Had a look at the Alcosense gizmos Mel. They seem to be the benchmark models. Basic, middle and top of range at £25, 35, 55.
The spec. says that it tops out at 1.2 whatsits , being 1.5 x legal fail mark. Anything over that and it still read 1.2
Waste of time then...

That's ridiculous. Are you sure?

How stupid.

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mel

It seems that they are fairly exact between nothing and 1.5 x legal limit. If one is 4 or 5 times over the limit it still shows 1.5. That's my reading of it, may be wrong.

Ah, I misunderstood you. 1.5 x is fair enough. All you want to know is if you're over, and it's useful to know if you're just a shade or a bit more. It would be interesting to know that you're 5x, for example, but I expect at 5x you would slightly miss the point.

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mel

A police officer, who was hoping to catch someone drunk, waited outside a bar. Now most bars close at midnight, so he parked himself right around the bar and waited for someone to come out drunk and try to drive.

Sure enough, at 11:45pm, a man came stumbling out of the bar. It took him five minutes to get to his car and another five to turn the car on. The police officer sensed victory and let the man start driving.
He pulled the man over only 50 feet away from the tavern.
He walked up to the man and said, "I just saw you come out of that bar and you were pretty loaded."
"Daknguifshregjdgfnfdjgn," said the drunk man.
"How many beers did you have?" asked the police officer.
"Anoout fiften," said the man.
"FIFTEEN! And you're trying to drive?!? You will get life for this," said the officer.
"Hop out of the car. I am going to run some tests on you," said the officer.

The man hopped out of his car with perfect grace, he smiled and stood on one foot, hopped up and down and said his ABCs fowards and backwards. The police officer didn't get it.
"Okay, let me smell your breath," said the officer.
"Sure," said the man.
He exhaled right into the officers nose and the officers smelled no beer on his breath.
"Well, I guess I am gonna have to let you go, but why did you stumble out of the bar so drunk?"
"Oh, I'm the DD," said the man.
"A designated driver?"
"No, a designated decoy," said the man.

Fairy Nuff.
However for a male of my dimensions and proclivities, I reckon that about 1.5 pints of decent beer is the legal limit. Ergo somewhat under 2.5 pints (way under a bottle of wine) is this m/c's topping-out point. So it can't differentiate between (say) 2.5 pints and 4.