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Dimensional measurements

Reference master thread gauges (API and VAM)

Compliance to American Petroleum Institute specifications

Our UKAS accredited service provides certification of compliance for Reference Master gauges to American Petroleum Institute (API) Specifications. Under a EURAMET agreement, traceability for API gauges throughout Europe has been devolved to NPL who are the sole agency in Europe authorised by API to certify reference master screw gauges. NPL also provides calibration services for VAM specification gauges.

API regional masters

Approved Regional Master gauges held by NPL are as follows:

Style Size
Numbered connections NC 26 NC 31 NC 35 NC 38 NC 40 NC 44
NC 46 NC 50 NC 56 NC 61 NC 70  
Regular right-hand (Reg) 2 3/8" 2 7/8" 3 1/2" 4 1/2" 5 1/2"  
6 5/8" 7 5/8" 8 5/8"      
Regular left-hand (Reg LH) * 4 1/2" 6 5/8"        
Full-hole right-hand (FH) 3 1/2" 4" 4 1/2" 5 1/2" 6 5/8"  
Internal flush right-hand (IF) 2 3/8" 2 7/8" 3 1/2" 4" 4 1/2"  
5 1/2"          

* Regular left-hand (Reg LH) gauges have been temporarily withdrawn from service

NPL provides certification of compliance for Reference Master gauges to the following API Specifications - 5B Casing, Tubing and Line Pipe threads, 7 Rotary Drilling Equipment and 11B Sucker Rods.

VAM master thread gauges

Calibration of VAM gauges are available on request.

Measurement Uncertainties

Major diameter ± 0.000 20 inch
Hardness ± 1.2 HRC up to 5"
± 2.5 HRC above 5"
Pitch of threads ± 0.000 12 inch
Taper on diameter ± 0.000 11 inch
Pitch diameter at gauge point ± 0.000 37 inch
Semi-angle of thread ± 6 minutes of arc
Stand-off measurement ± 0.000 34 in for fast tapers, e.g. Rotary gauges
± 0.002 in for slow tapers, e.g. Line pipes, tubing

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