Just to throw my hat into the ring.
Another Plod.
I have to disagree with bjcc.
Many is the time I have used an ESD (Electronic sampling device) at the roadside and the result to bypass the pass and warn lights and shoot straight to positive, only to find that the subject blows under the limit when tested on the EBM at the station a short while later.
These machines are merely indicators that a person MIGHT be over the prescribed limit and they are not infallible.
LM