The measurement uncertainty for thread ring gages is calculated from two factors:
The measurement uncertainty of our set plugs which is around 0.000070 depending on size
- ANSI/ASME B1.2-1983 Section A3 (p. 164) Section A3.1.2- the measured pitch diameter on rings fitted to a setting plug may be 0.0001 to 0.0002 in. larger than the measured pitch diameter on the plug because the pitch diameter equivalents from permissible pitch, lead, and flank angle tolerance on matched plug and ring cause some unavoidable discrepancy. So in other words- the standard error between the plug and the ring as stated in the standard. We err towards 0.0002 as the worst case scenario.
Since there are no established torque values for the degree of drag, the end result is up to technician and ultimately does not constitute a measureable result, like a thread plug or a gage block would.
Please note that ISO/IEC 17025:2005 Section 126.96.36.199 Note 2 “ In those cases where a well-recognized test method specifies limits to the values of the major sources of uncertainty of measurement and specifies the form of presentation of calculated results, the laboratory is considered to have satisfied this clause by following the test method and reporting instructions”. Calibration of thread rings using set plugs is most assuredly a “well-recognized” test method.
Basically, you have to take into consideration the uncertainty of the set plug and add 200µinches per the standard.