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As far as I can tell it's accurate, but I don't have anything else to verify it. I used it on a brand new lockout stringer that was calibrated at the factory and it was dead on. I would assume the factory calibrated the stringer accurately so it seems like the Gamma calibrator works perfectly.
The gamma tension calibrator is very useful to have along with your string machine. used it once I received it and learned my crank tensioner was accurate. The calibrator does not come with any instructions but I guess it is self explanatory. Just be careful to make sure your clamps are tight enough before adding tension or the strings will slip