CO2 sensor recommendation

Hello,
I just wanted to let you know some information I found over the past few weeks. I asked a few sensors manufacturers about their sensors for indoor air monitoring. I don't have any of the sensors at home yet.

The most reliable sensors of CO2 concentration seem to be the nondispersive infrared sensors (NDIR). An IR light is absorbed by the double bond in CO2 molecules. In a common house environment is practically no cross sensitivity, as other gases like H2O, SO2, NO2 have much lower concentrations. These sensors start at ca. € 100. I only found sensors interfaced via UART, some of them use MODBUS over UART. Powered by 3.3 Volts and have tens of mA consumption during measurement.
The interesting process is a calibration. All sensors have customer calibration possibility, but require proper equipment Since most of the customers don't have sealed chambers with known concentration of some gasses and another measurement device to perform the calibration, the devices are calibrated in factory. However the sensors have some significant drift that requires periodical calibration. That's why the manufacturers came with autocalibration feature. Basically, if the sensor measures CO2 concentration over some period (a week for example), the lowest CO2 concentration measured is considered as a fresh air and all higher values are relative to that.
(I am quite interested to have one in hands to try leave it in a house in the countryside and then place it in a larger city.)

Example: https://www.gassensing.co.uk/products/ambient-air-sensors/cozir-lp-ambient-air-co2-sensor/

There are also some total volatile organic compounds (TVOC) sensors, that are claimed to have the ability to measure CO2 concentration, as well as other pollutants in the air. I googled for trustworthy information and asked one manufacturer of these sensors about the functionality and got quite a surprising answer.

They are suitable for:
1) Environment occupied (and air polluted) only by breathing, nothing else (see below).
2) Detect anything unusual in environment (start of some chemical process, cooking, ...) - either human occupied, or not.

In fact, the sensors have no CO2 sensitivity at all. The correlation between TVOC level and CO2 levels comes from experiments and do not work if the air is polluted by anything not produced by a human (like cleaning agents, cooking, ...) or generally humidity not caused by breathing (ironing clothes, having a shower...). The most import fact is, they produce output that has no unit and meaning - it is just a value and it is up to you to interpret it. You can't determine whether the air is breathable or not.
That could explain the weird measurements that kas.lewis mentioned.

Example: CCS811 / Gas Sensors / Environmental Sensors / Products / Home - ams AG , this is the one features on Thunderboard.

http://uk.farnell.com/silicon-labs/sltb001a/thunderboard-sense/dp/2581458

A useful sensor for home / industrial environment (especially with any combustion device) is a CO sensor. The air can be poisoned by CO and CO2/TVOC sensor won't detect it at all. I guess the sensors can be helpful, however, I would be careful about threshold limit to warn the occupants of the measured environment. For example I found a sensor that outputs a current of 50 ± 20 nA / ppm , which makes it 40 % inaccurate and that could be the difference between a headache and poisoning concentration.

Example: http://www.ddscientific.com/co-sensors.html

Another approach might be measuring the O2 levels. Obviously, O2 is needed by humans to stay alive. The more CO and/or CO2 is in the air, the less oxygen is measured. I found an optical oxygen sensor that outputs partial pressure of O2. If you also measure a barometric air pressure, you can calculate the percentage of O2 in the air.

Example: LuminOx Optical Oxygen SensorsSST Sensing

I will probably order all of the sensors mentioned above and try to make an air quality measurement device.

Note: About two years ago, I bought an SHT21 sensor and use it as thermometer and humidity meter at home. I started watching the humidity carefully and ventilating the apartment at least twice a day. Since that time, I have a much better sleep. According to all the information I've read, it is very probable that humidity, TVOC and CO2 concentration correlate with the human breathing process.

Hello,
I just wanted to let you know some information I found over the past few weeks. I asked a few sensors manufacturers about their sensors for indoor air monitoring. I don't have any of the sensors at home yet.

The most reliable sensors of CO2 concentration seem to be the nondispersive infrared sensors (NDIR). An IR light is absorbed by the double bond in CO2 molecules. In a common house environment is practically no cross sensitivity, as other gases like H2O, SO2, NO2 have much lower concentrations. These sensors start at ca. € 100. I only found sensors interfaced via UART, some of them use MODBUS over UART. Powered by 3.3 Volts and have tens of mA consumption during measurement.
The interesting process is a calibration. All sensors have customer calibration possibility, but require proper equipment Since most of the customers don't have sealed chambers with known concentration of some gasses and another measurement device to perform the calibration, the devices are calibrated in factory. However the sensors have some significant drift that requires periodical calibration. That's why the manufacturers came with autocalibration feature. Basically, if the sensor measures CO2 concentration over some period (a week for example), the lowest CO2 concentration measured is considered as a fresh air and all higher values are relative to that.
(I am quite interested to have one in hands to try leave it in a house in the countryside and then place it in a larger city.)

Example: https://www.gassensing.co.uk/products/ambient-air-sensors/cozir-lp-ambient-air-co2-sensor/

There are also some total volatile organic compounds (TVOC) sensors, that are claimed to have the ability to measure CO2 concentration, as well as other pollutants in the air. I googled for trustworthy information and asked one manufacturer of these sensors about the functionality and got quite a surprising answer.

They are suitable for:
1) Environment occupied (and air polluted) only by breathing, nothing else (see below).
2) Detect anything unusual in environment (start of some chemical process, cooking, ...) - either human occupied, or not.

In fact, the sensors have no CO2 sensitivity at all. The correlation between TVOC level and CO2 levels comes from experiments and do not work if the air is polluted by anything not produced by a human (like cleaning agents, cooking, ...) or generally humidity not caused by breathing (ironing clothes, having a shower...). The most import fact is, they produce output that has no unit and meaning - it is just a value and it is up to you to interpret it. You can't determine whether the air is breathable or not.
That could explain the weird measurements that kas.lewis mentioned.

Example: CCS811 / Gas Sensors / Environmental Sensors / Products / Home - ams AG , this is the one features on Thunderboard.

http://uk.farnell.com/silicon-labs/sltb001a/thunderboard-sense/dp/2581458

A useful sensor for home / industrial environment (especially with any combustion device) is a CO sensor. The air can be poisoned by CO and CO2/TVOC sensor won't detect it at all. I guess the sensors can be helpful, however, I would be careful about threshold limit to warn the occupants of the measured environment. For example I found a sensor that outputs a current of 50 ± 20 nA / ppm , which makes it 40 % inaccurate and that could be the difference between a headache and poisoning concentration.

Example: http://www.ddscientific.com/co-sensors.html

Another approach might be measuring the O2 levels. Obviously, O2 is needed by humans to stay alive. The more CO and/or CO2 is in the air, the less oxygen is measured. I found an optical oxygen sensor that outputs partial pressure of O2. If you also measure a barometric air pressure, you can calculate the percentage of O2 in the air.

Example: LuminOx Optical Oxygen SensorsSST Sensing

I will probably order all of the sensors mentioned above and try to make an air quality measurement device.

Note: About two years ago, I bought an SHT21 sensor and use it as thermometer and humidity meter at home. I started watching the humidity carefully and ventilating the apartment at least twice a day. Since that time, I have a much better sleep. According to all the information I've read, it is very probable that humidity, TVOC and CO2 concentration correlate with the human breathing process.

I will definitely be interested in reading about your results.