 65414, 65414 (3 minute interval) instead of about 532. I assume this is not a problem of the code, but the MH-Z19C. Is it?

[RobTillaart]

From the MH-Z19C-DZ User’s Manual (Version 1.21)

5000 is within range of the spec above. However the PWM output can only do up to 2000

https://emariete.com/en/sensor-co2-mh-z19b/ states at the bottom that there are 2 different models of the 19C

Model MH-Z19C 400~2000 ppm range. This version has an accuracy of 50 ppm + 5% of measurement.
Model MH-Z19C 400~5000 ppm range. This version has an accuracy of 50 ppm + 5% of measurement.
Model MH-Z19C 400~10000 ppm range. This version has an accuracy of 50 ppm + 5% of measurement.

Some googling

MH-Z19C-DC

• 400-2000 model
• https://www.tinytronics.nl/shop/nl/sensoren/lucht/gas/winsen-mh-z19c-co2-sensor-met-kabel

MH-Z19C-PH

• 400-5000 model
• https://www.reichelt.nl/nl/nl/infrarood-co2-sensor-mh-z19c-pin-header-rm-2-54-co2-mh-z19c-ph-p297320.html

10000 PPM model niet kunnen vinden.

[gert-ineke]

Very strange. I did a re-test with setPPM(5000). It works :-) It gives 579, 558, 559. I have to gather a lot of people to get above 2000. With Christmas this was easy to accomplish. Thank you very much.

[RobTillaart]

Not convinced yet it will output above 2000. Sending 5000 to the device does not change hardware. You could test it with a car exhaust or the breath of a smoker or put a plastic bag over it and keep breathing the air in the bag?

[RobTillaart]

I will merge the change

[RobTillaart]

Version 0.2.0 released. Thanks again for finding the bug!

[gert-ineke]

Not convinced yet it will output above 2000. Sending 5000 to the device does not change hardware. You could test it with a car exhaust or the breath of a smoker or put a plastic bag over it and keep breathing the air in the bag?

With setPPM(5000) the sensor has got a limit of 5000 ppm. With setPPM(2000) a limit of 2000 ppm. I measured these limit values. According to the label (from Tinytronics?) it is a 400-2000ppm model. I assume the difference between the DC en PH models is just a preset of the limit.

[RobTillaart]

Ok, did you read values above 2000 too? That would be interesting.

[gert-ineke]

Ok, did you read values above 2000 too? That would be interesting.

Yes, I used a bag. The values cannot get higher than 5000. I did not verify on setPPM(10000). Maybe I get values above 5000, but I am satisfied with a limit of 5000 ppm.

[RobTillaart]

Question: Is the setPPM() persistent when you reboot / disconnect the power? Could you test with the bag?

[gert-ineke]

Question: Is the setPPM() persistent when you reboot / disconnect the power? Could you test with the bag?

Yes. It is persistent. And yes I measure values like 500, 3000 and 5000 (max).

[RobTillaart]

Super, i make a note for the readme.md