Optical realization of the pascal—Characterization of two gas modulated refractometers

Abstract

By measuring the refractivity and the temperature of a gas, its pressure can be calculated from fundamental principles. The most sensitive instruments are currently based on Fabry–Perot cavities where a laser is used to probe the frequency of a cavity mode. However, for best accuracy, the realization of such systems requires exceptional mechanical stability. Gas modulation refractometry (GAMOR) has previously demonstrated an impressive ability to mitigate the influence of fluctuations and drifts whereby it can provide high-precision (sub-ppm, i.e., sub-parts-per-million or sub-10−6) assessment of gas refractivity and pressure. In this work, two independent GAMOR-based refractometers are individually characterized, compared to each other, and finally compared to a calibrated dead weight piston gauge with respect to their abilities to assess pressure in the 4–25 kPa range. The first system, referred to as the stationary optical pascal (SOP), uses a miniature fixed point gallium cell to measure the temperature. The second system, denoted the transportable optical pascal (TOP), relies on calibrated Pt-100 sensors. The expanded uncertainty for assessment of pressure (k=2) was estimated to, for the SOP and TOP, [(10mPa)2+(10×10−6P)2]1/2 and [(16mPa)2+(28×10−6P)2]1/2, respectively. While the uncertainty of the SOP is mainly limited by the uncertainty in the molar polarizability of nitrogen (8 ppm), the uncertainty of the TOP is dominated by the temperature assessment (26 ppm). To verify the long-term stability, the systems were compared to each other over a period of 5 months. It was found that all measurements fell within the estimated expanded uncertainty (k=2) for comparative measurements (27 ppm). This verified that the estimated error budget for the uncorrelated errors holds over this extensive period of time.

Thermodynamic Effects in a Gas Modulated Invar-Based Dual Fabry-Pérot Cavity Refractometer Addressing 100 Kpa of Nitrogen

“…Since the instrumentation scrutinized in this work has been assessed to have an extended uncertainty of − + [3], this scrutiny, which has concluded that the influence of thermodynamic effects on the assessment of pressure when 100 kPa of nitrogen is addressed is in the sub-ppm level, has indicated that, despite the use of gas modulation cycle times of 200 s, as can be used when the GAMOR method is utilized, the Invar-based DFPC system is not significantly affected by thermodynamic effects related to the gas exchange process. Hence, this work further strengthens the conclusion that thermodynamic effects (pV-work) will not be a limiting factor when the Invar-based DFPC GAMOR system is used for assessments of pressure or as a primary pressure standard for pressures up to 100 kPa.…”

‘Quantum-Based Realizations of the Pascal’: Status and Progress of the Empir-Project: Quantumpascal

“…A close scrutiny of the Invar-based system has revealed that the system is capable of assessing pressure with an extended uncertainty (k = 2) of [(10 mPa) 2 + (10 ×10 −6 P) 2 ] 1/2 [18], which is in pair with the so far, most accurate FP-refractometry system developed by NIST.…”

Circular Comparison of Conventional Pressure Standards Using a Transportable Optical Refractometer: Preparation and Transportation

“…The refractometry system, denoted the Transportable Optical Pascal (TOP), has previously been described in detail [3], [4]. Besides a brief IMEKO 24 th TC3, 14 th TC5, 6 th TC16 and 5 th TC22 International Conference 11 -13 October 2022, Cavtat-Dubrovnik, Croatia 2 of 7…”

“…The system differs from a previously constructed stationary system (SOP) [3], [5], [7] primarily by the way the temperature is assessed; instead of assessing the temperature with respect to a Ga fixed point cell, it utilizes calibrated Pt-100 sensors. This contributes to its uncertainty by 20 parts-permillion, which is on par with the uncertainties of pressure balances used in the circular comparison.…”

“…Although the refractometer in principle can operate as a primary standard and has, in an earlier work [3], been evaluated in terms of uncertainty, it was in this work operated as a transportable standard. The main reason is that this could significantly reduce the complexity of operation.…”