The devices used by the police fall into two types. The one used at the roadside is not claimed to be infallible, but gives a screening test to allow the officer to make a decision on whether to arrest. The device used at the station is properly installed and maintained. The maintenance is carefully recorded. There can be errors in anything (see the worrying errors made by speed cameras) but you would find it difficult to challenge the Intoximeter. The blood/urine test is not normally relevant because the law gives a maximum level for alcohol in the breath. The period of 20 minutes is allowed by the police as a matter of course, but that would only affect the roadside screening test anyway.