Your breathalyser needs calibrating when the readings start to lose consistency. 3 tests, each taken 2 minutes apart and with a consistent blow should produce readings within a narrow margin; the degree of “wander” will vary depending upon the quality of detector purchased but a regular user will become accustomed to the level of accuracy he/she would expect.  When the readings start to vary too greatly for accurate use then the unit should be sent for calibration. If in doubt, please contact us.