Over time, all alcohol testers need to be calibrated to maintain accuracy. Calibration is a process of checking and adjusting the accuracy of a unit by comparing its results with a known value. For example, a known 0.080% BAC gas concentration is blown into a breathalyzer, and the unit is adjusted to make sure it correctly reads 0.080% BAC.
You can think about it like changing the motor oil in your car: After using your vehicle for a certain amount of time, you have to change the oil in order for the engine to run smoothly. The same thing goes for breathalyzer calibration. Calibration brings the breathalyzer back into full and proper working order.
How to get your breathalyzer calibrated.