I have my own Question I'm using a Hobbico mini tach one of the ones with the button on the side well last year the batteries died and i pulled them out now I can't find them and don't know what kind they are LOL i lost the old ones the new has AAA but this i thought had button type but what size? AS far as you question lighting has a lot to do with how it reads, somes getting close I dare helps it read better, face it towards the brighter light source and the shadow of the prop will do the rest be sure settings are correct for 2 & 3 Bladed props