Never had one. My experience with Hanna meters is that you get what you pay for. If it cost enough to make you think a time or two it's a good meter. Why not call or email Hanna and get someone with a lot of experience with these Nitrate meters. BTW Don, color strips are crap and drop tests can be hard for some men to read because a good percentage of men have a degree of colorblindness they are unaware of.