Some 15 years ago now I had the local council building inspector in to pass a double glazed window I'd installed. My memory is a bit vague but he used something like a small tube which he pointed at or maybe held against the glass of the window and which apparently indicated whether it was a pass or fail.
My new window passed but he also checked some other already installed windows that had been in for about 10 years and they failed.
Anyway, can someone tell me what this magic device he had was and what exactly it was measuring?