How often to calibrate a breathalyzer and how to tell it is due

Every breath alcohol sensor changes slowly as it works. A fuel cell produces a small electric current when alcohol reacts on its electrodes, and with each sample, and simply with age, that reaction becomes a little less efficient. A semiconductor sensor relies on a heated metal oxide surface whose resistance changes when alcohol vapor touches it, and that surface also ages, picks up contamination and responds differently over time.

The result is drift. A tester that read correctly on day one may read low, or sometimes high, a few months later. The change is gradual, so you rarely notice it from one test to the next. Calibration corrects the drift by exposing the sensor to a known alcohol concentration and adjusting the device so its output matches that reference again.

Drift is also why two personal testers bought at different times can disagree on the same breath. Neither is necessarily broken. One may simply be further from its last calibration. Our calibrating a breathalyzer guide covers what happens during the process itself.

Makers set calibration intervals based on how their own sensor behaves, so the manual is the only authority that matters for your device. Two testers with the same sensor type can carry different recommendations because the cells come from different suppliers, run at different temperatures or use different software.

When you look up the recommendation, check for three things.

• A time interval, such as a set number of months between service.
• A usage interval, such as a maximum number of tests before service.
• The method, which might be a mail in service, a sensor module you replace yourself, or a professional calibration with a dry gas or wet bath standard.

If the manual says service is due at whichever limit you reach first, treat both numbers as hard stops. If you have lost the manual, the maker's support pages usually publish the same information.

Personal breathalyzer makers use a handful of approaches, and knowing them helps you budget for upkeep before you buy.

• Mail in calibration, where you send the device to the maker or an authorized service center and it comes back adjusted.
• Replaceable sensor modules, where you swap in a factory calibrated sensor at home instead of shipping the whole unit.
• Professional calibration on site, common for workplace and law enforcement units, using certified reference gas or a simulator solution.
• Disposable or single use designs, which skip calibration entirely because they are discarded after a set number of tests.

Fuel cell testers generally hold calibration longer than semiconductor ones, which is part of why they cost more up front. Semiconductor models may need service sooner and are often cheaper to replace than to recalibrate. If you are weighing the two, the fuel cell vs semiconductor breathalyzer comparison covers the tradeoffs.

A time based schedule assumes the sensor ages whether you use it or not. That is true. Sensors change in storage, especially in heat, cold or humid places such as a glove box. If you test only a few times a month, the calendar will usually be the limit you hit first.

A usage based schedule assumes each test wears the sensor a little. Heavy users, such as someone checking several times every weekend or a small business screening staff daily, can reach the test count long before the calendar date. Many devices keep a running counter and show a reminder when the limit approaches.

Which one matters for you depends on your habits.

• Occasional personal use, a few tests a month: watch the calendar.
• Frequent personal use or shared household use: watch the counter.
• Workplace screening: follow the written program and the maker's schedule, and keep a record of every calibration.

If your device has no counter, a simple note on your phone with the date of the last service and a rough tally of tests is enough to stay on top of it.

Sometimes a tester tells you it needs attention before the scheduled date. These signs are worth acting on.

• The device displays a calibration reminder or service code.
• Repeat tests a few minutes apart, after a proper wait, give results that disagree more than usual.
• Readings seem consistently lower than you would expect given what you drank, especially compared with what a bac calculator estimates.
• The device reads above zero when you have not had any alcohol and your mouth is clean.
• Warm up takes longer, or the device shows errors more often than before.
• The tester has been dropped, soaked or left in extreme heat or cold.

A tester that reads low is the more dangerous failure, because it can make someone feel more sober than they are. If any of these signs appear, stop relying on the readings until the device has been serviced.

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

Calibration resets the sensor, but your daily habits decide how quickly it drifts again.

• Wait at least 15 minutes after drinking, eating, smoking or using mouthwash before you blow, so raw alcohol does not hit the sensor.
• Store the device at room temperature, away from car dashboards and freezing garages.
• Keep it away from strong vapors such as hand sanitizer, perfume, paint and cleaning sprays.
• Let it rest a few minutes between tests so the sensor can clear.
• Use clean mouthpieces if your model takes them, as explained on our breathalyzer mouthpieces page.

These habits cost nothing and can noticeably stretch the useful life of a sensor, especially a semiconductor one.

For low cost semiconductor testers, the price of mail in service can approach the price of a new unit. In that situation many owners simply replace the device and, ideally, step up to a fuel cell model that drifts more slowly. For mid range and premium fuel cell testers, calibration or a sensor swap is almost always the better value.

Whatever you own, the principle stays the same: a tester is only as trustworthy as its last calibration. Put the next due date somewhere you will see it.