Tachometer question

I hope to hook up another tach tonight from my timing light to compare. If it's the same I guess I wont worry about it. If it's different maybe I will need another tach. What was really strange was that if I connected and disconnected the hot wire it would go from 0 or 10. As long as its not doing damage to anything I'll use the one that's on it. New ones are $250-$300!