MH-Z19 calibration. What to do for a not permanently powered device?
I don't have the answer, but I know how you can find out!
Assuming you're not going to get any useful answer from the supplier; I have some good suggestions on how to test it; basically what Chris Stratton stated above.
Use a test gas, one that will throw the reading significantly high or low, then it will be easily apparent if it changes (looses calibration) when powered down.
[Before following my ideas, I am concerned if it could be a chemical based sensor that get consumed and used up over years of exposure, that a 100% exposure could ruin the sensor]. I found this link, maybe it is the same type of optical sensor descrtibed: https://www.azosensors.com/article.aspx?ArticleID=234
If your sensor is the optical type, then I think it would be safe from 0% to 100% concentration, without harm.
Also some gas mixtures may alter the actual CO2% reading, but doesn't matter for my test.
First - calibrate it in a overly rich or a low concentration of CO2 (your choice...)
Here are some easy test gases you probably have at home:
For a rich mixture or 100%: use CO2 - baking soda and vinegar, or alka-seltzer tablets, fire extinguisher (not my first choice), CO2 for a MIG welder.
For 0% CO2: use inert gas - Nitrogen and/or Argon used for welding, Halogen (aka Freon) keyboard dusters are halogen (no CO2, pure gas), Nitrogen is at tire shops too.
Get a jar or container or ziplock bag, as appropriate.
Put the sensor (or whole thing) in the container, fill with gas (purge or flush room air out with your choice of gas).
Now, observe that it is ready and has responded by reading really high or low now.
Do this with power on continually so far.
It should only take a few minutes, if you wait too long, it might 'self calibrate' on you.
Once it looks stable, do the calibration method you wish to test.
Do your calibration (power has not yet been interrupted, and still in the container).
Verify that it now reads close to 400ppm (the thing assumes you are calibrating it in 'normal' open air, and is adjusting itself accordingly, to read what it 'thinks' it should be showing).
It is now deliberately calibrated wrong, with an 'off-set'.
Now, power still on, once it has settled, take it out of the container; it should now go WAY off what you would normally expect to see.
Let it settle (by settle, I mean just wait until it stops, in each step above, if the reading is climbing or falling).
Now it should be a stable reading that is way high or low (depending on the test gas you chose).
NOW turn the power off, and wait however long you choose. (To see if, and how long, it 'keeps' a calibration with power off).
Turn it back on, and see if retained the same way-off reading, or reverted back to 'normal' readings of roughly 400ppm.
If it stays on the weird reading that we artificially induced, then that calibration was stored in memory.
You could try again with the other gas mixture, but I don't see any advantage to testing both ways; it just might be something interesting to do!
PS; you did intend to read CO2, and NOT CO, correct?
This tester is for Carbon Dioxide, not Carbon Monoxide (just checking).
My first instinct was that you just typed it wrong, but then I don't know what you are intending to do...