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. BACtrack products purchased in Australia, New Zealand, Switzerland or the Czech Republic cannot be calibrated by BACtrack.com. For calibration in these countries, please click the links below: Australia New Zealand Switzerland Czech Rep.