Yep, I'm good to drive . . . OR AM I?

CONSUME: ANZ Herald staffer uses a breathalyser to see how many drinks he can consume before he goes over the limit.

Personal breathalysers have flown off the shelves in Masterton since drink-driving limits changed - but police are cautioning drivers not to rely on them to escape conviction.

The warning comes after a Hamilton man received a criminal conviction when a store-bought breathalyser falsely told him he was sober enough to drive.

Masterton retailers said they saw a spike in the sale of breathalysers when the drink-driving limit changed in December, with one store selling out.

Masterton Traffic Sergeant Chris Megaw said he was aware breathalysers were available for sale in Wairarapa but recommended drivers did not rely on their accuracy.

"My advice would be not to rely on it as an accurate reading and certainly don't rely on it as a defence if you get caught.

He said he could not understand why someone would choose to use such a device.

"The police line would be if you are going to drink or drive, don't do both. Once you've had two or three your desire to stop and that part of your brain that controls the ability to stop has been compromised."

He was not aware of any recent cases of drink-drivers using store-bought breathalysers in Wairarapa.

Consumer's head of testing Paul Smith said while there were a variety of breathalysers available in New Zealand, ranging from cheap "toys" to workplace testers, the best advice was not to drink at all.

"Ultimately, the only thing that matters is the test taken by the police on their equipment. If you think anything else is useful, you are mistaken.

"If you think you are close enough to the limit to need to check with one of these devices, you certainly shouldn't drive."

Products on the market ranged from single-use breathalysers that were available at service stations and liquor stores to devices similar to ones police used.

"The best devices use 'fuel cells', just like the police roadside ones. These tend to be $200 plus. Cheaper ones use semiconductors, which are less accurate and can give false positives through contamination. Many of the cheapies don't give a breath alcohol concentration value, just indicate if you are over or under a pre-set limit. All but the cheap 'toys' need calibration and recalibration."

But the bigger problem was that everyone metabolised alcohol in a different way, Mr Smith said.

"It takes time for alcohol to enter the system and breath alcohol concentration rises for a while (something like 30-90 minutes) after drinking, before gradually falling away. Using one of these to check before driving, then being stopped half an hour later can make a huge difference, and makes the accuracy of the device a bit meaningless."

Sober Check manager Gavin Foster, who specialise in certified drug and alcohol testing products, said the company had noticed an increase in breathalyser sales to the public since the law changed.

It was important drivers invested in accurate products, he said.

"We always say if you going to use a breathalyser, it needs to be accurate. When you have a good breathalyser you can test yourself and it will come up with what your level is, and you know if you drive your car and go through a checkpoint, you aren't going to get caught. It gives you that peace of mind that you are staying under the drink-driving level."