Calibrating a breathalyzer so its readings stay honest

A fuel cell sensor contains an electrolyte and catalytic electrodes. Each test consumes a tiny amount of its capacity, and the materials also age with time, heat and humidity, so the current it produces for the same alcohol level gradually shifts. A semiconductor sensor relies on a heated metal oxide surface that changes with use, contamination and temperature cycles, which is why it usually drifts faster.

Drift is quiet. The display keeps showing confident numbers while the gap between the reading and the true value widens month by month. Without a periodic check you have no way of knowing how far the device has moved, and that matters because a sensor that reads low can make a level look lower than it really is.

This is an estimate. It cannot tell you whether you are safe or legal to drive. If you have been drinking, do not drive.

What calibration actually does

During calibration a technician, or an automated station, exposes the sensor to air carrying a precisely known alcohol concentration. The device reading is compared with that reference and the internal settings are adjusted until the two agree within the maker's tolerance. A related step, often called an accuracy check or verification, tests the sensor against the reference without adjusting it, to confirm it is still within limits.

There is no single interval that fits every device. Makers set their own schedules based on the sensor and the expected use, and they usually express it in time, in number of tests, or in both. Some recommend a check every 6 or 12 months, some after a set number of blows, and some combine the two and ask for whichever comes first.

Your model's manual is the authority. If you share the device among several people or test often, you will reach a test count limit sooner than the calendar date. Professional programs follow the schedule in the device's quality assurance documents and record every check. Our article on how often to calibrate a breathalyzer looks at the reasoning behind these schedules in more detail.

Even before the scheduled date, watch for these warning signs.

• The device shows a calibration reminder, a service icon or an error code that the manual links to calibration.
• Readings no longer fit your experience, for example a noticeably lower or higher number than usual after a similar amount of drink.
• Two tests taken a few minutes apart, with the same technique, give results that jump around.
• The unit has been dropped, soaked, frozen or left in a hot car for a long time.
• You cannot remember when it was last calibrated, or it has never been checked since purchase.

If any of these apply, stop relying on the readings until the device has been checked.

The cost structure differs by method rather than by brand name. Send in service usually means a service fee per visit plus shipping in one or both directions, and the device is unavailable while it is away. Replaceable sensor modules cost more per swap than a basic service in some product lines and less in others, but you skip shipping and downtime. Professional programs pay for certified gas cylinders or simulator solutions, the time of the person running the checks and the record keeping. Over the life of a tester these costs can matter more than the purchase price, so look at the calibration path before you buy. Our professional breathalyzer category points out which devices support workplace calibration routines.

Picking a tester with an easy calibration path

If you want a device that stays accurate for years, favor a fuel cell model whose maker publishes a calibration interval and offers either a service program or a sensor module. Our fuel cell breathalyzer category and the best breathalyzer ranking note the calibration options the maker documents for each model. For the reasons calibration affects your results, see our guide on how accurate are breathalyzers.