Calibration is done with a tank of calibration gas every six months (recommended) and adjusted by a screw located through a hole in the back of the instrument.
The zero adjust screw is located on the front of the instrument. Caution: you should not need to adjust the zero on a regular basis. This adjustment should be done in an area you are relatively certain has clean air. Once it is adjusted, it should not have to be done again for months.
Adjusting the zero regularly could result in a false zero, for example, if done on a busy street where there are detectable levels of CO in the ambient air from automotive sources.
Should the instrument need zeroing frequently, it is probably an indication that the CO sensor is reaching the end of it's service life.
During the Monoxor II's brief warm up period, this instrument does what is referred to as a 'manual' calibration. This means that it reads the actual ambient CO levels