Back in the 90s I had 3x Traffic Cop mates and I was out with them one night.
We got in the car and I had a go at Dave the driver saying he should be disgusted with himself because he's had 6 pints and he'd be way over.
An argument ensued he he gave his keys to Sarge and said get my spare breathalyser out the boot in my kit bag.
He blew under the limit and I was gobsmacked.
He said in the peak fitness condition he was in he knew he could have 6 pints and not be over however earlier that year when he was on the sick for 3 months he could only have two.
This is when they told me they had official weekends away to test breathalysers by drinking/testing/recovering and it's something I've always wanted to try.
I also remember a Nicky Campbell documentary where he waited outside pubs for drinkers to come out but none of them were going to drive.
He asked one bloke how many drinks he had and he said 4 pints of carling so he asked the bloke if he thought he was over to which he laughed and said no.
He blew around 78 and argued about it.
A woman came out and she had drunk around 8 vodka & tonics (or similar) so he asked the same question.
She said I'm way over, I can hardly see.
She blew under.
This whole buying a breathalyser experiment idea/assignment is just to swing the time away from the missus isn't it you swine! 
