On a few occasions it's read too high in the morning and I didn't drive until it had cleared. Once I got a lift into work instead of driving and the Police were testing people driving into the industrial estate at 730am....
The Alcosense Elite has a purge facility too so you can use it soon after a test - the cheaper ones you need to wait an hour or more after a test to get an accurate reading.
They're at Halfords for £59 I've just seen.
Current: 2013 F10 520d M-Sport, Professional Media, Visibility Pack, Reversing Camera, DAB
Previous: 2011 F10 520d M-Sport, 2008 320i M-Sport Coupe, 2002 325i, 2001 318i valvetronic, 1998 318i, 1996 525i, 1990 Porsche 944S2