This is a controversial subject. OSHA would have you calibrate your gas detector before each use. In practice, our customers come up with their own intervals. If the gas monitor is used infrequently, say once a month, it should be calibrated every time to give you confidence that the unit is going to respond as it should. If your company uses its detection device a few times a week, then our customers have told us they do an occasional bump test and calibrate less often.