Compliance to American Petroleum Institute specifications
Our 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 |
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| Regular right-hand (Reg) | 2 3/8" | 2 7/8" | 3 1/2" | 4 1/2" | 5 1/2" |
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| 6 5/8" | 7 5/8" | 8 5/8" |
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| Regular left-hand (Reg LH) * | 4 1/2" | 6 5/8" |
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| Full-hole right-hand (FH) | 3 1/2" | 4" | 4 1/2" | 5 1/2" | 6 5/8" |
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| Internal flush right-hand (IF) | 2 3/8" | 2 7/8" | 3 1/2" | 4" | 4 1/2" |
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| 5 1/2" |
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* Regular left-hand (Reg LH) gauges have been temporarily withdrawn from service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 |
