Orifice plate specifications are covered in API MPMS, Chapter 14.3/AGA Report No. 3 (1992) and include for example, the following:
• Plate thickness: The minimum and maximum thickness depend on plate size, but 1/8 inch is the minimum.
• Plate flatness tolerance: The maximum departure from flatness varies with orifice diameter (d) and size (D). It may be calculated as 0.005(D-d).
• Roughness: The orifice plate surface roughness should not exceed 50 microinches.
• Upstream edge: The square-edge (sharp-edge) type should not show a beam of light when checked with an orifice edge gage. Alternately, the square-edge type should not reflect a beam of light when reviewed without magnification. The orifice should not have a burred or feathered edge. Nicks on the edge can be expected to increase the measurement uncertainty.
• Centering: The concentric orifice should have the bore centered within 3% of the inside diameter of both the upstream and downstream sections of the orifice meter run.
• Beta ratio: A recent study by Chevron Petroleum Technology Company (CPTC) suggests that the optimal beta ratio is 0.4 to 0.55 for natural gas. Beta ratios departing from this range increase measurement uncertainty.