My tach (Cermark One Touch - black case) appears to be the same thing as the Hobbico (red case) and generally it has been reliable. I only use it a couple times per year. But yesterday on a 3w 100 gasser (26 inch prop) at full throttle, the tach varied from 6,000 to 8,000 during about 60 seconds. The engine was not changing rpm by more than a few hundred rpm at most. Yet the tach would not stop on a number consistently. I had just put in a new 9-VOlt. It reads 3600 pointed at the flourescent. Usually the tach shows a number and it does not vary by more than a few hundred rpm.

The sky was very clear and bright - around high noon. I don't use the tach often enough to know if perhaps it works better when the sun is lower in the sky.