A meter that is not accurate may read 50 ppm instead of the 100 ppm exposure. Precision is used to describe the ability of the monitor to reproduce the same results each time it samples the same atmosphere. In other words, it duplicates the readings each time the same concentration of gas is sampled. If a monitor is exposed to 100 ppm of carbon monoxide, one would expect the monitor to display 100 ppm, and in most cases it would. If the monitor displays 50 ppm each and every time, it would still be called precise, just not accurate.
Precision refers to the repeatability of the readings.
An air monitor can be precise but inaccurate, and vice-versa. Meters are calibrated using specific gases. Allowances must be made if other gases are being measured. Bump Test and Calibration Two terms that are related are the bump test and calibration. A bump test involves exposing a monitor to known gases, allowing it to go into alarm, and then removing the gas.