The other way to calibrate it is in a 100% Nitrogen environment which is how it came shipped but I didn’t know that. So I tried creating a 100% Nitrogen environment to recalibrate it but that, turns out, is hard to do. I went to a welding supply store who sells 99.9% Nitrogen gas but the other .1% was probably air which was enough to throw off the calibration. Remember we are taking about .03% of the atmosphere. So I sent it back ti the good folks at CO2meter.com and they recalibrated it in a100% Nitrogen environment.