Black sword.
The amount of alcohol in your blood is the point at issue. There are 3 ways of mesuring that. Breath, the amount of alcohol in your blood is proprtional to that in your breath. This is how breath tests are useable in Road Traffic Cases.
Blood, obvious really about that one
Urine, again contains alcohol in proportion to your blood alcohol content.
None of the above show impairment. They mearly show you have a blood alcohol content above or below a prescribed limit. In my experience and that of other Police officers the blood and urine tests often give a slightly higher reading than breath.
As has been pointed out there are 2 seperate offences. If you are rolling drunk then you clearly unfit to fly, although you don't have to be 'rolling drunk, just obviously incapable of doing the job. Thats the first offence, section 92. It also applies to all forms of drugs, I am sure you have seen the warning 'may cause drowseness if affected do not drive' on some drugs, Night Nurse used to be one that affected lots of people, thats whats meant by the lower end of the drugs side of this section. ie the side effect means you are not fully capable of doing your job. Of course it also applies to hard drugs you wont get at the chemists.
The second offence is doing whatever function with a blood alcohol concenration above a prescribed limit. Does not matter what effect it has on you, in exactly the same way as driving with a BAC above a prescribed limit. No one is saying you are drunk or that you are impaired, just you have had more to drink than you are allowed. Blood Breath and Urine can all mesure the levels and be used as evidence.
As for what a Police Officer will do if you blow a level close to the limit, well speaking from the Road Traffic side I would warn them it was not in thier interests to continue to drive, given that thier BAC COULD still be going up, and that if they are stopped and tested again they could find they were over the limit. If they chose to drive there is nothing I could do to stop them. The same applies here, no power to stop you doing anything. However if there is reason to test you again you have been warned about the possible consequences.
As regards to the machines ability to mesure low levels of alcohol, yes they can, the Sweeds and the Australians manage with comercialy available equiptment. They Swedish level for driving is the same as under this law, and the australians as I recall is ZERO.