Bourbon tubes get knocked off zero all the time. It changes the starting point of the scale but doesn't affect accuracy at all except to add a constant. I have a lot more confidence in my pump that shows 5 psi at rest then the one where the needle rests on a pin stop at zero. If your needle is resting at 20, just pump to 120 when you want 100. Trying to reset it will only risk damaging it, and won't improve accuracy.
If you want to be more accurate then that, you will need to find a lab somewhere to calibrate it for you.
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Last edited by eddy m; 10-11-09 at 08:48 AM.