Cheap microelectronics are democratizing the world of blood alcohol measuring equipment. A field once dominated by $10,000 machines owned only by police departments and requiring significant training is now filled with devices that cost as little as $30 and can hang from a key chain.
A recent proposal by the National Transportation Safety Board to drop the legal definition of drunk to 0.05 percent blood alcohol from 0.08 percent has kindled interest in the personal units, some of which are sold as smartphone accessories, app included.
The devil, of course, is in the details. Safety experts warn that a reassuring reading on one of these consumer-grade measurement devices does not guarantee someone is safe to drive, partly because impairment from alcohol varies from person to person. And manufacturers, wary of liability, don't endorse the idea, either.
These machines do offer some form of measurement beyond counting drinks, however, and they could be useful to parents trying to make sure their offspring aren't drinking.
Keith Nothacker, whose company, BACtrack, sells a so-called fuel cell unit that displays its results on an iPhone via a Bluetooth link, argues that "law-abiding citizens can't possibly be expected to know the difference" between 0.05 blood alcohol content and 0.08 blood alcohol content without doing a test. BACtrack displays a graph that predicts blood alcohol levels in the hours to come, all the way down to zero. The device sells for $150.
Inside the hand-held, battery-powered unit is a metallic catalyst that starts a chemical reaction, breaking alcohol down into an acid and water. The reaction also gives off electricity, which is easy to measure and which the unit uses to calculate blood alcohol.
Mr. Nothacker's company describes its product as "pocket-sized peace of mind," but in a telephone interview he insisted that its purpose was not to assure a drinker that he or she would be under the limit if pulled over by the police. He said it was simply an "an educational tool."
A competitor, Alcohoot, is promising shipment in September of a $75 model using the same catalytic method that plugs into an iPhone. Alcohoot's product includes an app that displays a list of local restaurants where people can eat and sober up and a list of taxi companies.
Another type of measurement device, which typically costs $30 to $70 and is made by manufacturers including Bactrack, relies on a tin oxide semiconductor. Applying alcohol to the sensor changes its ability to conduct electricity, giving an easy indication of blood alcohol. But those sensors may go bad after three to six months. They also can be prone to giving false positives, mistaking acetone, hair spray or other common contaminants for alcohol.
The machines are all calibrated by the manufacturer using a device approved by the Transportation Department that takes in a solution with a known alcohol concentration and puffs it out in a way similar to human breath.
Bactrack recommends annual recalibration of its more expensive models, which it will do for $19.95, including prepaid return shipping.
The market for these devices varies. Most of the cheap semiconductor models are probably bought for personal use, manufacturers say. The fuel cell units are sold to drinkers for their own use as well as to employers and parents.
The testing equipment used by police, on the other hand, is usually not sold to private citizens, nor could the average person easily use them without training.
A driver pulled over by the police and subjected to a blood alcohol test back at the station will most likely blow into a device about the size of a desktop printer. Inside, an infrared light shines through a breath sample, and a sensor measures how much is absorbed; the higher the absorption, the higher the blood alcohol.
Most police departments use a model called the Intoxilyzer, made by CMI, of Owensboro, Ky. It sells for about $10,000 and its results are widely accepted in court. "We have instruments, as opposed to machines," said Alan C. Triggs, a CMI spokesman.
Machines like the Intoxilyzer can detect blood alcohol levels at 0.08 percent, or 0.05, or the lower levels applicable to commercial drivers, teenagers and others. But before police officers haul a driver back to the station to take the test, the officer often will stick a flashlight-shaped device through the driver's window. The light given off by this $600 mobile detector mostly serves to disguise its true purpose: an air sensor that sniffs for alcohol.
The mobile units do not get the same respect in court as the big systems seen at police stations, although their manufacturers claim that they have the same accuracy. But they do provide an officer with probable cause to order a driver out of the car for a field sobriety test or further action. The N.T.S.B., which recently called for the stricter definition of drunken driving, recommended more use of the flashlight sensors, because studies have shown that even at a sobriety checkpoint, officers often fail to spot the drunks.
J. T. Griffin, a lobbyist at the Washington office of Mothers Against Drunk Driving, warned that the consumer models were sometimes used by people who wanted to skate as close to the edge as legally possible. And, he said, "If the device isn't calibrated properly, it could give them the wrong reading and they might drive anyway."
The devices might be accurate when new, he said, but they all require follow-up testing, and "The average person isn't going to get one calibrated."
"From our standpoint, it's not a good a good way to test" blood-alcohol content, Mr. Griffin said. A better idea, he added, is to pick a designated driver. And even if a person is below the legal standard for drunkenness, he said, "Impairment does begin before 0.08."
This article originally appeared in The New York Times.