Calibration of the Proving Ring
Proving rings are calibrated in accordance with procedures established by the American Society for Testing and Materials in a standard designated as ASTM E 74 - 95 entitled "Standard Practice of Calibration of Force-Measuring Instruments for Verifying the Force Indication of Testing Machines." This standard can be purchased through the American Society for Testing and Materials. The standard clarifies the method to be used in the calibration of all force measuring instrumentation, including proving rings. It also specifies the requirements that force standards must meet as well as the procedures used to determine their accuracy.
The National Institute of Standards and Technology (NIST) realizes and disseminates the unit of force to industry, government, testing laboratories and academia. To this end, the NIST's Force Group realizes discrete forces in the range of 44.822 N (10 lbf) to 4.44822 MN (1,000,000 lbf) with a relative standard uncertainty of 0.0005 %. To accomplish this, NIST maintains a unique force laboratory that includes six deadweight machines.
In the deadweight machines, static forces are generated by suspending weights of known mass in a known gravitational field. Above 4.44822 MN, NIST's Force Group provides force calibration services in compression up to 53 MN with a relative standard uncertainty of about 0.01 % by comparison with NIST transfer standards using a 53 MN capacity universal testing machine .