A supplementary bilateral comparison was carried out to compare two different pressure standards at a nominal pressure of 0.05 Pa. The two standards used different principles for generating the nominal pressure; one based on the static expansion method (NPL, India), and the other based on the orifice flow or dynamic expansion method (NIST, USA). The transfer standard consisted of a pair of spinning rotor gauges whose accommodation coefficients were measured at a nominal pressure of 0.05 Pa at the two laboratories. The nominal pressure value for the comparison was chosen because (i) it can be easily generated by the respective standards with high accuracy and (ii) it had not been covered in the earlier two key comparisons, namely, CCM.P-K3 and CCM.P-K4, which the two NMIs had participated in. NPLI served as the pilot lab, and provided the stainless steel rotors; each lab used its own thimbles and electronics. The rotors were hand-carried from NPLI to NIST at atmospheric pressure in small, insulation-packed glass vials to prevent the rotors from moving during transit; otherwise no special transit precautions were taken.

The pressure value of 0.05 Pa generated by the NPLI and NIST showed good agreement within their combined uncertainties, proving thereby that the two standards were equivalent to each other for the pressure of 0.05 Pa.

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