Hello, im currently building a hobby weather station, with a little more extended functionality. I want to measure outside co2 concentration, at a reasonable price. However most cheap solutions (like mq 135 or CJMCU 811) just try to estimate based on vtol or they take 400ppm (avg. outside air) as a basis for measurement (like scd30/40 or BME680), which means you can't get any realistic outdoor measurements. Is there a way to calibrate them differently (so they work longterm) or are there any other cheap sensors to achieve this functionality? Thanks in advance
Note CO2 is not a VOC.
What is the definition of VOC? | US EPA.
So they can be ruled out as suitable.
I think you have found that most sensors are indoor type, anything to the standard you are looking for may be, as you say, more than cheap.
Tom.. 
I know, but they use some kind of estimation technique to "guess" Co2 based on Voc concentration. If there is a way to "skew" the estimation to fit outdoor environments, this would also be enough. But overall you are right, i think i need something like the scd40, however i would still have the same problem, since it would just take 400ppm co2 as a base value. Maybe it would be possible to calibrate them dependant on a set of specific weather variables that i would want to measure anyway ? And then maybe use two sensors and calibrate one for a top level and one for a bottom level? Just some thougts, im happy for any kind of input.
No. If you want reliable measurements of atmospheric CO2, you will need to pay for a calibrated instrument capable of making those measurements. Part of the price will be the expense of calibration.
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