PaulB said:
I've been breathalysed but only unofficially. One new years eve, we were having a party at my mates house and one of the guests was married to a copper who was on duty yhat night. While it was still quiet out, he came round to wish us all the best and that and the host asked him if he had his breathalyser. Of course he did and he let us have a go! I'd drunk four pints over the course of the evening and the host reckoned he'd had about six. To our astonishment, we both registered we'd had a drink but were safe to drive! The copper (Horace he was called) reckoned it's 'cos we were fit lads who ran and could inflate our lungs to a greater capacity than most people but I don't know if that's true. My wife who's tiny registered over the limit after having only one vodka and tonic. After that, as my job depends on my licence, I am always ultra cautious as you never know.
The roadside device, as well as the evidential machine measures air from what it reckons to be the bottom of the lungs. That is where the concentration od alcohol molecules is likely to be at its strongest. When breathing normally the you create a tidal flow of about 3 litres of air per breath, and this is sufficient to carry out the necessary oxygen/gas exchange in your lungs. The three or more litres of air remaining in your lungs more or less stagnates, where if you have alcohol in your bloodstream, the stale air picks up more and more alcohol molecules. The breath devices are designed to discount the tidal volume and only sample the staler air, as it correlates more closely with the level of alcohol you actually have on board.
The fitter you are, and the larger your lung capacity, the cleaner that fourth litre of air is going to be. Similalrly, if you breathe out thoroughly and clean the stagnant air from the bottom of your lungs, and then inhale massive quantities of fresher air before you provide that 4 litre sample, the lower the reading.
I was a singer for many years, and have a lung capacity of getting on for 9 litres. I was once breathalysed by a mate who was on duty, when I was out for a night out and had had well over 8 pints. By adjusting the sample as above I was able to blow clear.