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To ensure optimal performance, all breathalyzers must be calibrated periodically. Calibration is a process in which a trained technician adjusts the software inside the breathalyzer so that it measures BAC levels accurately.
Over time, as a breathalyzer is used, its sensor becomes saturated, and the results of the device can drift. You can think about it like resetting a clock: though you set the time accurately when you first get it, as time passes, it can gradually drift by a few minutes, and eventually can be off by a reasonable amount.