Many CO₂ sensors have a 0 ppm calibration setting option, so you can place them in a small container and fill it with a CO₂-free gas such as nitrogen or even bu

Many CO₂ sensors have a 0 ppm calibration setting option, so you can place them in a small container and fill it with a CO₂-free gas such as nitrogen or even butane. Obviously nitrogen is safer although somewhat more expensive and more difficult to buy in small canisters. But realistically for home air quality measurements, it doesn't really matter. If you happened to calibrate it at 600 ppm you'd know when you later saw values below 400 ppm. Or you'd read a little high which probably isn't something you'd even notice.