thats what I found Nelly, when i first got mine I inserted the No.5 button and we tried it on my mate Dallas's 748RS on which he had just had the belts tensioned by Brancato, after getting the wron reading I swapped to the No.4 button and got the correct reading, I put it down to the belts containing Kevlar in their construction but there is probably some other reason.
Kev

Thanks Nelly and Skidlids for answering. Havent used one yet myself. Would def go for the AST one though.

Thanks for all the info guys.
Especially re: the button 4 vs 5

AST one it is then.

Paul