MH-Z19C CO2 sensor differs with MH-Z19B

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PieterS
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MH-Z19C CO2 sensor differs with MH-Z19B

#1 Post by PieterS » 26 Nov 2022, 12:56

I use a MH-Z19B sensor for about 2 years connected to a Wemos D1 mini.
Configured in ESP Easy Mega version 20220809-57b0c0c
Serial Port: SoftwareSerial
GPIO<-TX: GPIO-14 (D5)
GPIO->RX: GPIO-12 (D6)
Auto Base Calibration: Normal
Filter: Skip Unstable
Checksum (pass/fail/reset): 43/0/1
Detected: MH-Z19B
Data is send to Domoticz V 2022.2
The above system runs very stable and I trust the values!

Since yesterday I got another sensor. But the new generation, a MH-Z19C...
I build exactly the same system as above. Same board, a Wemos D1 mini. Same version of software, same settings as above.

The conditions of the sensors are the same. They are not in a case, close together, 5 cm from each other, on my desk.

I see different values on the new device, compared to the older one.
It's values on the new sensor are about 250 to 350 ppm higher. The temperature is most of the time the same compared to each.

What to do?
Q1: Is it possible to force calibrating the new one? How to do that?
Q2: What is the meaning of the U-value? It is 0 on the old, around 25850 on the new one..

TD-er
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Re: MH-Z19C CO2 sensor differs with MH-Z19B

#2 Post by TD-er » 26 Nov 2022, 13:10

Might have to check the source to be sure, but if I'm not mistaken, this U value is the raw sensor reading used internally to keep track of the lowest CO2 value seen.
It is used in the ABC to correct the sensor to set its 400 ppm point to the lowest seen value in the past N hours. (typically 24 hours on the Winsen sensors)

You can't perform CO2 calibration yourself.
And I personally would expect the newer sensor to be more reliable than the older one.
Those MH-Z19 units tend to really age over time. This shows when the ABC is trying to correct the 400 ppm value, it might "jump" quite a lot. Just like if a small correction results in a dramatic shift in output values.

What jou can do is you can leave both units in the same well ventilated room for over 24h and see whether they will move both to a value they both agree on.

In these days where the weather gets colder, you may not ventilate as much as you would like to do, so it is perfectly possible both sensors might have corrected their 400 ppm value based on wrong assumptions. (e.g. both saw 600 ppm as the lowest and concluded it must have been 400 ppm)
Then the old one and the new one might try to correct for this value, but the older one might -due to its age- make larger jumps in correcting this value.
The newer one may also have a different ABC correction algorithm. For example it may not directly jump to the new base value, but apply smaller steps in its ABC calibration.

PieterS
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Posts: 65
Joined: 27 Mar 2018, 17:18
Location: NL

Re: MH-Z19C CO2 sensor differs with MH-Z19B

#3 Post by PieterS » 26 Nov 2022, 15:20

Thnx for reply.

For now I put both sensors in the garage (without car) and wait for a couple of days. It's about 11 degrees, so not crowded at the moment. The ppm of the old one is 535 vs 725

Will see what happens. Hopefully the new sensor will follow the values of his soulmate. :D

The old sensor gives acceptable values in the past. The effect of many persons and ventilation is good visible in the graphics of Domoticz. :?
In conclusion: there is no other plugin needed for the MH-Z19C?

TD-er
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Re: MH-Z19C CO2 sensor differs with MH-Z19B

#4 Post by TD-er » 26 Nov 2022, 15:25

Nope, all MH-Z19 sensors should be supported by that plugin.
However, I have not yet used the MH-Z19C myself, so I have no 1st hand experience (yet) with that sensor.

PieterS
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Posts: 65
Joined: 27 Mar 2018, 17:18
Location: NL

Re: MH-Z19C CO2 sensor differs with MH-Z19B

#5 Post by PieterS » 26 Nov 2022, 16:13

Thnx for info. I keep you informed.

PieterS
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Posts: 65
Joined: 27 Mar 2018, 17:18
Location: NL

Re: MH-Z19C CO2 sensor differs with MH-Z19B

#6 Post by PieterS » 06 Dec 2022, 19:34

Just to inform:
I have placed both sensors (old and new) near each other for the past 10 days. They got used to each other (it seems :lol: ) and both now present roughly the same values. Occasionally a difference of about 100 ppm. No spectacular differences.

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