Nate, we use
the
Gamma calibrator and as esgee48 says regarding lock-out machines (e.g., spring crank), you'll want to check the reading right as it locks-out (because the tension will drop as the string relaxes). I'd start by checking the calibration every 20/25 racquets. You can always adjust your check rate based on your findings. It's also smart policy to check calibration if you switch to radically different tensions, (e.g., badminton), or if you transport the machine. I agree with esgee48 about independent verification with a known weight.
Hope this helps,
Jon