All good things must come to an end, and when it comes to consuming alcohol, that end typically comes before with the ride home. But are you really in shape to drive? A personal breathalyzer can give you some much needed guidance that less quantitative tests (close your eyes and put your finger on your nose!) just can’t provide.

Before we get started, let’s acknowledge that there is significant controversy around the use of breathalyzers. Detractors hold that if you’ve been drinking enough to need to blow into a breathalyzer, you’ve been drinking enough to call a cab. That might be an overly simplistic viewpoint, but I’m not going to try to solve the debate in this roundup. Ultimately, it’s my position that the more information someone has about any alcohol-related impairment they might be experiencing, the better. In general, law enforcement tends to encourage such devices, provided they are accurate and easy to understand.

While breathalyzers utilize a handful of technologies to operate, consumer units rely on two main types of sensors: Fuel cell sensors and semiconductor oxide sensors. Not to get too geeky, but here’s how they both work. Fuel cell sensors oxidize alcohol in a breath sample, then measure the strength of a current that is run through that sample. The strength of the current varies based on the amount of alcohol in the sample, and it can be used to calculate a blood alcohol content. These are what the cops generally use, and it’s the tech inside all four of the units tested here. Semiconductor sensors are smaller and less accurate and are likely to be what you find in keychain-sized convenience store units. These rely on a heated, metal mesh inside the sensor. When you blow onto the mesh, any alcohol in your breath changes the electrical resistance of the mesh. This change in resistance can also be used to calculate your BAC, albeit more crudely.

In use, handheld breathalyzer units all work roughly the same way. First, wait 15 to 20 minutes after your last drink. This is crucial, otherwise the alcohol vapor in your mouth will give you an off-the-charts reading—and even an extra minute or two can result in wildly different results. Turn the unit on and wait until prompted (this is an important calibration step). Then blow until directed to stop (most units make a Kafkaesque clicking sound to indicate when you can finally inhale). The unit processes for a couple of seconds and displays your BAC on the screen. Afterwards, often, an audio cue will alert you if you’re above a certain threshold. A newer class of breathalyzers interface with your smart phone and provide info through its LCD, but otherwise they use the same process and technology.

All of the four units reviewed below promise government-grade accuracy. To test those claims, I worked under the supervision of a local police department officer to perform side-by-side testing with the consumer devices and an Intoximeters Inc. Alco-Sensor IV breathalyzer which had been freshly calibrated. I took multiple data points over a range of BAC levels and spent several hours testing all of the devices in tandem.

Here’s how the personal models stack up against what the cops—er, the pros—use.

breath-inline2 ALCOMATE

AlcoMate Revo TS200

$250, Rating: 5/10
The Cadillac of breathalyzers, at least if you go by price. The Revo is one of the most handsome and compact standalone units on the market, and it comes complete with a padded plastic briefcase that would make it right at home on any CSI series. The stylish unit is idiot-proof, with only one button (power). Dim backlighting means you can read the LCD in the dark—but you won’t attract a lot of attention aside from the long beep it emits afterwards if your BAC is above 0.05 percent.

The big news with the Revo is actually under the hood. All breathalyzers need to be recalibrated periodically—usually once a year—which involves mailing them back to the manufacturer. Knowing that most users are unlikely to ever do this (and risk getting inaccurate results), Revo lets you replace the sensor yourself. The bad news: At $90 per sensor, the price could nearly fetch you a brand new breathalyzer from another vendor.

Unfortunately, the Revo didn’t turn in the best accuracy numbers in my testing. Compared to the police unit, the Revo’s average reading was 10 percent low, including one reading that was 22 percent below what the police unit measured. That may not sound like much, but 10 percent is the difference between blowing a 0.072 percent and actually being at a 0.08 percent BAC—legally drunk. Inaccuracy is a problem, but inaccuracy on the low side is an even bigger one, as users might assume they are less impaired than they are.

breath-inline1 AlcoHAWK

AlcoHawk PR500

$150, Rating: 8/10
The size and design of the AlcoHawk PR500 date it a bit, but you can chalk at least some of that space up to the more helpful, amber-backlit LCD. With a total of three buttons, the PT500 has more options than most standalone breathalyzers, including a feature that lets you customize the audible alarm point (the default is 0.02 percent, after which a short chirp is emitted if you’re over). My only real complaint: a lengthy nuisance of a klaxon buzzes seemingly forever for no other reason than the unit is about to auto-shutoff.

While the AlcoHawk is too bulky to fit easily into a pocket, it makes up for its design flaws by turning in very accurate results. Only one of my readings was below the police unit (by just 2 percent), and the rest were just a bit above or right on target. All told, the PR500 had an average variation of just 4 percent above the police unit, making it the most accurate device in the roundup.

breath-inline4 BACtrack

BACtrack Mobile Pro

$100, Rating: 6/10
The BACtrack Mobile Pro is a smartphone-connected breathalyzer. It connects via Bluetooth to iOS or Android devices, and works with a companion app that you install in advance. Unlike all the other devices in this roundup, which are powered by regular batteries, the BACtrack has in integrated rechargeable battery that you juice up via a USB cable.

The BACtrack device is very small, slightly larger than a Zippo lighter, and it offers no audio cues at all (which is refreshing in what is a very noisy field). It does feature bright blue internal lighting, however, so it’s hardly discreet. My only complaint with the hardware itself is that the mouthpieces (disposable plastic nozzles that can be discarded, because cooties) don’t fit well and simply fall out of the device. I lost two of them the first day I had the device—and you only get three with your purchase. (Pro tip: Put the nozzles in upside down and they’ll stick!) There’s no auto-off feature, so make sure you keep an eye on the power button.

The smartphone angle is a neat one, as it opens up additional information about your inebriation, including a graph that estimates when you’ll be sober, an explanation of the likely impact on your body and mind by your BAC, and—strangely, an ill-advised “social” feature that maps other users’ BACs so you can compare your drunkenness with friends’.

Sadly, the BACtrack scanner was the most inaccurate scanner in my testing, with readings coming in between 3 percent and 17 percent over the police unit. None of the readings were low—which is always a good thing—but its average reading of 12 percent above that of the police unit makes it hard to fully trust the data it provides.

breath-inline3 Trubreez

TruBreez Premier

$100, Rating: 6/10
This Brutalist monolith won’t win any design awards, but it is relatively compact, simple to use, and fast. A nice backlight and fast operation are also plusses, but those benefits come at the expense of your ear canal, as the TreBreez emits a shrill and sustained beep while you’re blowing into it, making it impossible to use in mixed company. A memory button lets you cycle through the last five measurements, but you can only use this feature after you complete a reading. Blow over a 0.05 percent BAC and the screen turns an angry red, warning you not to try to operate heavy machinery.

TruBreez tied the AlcoHawk unit for accuracy but went the other way, turning in averages that were 4 percent below the police unit. That 4 percent average is extremely close, but I did encounter a couple of outliers with this unit, including one result that was 14 percent too low, which is a bit too far off for comfort. As a side note, I also encountered more errors with this unit than any of the others I tested (recovery just means restarting and trying the process again), though it still had a roughly 90 percent success rate on the whole.

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One Too Many? These Breathalyzers Will Tell You