Some time ago I purchased the BACtrack Keychain Breathalyzer which I have used for myself and my friends. When I lost the mouthpiece, I immediately moved to replace it, because it allows me to share this device with those who have been drinking. I already have a friend who has purchased a Breathalyzer for himself. We have also discovered that body mass makes a great difference. A female friend who had been drinking less than her study husband, suggested she drive home. He asked that I check them both and we discovered that his BAC was significantly lower. THis is a great investment.
While it requires a companion app on your smart phone, it works quickly and accurately. Also, the privacy (no sharing) option in the app allows you to keep all results ... private.