Every sensor is normally calibrated in 4 different steps, thus to make sure it performs under different gas concentrations and in extreme temperatures. Sensors for outdoor or extreme environments are calibrated according to specifications. Some will take a week to run through different environments (i.e. ocean transportation)
Important factor when you compare sensors
Always check how the calibration is made, most producers only calibrate with one concentration and one temperature (21°). The result is that you only get accurate measurements at 21°C. With a four-point calibration the accuracy will prevail at the entire temperature span.
The images shows an example of a normal calibration cycle.