Know this instrument shouldn’t be used to determine whether you can legally drive & may not be 100% accurate, but it does take the guess work / doubt out of the equation. I Drive for a living & feel with complete confidence that this was the best purchase for me in a long time ! I have brought it with me everywhere I go since the day of the purchase. It’s small / compact size is very easy to keep on yourself, I let other friends use it to prove they shouldn’t be driving under any influence. By all means it doesn’t cure the DWI problem , but I’m positive the instrument has already saved lives. Thank you for such a great product. Rich C