My Davis V2 rain gauge was reading 20% less than a 4" manual gauge placed beside it. I checked the balance between the 2 buckets, and it was way out - 1 side tipped at about 48 drops, and the other about 83 drops! I had to turn one screw 3 whole turns before they were about equal - and then proceeded to tweak both together to get the calibration about right (4.28ml/tip).