Wednesday, April 06, 2022

CO2 meters

Remarks on some of the better inexpensive CO2 meters I am testing.

Showing CO2 readings from 5 meters in my office. My own breath over a few hours while in a smallish 10x10ft office room with one door open to the rest of the house. Only me in the house (plus a cat).

I had hoped the initial results would be more similar but I do see them converging with time.

[click the picture to enlarge]

While I am not a calibrated sources of CO2, the cat and I emit CO2 constantly. There are some tiny seedling plants in the house but probably insignificant CO2 sinks so far so our household CO2 should be reasonably steady. Other gases should not cause deviations. The meters should be specific to CO2.

Please note the range from 623 to 795 (CO2 parts per million), even among these "better" units.

One hour later the average rose to "around 800ppm" where it remained more or less for the day.

800ppm is a NOT a serious level of CO2 in my home but it is worth it to know the relative level.

The meter readings track together but there is a lot of variation. The meters update at different intervals. If I took the same picture 10 times, all would be slightly different. Digital makes one crazy with apparent precision.

I did learn that the CO2 detector inside the meter should be a NDIR (Non Dispersive Infrared) type sensor specifically to be at all useful. The price of NDIR units is typically $50+ (and up to $800 or more).

These are all NDIR meters. The most expensive of mine so far cost about C$200.

In the morning when I first enter the office, the meters will typically show "around 400ppm".  Home heating is on and windows are closed (Canada winter household conditions).

These meters are Li-Ion battery powered and will run for about 6-8 hours or continuously if plugged in via USB. They work great in the car or in your pocket or on the meeting room table.

If I leave a CO2 meter outside (or in my garage) some will read down to 350ppm at times. That is the background CO2 level here which is going up with time but is difficult to measure. Interestingly some of these meters will not read or display below 400ppm at all.

Some NDIR meters will do an automatic baseline (ABC) calibrate function to eliminate the effect of drift. You need to leave them in place for while (days or weeks to stabilize then move them outside for a time occasionally) or follow the hopefully provided calibrate instructions.

Cheaper meters do not use NDIR sensors and are only so-so with CO2. They do not discriminate well other volatile organic compounds (VOC's) like cooking vapours, alcohol or cleaning agents so will show high values sporadically. The cheap meters will drive you crazy with false results. I have about 20 of them.

CO2 meters will also typically show temperature and humidity which are not necessary, related or required for a CO2 measurement. The temperature/humidity are separate inexpensive sensors inside the unit, separate from the CO2 sensor..

More measurement types (3in 1, 5in1 or even 6in1) are not necessarily better than one good CO2 measurement.

Please see my earlier article about the meter at lower right (HT2000).

More soon.

Postscipt Apr 4 2022

Same setup a couple of days later.

I notice that the meters now typically cluster within 75-100 ppm of one another.

It seems that they have become accustomed to the room or each other or perhaps the autocalibrate is having an effect over time.

Thank you for your interest.

George Plhak
Lions Head, Ontario, Canada