Guides
Police breathalyzer devices and how roadside and station tests work
When people talk about a police breathalyzer they usually mean one of two very different tools. The first is a small handheld unit an officer uses at the roadside for a quick screening. The second is an evidential instrument, normally at a station or in a testing vehicle, that produces the result used in court or in a license case. This guide explains how each one works, why a personal tester will not reproduce their numbers, and what implied consent means in general terms. It is general information, not legal advice.
Roadside screening with a preliminary breath test
A preliminary breath test, often shortened to PBT, is a handheld screening device. Most modern PBTs use a fuel cell sensor, give a result within seconds and help an officer decide whether there is probable cause for an arrest and a formal test. In many states the PBT number itself has limited use in court, and its main role is to support the decision to go further. Rules on whether a driver must take a roadside screening test, and what happens on refusal, vary from state to state.
PBTs are looked after on a schedule set by the agency and the maker, with regular accuracy checks against a known alcohol standard. They are still screening devices, which is why a separate evidential test normally follows.
Evidential breath testers at the station
An evidential breath tester is a larger instrument built to produce a result that can stand as evidence. Many use infrared spectroscopy, some pair infrared with a fuel cell, and all run internal checks before and after each subject test. The procedure around the instrument is as important as the hardware.
- An observation period, commonly about 15 minutes, during which the person is watched to make sure nothing enters the mouth and no belching or vomiting occurs.
- Two separate breath samples, which must agree within a set margin for the result to be valid.
- Blank tests of room air and checks against a reference standard built into the sequence.
- A printed or electronic record with the instrument identity, the operator and the times of each step.
- Operators trained and certified under the state program, and instruments maintained under that program's rules.
Federal guidance supports this system. NHTSA publishes conforming products lists for evidential breath measurement devices and for alcohol screening devices, and states choose approved instruments from those lists and add their own requirements.
| Feature | Preliminary breath test | Evidential breath tester |
|---|---|---|
| Where it is used | Roadside, handheld | Station or testing vehicle |
| Main purpose | Screening and probable cause | Evidence and license decisions |
| Typical sensor | Fuel cell | Infrared, often with fuel cell |
| Procedure | Single quick sample | Observation period and two samples |
| Records | Limited | Full printed or electronic record |
Why personal breathalyzers differ from police devices
A personal tester and a police instrument may share a sensor principle, yet they are not doing the same job. Police and evidential devices are checked on a documented schedule set by agency rules, while a personal unit may go a year or more without a check. Evidential instruments use an observation period and two samples, while a personal test is usually a single blow taken whenever the owner chooses. Even the moment differs, since an official test may happen an hour or more after a roadside stop, while alcohol in the body is still changing.
Marketing phrases such as police grade or used by law enforcement do not make a personal tester an evidential device. Some personal and professional handhelds do use the same class of sensor as PBTs, and a few appear on federal screening device lists, but none of that turns a home reading into a prediction of what the station instrument will show. Our guide on how accurate are breathalyzers covers the factors behind these gaps.
This is an estimate. It cannot tell you whether you are safe or legal to drive. If you have been drinking, do not drive.
Implied consent and refusing a breath test
Every state has an implied consent law. In general terms, by holding a license and driving on public roads you are treated as having agreed to chemical testing of breath, blood or urine when lawfully arrested on suspicion of impaired driving. Refusing the evidential test after arrest usually leads to an administrative license suspension that can be longer than the suspension for a failed test, and in some states refusal carries additional penalties of its own. The rules for the roadside screening test are often different from the rules for the evidential test.
The details change from state to state and are updated over time. For the rules that apply to you, check your state DMV or the state highway safety office, and speak with a licensed attorney about any specific situation. This page is general information, not legal advice.
Legal limits in brief
All states set the per se limit for adult drivers of regular vehicles at 0.08, except Utah at 0.05. Commercial drivers face a 0.04 limit, and drivers under 21 face zero tolerance laws, usually at 0.02 or lower. A driver can also be charged below these numbers if impairment is shown in other ways. Our legal alcohol limit page and the state table on bac legal limit give more detail.
Using a personal tester responsibly
The safest decision does not depend on any number. If you have been drinking, arrange a ride, use public transport or stay where you are. A personal tester is useful for understanding how your body handles alcohol and for checking the next morning, and a fuel cell breathalyzer gives the steadiest results for that purpose. Employers and organizations that need screening devices from federal lists can start with our professional breathalyzer category and our review of the draeger breathalyzer handheld. Drivers facing an interlock requirement can read our ignition interlock device guide.
A short checklist before you buy
If you are choosing a personal tester after reading about police devices, our checklist sums up what to compare.
Police breathalyzer questions
Can I buy the same breathalyzer the police use?
Some professional handhelds of the kind police use for screening are sold to businesses and the public, but evidential station instruments are sold to agencies and used under state programs.
Is a roadside breath test the same as the test at the station?
No, the roadside preliminary test is a quick screening, while the station test uses an evidential instrument with an observation period and two samples.
Why might my personal breathalyzer show a lower number than the police test?
Different sensors, calibration schedules, procedures and timing all play a part, and alcohol levels may still be rising when the official test is taken.
What happens if I refuse a police breathalyzer?
Refusing the evidential test after arrest usually triggers a license suspension under implied consent laws and can bring other penalties, so check your state DMV and an attorney for the rules that apply to you.
Which agency lists approved breath testing devices?
NHTSA publishes conforming products lists for evidential breath measurement devices and alcohol screening devices, and states select instruments for their own programs.