What is the calibration of the pressure instrument?

A pressure gauge is attached to the proper fitting on a bench with valves and tubing. A known weight pressurizes a piston. That pressure goes to the gauge being calibrated and the N.B.S. certified gauge. The two are compared at several points called out in the test procedure. Adjustments are made to the mechanical linkage between the gauges internal tube and the indicator needle to make it read within an allowable percentage of deviance.