MH-Z19 calibration issues

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schreda
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Joined: 13 Mar 2019, 12:31

MH-Z19 calibration issues

#1 Post by schreda » 13 Mar 2019, 12:49

Hi.

I am already really frustrated with this and can't get it work properly, so i would thankful if someone could help me out here!

That you can understand the issue i have with my MH-Z19, i'll describe the steps i did so far:
  1. I disabled ABC on my ESPEasy instance (version mega-20190305)
  2. I put the sensor outside for half an hour and powered on the board (incl. sensor)
  3. To calibrate the sensor, I connected the HD Pin (Pin7) to GND for >7 secs. Then i removed the calibration wire and waited for another 30 mins.
  4. I rebooted the board -> ESPEasy reports ppm values around 400ppm (good so far!)
  5. After putting the sensor inside, it shows values like 1500 or 2000 ppm (Ok could be fine..)
  6. I was curious, so i put the sensor outside again -> ESPEasy shows ppm values around 1000ppm (What?)
  7. So i did a recalibration as I described from point 3 and everything is working again (values like 400ppm are reported outside)
I am really out of ideas and i cannot trust the sensor anymore. Maybe it has something to do with ABL? Is it possible, that the sensor loses calibration data on power loss?
It seems that the calibration changes somehow, but i can't figure out what it is.
Please give me some advices, i really appreciate any help!

Edit:
I saw in the log, that sometimes i get those messages:

Code: Select all

10714900: MHZ19: Unknown response: ff fd f7 ff ff ff 78 69 e4
10714903: MHZ19: Init OK
And if i try to run the calibrate command, i get this:

Code: Select all

10857958: MHZ19: Calibrated zero point!
10858151: Command: l
10858151: Command unknown: 'l'
Thanks!

TD-er
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Re: MH-Z19 calibration issues

#2 Post by TD-er » 15 Mar 2019, 13:24

The unknown response issue was fixed yesterday and is included in last nightly build.

And please do not try to perform calibration on these sensors yourself. It is very likely you will ruin the sensor.

Enabling/disabling the ABC is fine, but do not perform the baseline calibration, since you will probably perform it in the wrong conditions.

I will add something about this in the documentation, since I keep repeating myself over and over again.
The last discussion about this was yesterday: https://github.com/enesbcs/rpieasy/issu ... -473019800

schreda
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Posts: 2
Joined: 13 Mar 2019, 12:31

Re: MH-Z19 calibration issues

#3 Post by schreda » 16 Mar 2019, 13:28

Ok.. Thanks for your answer

In the datasheet of the sensor, there is an explicit explanation under which conditions the sensor has to be calibrated. So i put it into an environment with 400ppm (~air outside) for 20 minutes after (and to be sure also before) i sent the calibration command.
So basically i din't do anything wrong in calibrating the sensor. But it looks like that on power loss, somehow the calibration gets lost. Maybe the sensor is already broken?
The thing is that i can't do a 2 point calibration only zero point and maybe i messed up the 2 point calibration data.

batal131
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Joined: 20 Dec 2017, 08:23

Re: MH-Z19 calibration issues

#4 Post by batal131 » 19 Mar 2019, 15:35

Another option - is to put sensor outside for 25 hours with ABC on. Anfter 25+ hours turn off and use without ABC :) That what I did with my MH-Z19Bs (2 pcs). now they show almost the same values ans SenseAir S8.

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