The Short-Term Performances of Two Independent Gas Modulated Refractometers for Pressure Assessments

Refractometry is a powerful technique for pressure assessments that, due to the recent redefinition of the SI system, also offers a new route to realizing the SI unit of pressure, the Pascal. Gas modulation refractometry (GAMOR) is a methodology that has demonstrated an outstanding ability to mitigate the influences of drifts and fluctuations, leading to long-term precision in the 10−7 region. However, its short-term performance, which is of importance for a variety of applications, has not yet been scrutinized. To assess this, we investigated the short-term performance (in terms of precision) of two similar, but independent, dual Fabry–Perot cavity refractometers utilizing the GAMOR methodology. Both systems assessed the same pressure produced by a dead weight piston gauge. That way, their short-term responses were assessed without being compromised by any pressure fluctuations produced by the piston gauge or the gas delivery system. We found that the two refractometer systems have a significantly higher degree of concordance (in the 10−8 range at 1 s) than what either of them has with the piston gauge. This shows that the refractometry systems under scrutiny are capable of assessing rapidly varying pressures (with bandwidths up to 2 Hz) with precision in the 10−8 range.

Validation of a PTB force-balanced piston gauge primary pressure standard

Experimental methods using different pressure standards were applied to verify theoretical results obtained for the effective area of the piston-cylinder assembly (PCA) and for pressures measured with a force-balanced piston gauge (FPG). The theoretical effective area was based on the PCA’s dimensional properties defined via diameter, straightness and roundness measurements of the piston and cylinder, derived by gas-flow modelling using principles of the rarefied gas dynamics, and presented as two values: one obtained for absolute and the other for gauge pressure operation mode. Both values have a relative standard uncertainty of 5×10^-6. The experimental methods chosen were designed to cover the entire operating pressure range of the FPG from 3 Pa to 15 kPa. Comparisons of the FPG with three different PTB pressure standards operated in different pressure ranges – a pressure balance, a mercury manometer and a static expansion system – were performed using the cross-float method and by a direct comparison of the generated pressures. For the theoretical and experimental effective area, as well as for pressures generated by the FPG and the reference standards, all the results demonstrated full agreement within the expanded uncertainties of the standards.

Verification of Gas Flow Traceability from 0.1 sccm to 1 sccm Using a Piston Gauge

Fluke Calibration is accredited for gas flow measurements in the range of 0.1 sccm to 6000 slm in nitrogen and air. Traceability is maintained directly through a gravimetric f low standard but only recently from 1 sccm to 10 sccm. The traceability of flow in the range of 0.1 sccm to 1 sccm is based on extrapolation of the use of laminar flow elements (LFE) below 1 sccm. This part of the range has never been completely verified through interlaboratory comparisons, proficiency testing or other means of measurement assurance. In an internal document from DH Instruments in the early 1990s it was suggested that a piston gauge might improve traceability for very low gas flows. In order to prove out traceability in this range an attempt was made to use a piston gauge using a piston-cylinder size of 35 mm diameter as a reference. One reason for choosing a piston gauge as a reference is its pressure control. This is crucial when measuring gas flow through a LFE in this design and range. In addition, the effective area is known to within 0.001 %, leaving the vertical displacement of the piston to dominate the uncertainty of the dimensional part of the flow test. This was a challenge because the measurements required absolute mode and the internal piston position sensor supplied with the piston gauge did not have sufficient precision. This paper describes the theory and design of the gas flow measurement system, the current results, and improvements desired or suggested. Two different designs are discussed, one with a single piston gauge as a reference and one with two piston gauges measuring flow on either side of the laminar flow element. Note: sccm (standard cubic centimeters per minute) is an industry accepted alternative to kg/s [1]. It is used out of convenience to normalize flow rates of gases with significant differences in density.

Establishment of air piston gauge as primary pressure standard at CSIR-National Physical Laboratory INDIA

The air piston gauge (APG) was established at CSIR-National Physical Laboratory, India (NPLI) since 2000. Later the same piston- cylinder(p-c) assembly was calibrated in NIST USA; however, it was never published for metrology communities. As per international protocol, the establishment of the APG as a primary standard, the effective area of p-c assembly, and masses must be directly traceable to SI units. The first time we have calculated the effective area and associated uncertainty of p-c assembly using dimension and mass metrology, traceability to the SI units, i.e., meter and kilogram. To realize the APG as primary pressure standards, we have calculated the effective area of p-c assembly of APG directly from dimension metrology, which is further supported by various other methods. The effective area values obtained in the pressure range of 6.5 – 360 kPa lie in the range of 3.356729 – 3.357248 cm² due to uncertainty limitation in the measurement of dimension of internal diameter of cylinder. The expected values of the effective area which are also measured from cross-float technique against ultrasonic interferometer manometer (UIM), primary pressure standards. The accuracy in effective area measurement is possible only when the resolution in the internal radius of the cylinder should at least be up to 5th decimal order and the uncertainty is 80 nm. The expanded uncertainty was measured nearly 11 ppm at <em>k</em> = 2 by considering the uncertainty in internal radii of cylinder and radii of piston around 80 nm.