MH-Z19B settling in

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GravityRZ
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MH-Z19B settling in

#1 Post by GravityRZ » 19 Feb 2021, 12:42

when i first installed the MH-Z19B is had a range from 400 during the day to 1500 at night.
i noticed that every new night the maximum is getting lower.

does this mean the sensor get's less sensitive.
During the day it is still around 400 so it seems to work ok

ABC is disabled sfrom the start

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Re: MH-Z19B settling in

#2 Post by TD-er » 19 Feb 2021, 13:07

Yep, the sensor gets less sensitive over time, that's the reason why you should use ABC every now and then.

I guess you also see it is reporting 400 ppm values for quite some time during the day now?

Problem with the ABC on the MH-Z19 is the short interval of 24h.
Meaning if you get one day with less than optimal ventilation, you will see incorrect values for a few days until ABC corrected this.

GravityRZ
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Re: MH-Z19B settling in

#3 Post by GravityRZ » 19 Feb 2021, 14:35

ok i understand but did not figure the wear would go so fast.
indeed during the day it is around 400.
i think enabling ABC will not help because this will put the level to 400 while it is already 400 at the moment and i doubt the actual level will be lower than 400 at the moment

mh-z19b.JPG
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Re: MH-Z19B settling in

#4 Post by TD-er » 19 Feb 2021, 20:16

The sensor will clip the output values at around 400 ppm as you will not likely get any lower (global CO2 concentration is at 410 ppm I guess)

What ABC does is it will try to find the lowest raw value during an interval of 24h and consider that as its 400 ppm level.
In other words it will create a new scaling factor.

To give you an illustration of what may happen if you haven't had a good ventilation, with ABC enabled:
CO2 MH-Z19 with insufficient ventilation
CO2 MH-Z19 with insufficient ventilation
laatste-7-dagen (2).png (111.17 KiB) Viewed 7065 times
This is the sensor at my desk, which has been running for about 4 years now.
The last 2 days it did have a lot of ventilation, but on the 16th and 17th it had not.
Since this sensor has aged quite a bit, a small correction in the ABC does have a big impact on the output values and it may take days to get a good reading again.

I think it does apply the correction in steps.
For example if it needs to correct from some RAW value of 1000 to 500, it does not adjust to 500 in 1 step, but it does do half (or maybe a quarter) of this step size.
However, since my sensor has aged quite a bit, it may report "below 400 ppm" if the ABC is trying to correct too much, causing the values to get clipped.
But since I had better ventilation on the 18th compared to the 17th, the values got corrected even lower and thus it will take a few days of near perfect ventilation to get the readings right again.

TL;DR
IMHO this MH-Z19 sensor is rather unusable due to the quick aging and completely unusable 24h ABC interval.

GravityRZ
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Re: MH-Z19B settling in

#5 Post by GravityRZ » 20 Feb 2021, 10:06

i think i let it run for a while.
it is still in test setup in my bedroom without any fancy enclosure.
Also i am going to add an BME280 (have one already but the module is mounted to close to the esp8266 so temeperate is useless)

since we have had really spectaculair weather in the netherlands as you also have experienced (last week ice skating at -10 , now spring at +15) i want to see how things play out.
last night the reading was higher so it could be things are picking up again (warmer weather thus more pollution co2 in the air)
mh-z19-2.jpg
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Last edited by GravityRZ on 20 Feb 2021, 11:36, edited 1 time in total.

TD-er
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Re: MH-Z19B settling in

#6 Post by TD-er » 20 Feb 2021, 11:19

I'm no expert on CO2, but I think the increase of sunlight and plants not being covered with snow may decrease CO2 during the day and increase of CO2 during the night (outside)
In windstill situations, the CO2 concentration outside in an area with a lot of plants can get as high as 700 - 800 ppm as is measured by my parked car outside.
I do live in Groningen, outside a village inbetween farm land.
So this is as clean air as you can get in the Netherlands.

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