Gas equilibration gas modulation refractometry for assessment of pressure with sub-ppm precision

Abstract

Gas Modulation Refractometry (GAMOR) has recently been developed to mitigate drifts in the length of the cavities in Dual-Fabry-Perot Cavity (DFPC) based refractometry. By performing repeated reference assessments with the measurement cavity being evacuated while the reference cavity is held at a constant pressure, the methodology can reduce the influence of the long-term drifts, allowing it to benefit from the high precision of DFPCbased refractometry at short time scales. A novel realization of GAMOR, referred to as Gas Equilibration GAMOR (GEq-GAMOR), that outperforms the original realization of GAMOR, here referred to as Single Cavity Modulated GAMOR (SCM-GAMOR), is presented. It is based upon the fact that the reference measurements are carried out by equalizing the pressures in the two cavities. By this, the time it takes to reach adequate 2 conditions for the reference measurements has been reduced. This implies that a larger fraction of the measurement cycle can be devoted to data acquisition, which reduces white noise and improves on its short-term characteristics. The presented realization also encompasses a new cavity design with improved temperature stabilization and assessment. This has contributed to an improved long-term characteristics of the GAMOR methodology. The system was characterized with respect to a dead weight piston gauge. It was found that for short integration times (up to 10 min) it can provide a response that exceeds that of the original SCM-GAMOR system by a factor of two. For integration times longer than this, and up to 18 hours, the system shows, for a pressure of 4303 Pa, an integration time independent Allan deviation of 1 mPa (corresponding to a precision, defined as twice the Allan deviation, of 0.5 ppm). This implies that the novel system shows a significant improvement with respect to the original realization of GAMOR for all integration times (by a factor of 8 for an integration time of 18 hours).When used for low pressures, it can provide a precision in the sub-mPa region; for the case with an evacuated measurement cavity, the system provided, up to ca. 40 measurement cycles (ca. 1.5 hours), a white-noise limited noise of 0.7 mPa (cycle) 1/2 , and minimum Allan deviation of 0.15 mPa. Furthermore, over the pressure range investigated, i.e. in the 2.8 -10.1 kPa range, it shows, with respect to a dead weight piston gauge, a purely linear response. This implies that the system can be used for transfer of calibration over large pressure ranges with exceptional low uncertainty.3

An invar-based fabry-perot cavity refractometer with a gallium fixed-point cell for assessment of pressure

“…It has previously been shown that for a DFPC refractometer the refractivity can be expressed as [4,6] 1 1 1 q , of the mode addressed in the empty measurement cavity when gas is let into the cavity.  is a normalized cavity deformation, defined as…”

“…To reduce temperature overhearing, its reference junctions at the cavity are wound around the spacer a multitude of turns. Figure 4 provides a comparison between the Invar-based system and a similar system based on a Zerodur cavity [4]. Before the Ga fixed-point cell was implemented, for each cavity system, two measurement series were taken at dissimilar pressures; one when the system was evacuated, indicated in the form of Allan deviations in Fig.…”

“…assess cavity temperature; the Pt-100 sensors and the thermocouple system To demonstrate the performance of the temperature regulation and assessment system, a series of measurements was performed at a pressure of 11 620 Pa (produced by a dead weight pressure balance) over 4 Assuming an error of 10 mK in the assessed block temperature. Correction formula 0.001 x ( Block Ga TT − ).…”

“…The refractometry system and the piston gauge was exposed to 3600 gas filling-and-empting cycles according to the normal procedures of the GAMOR methodology [3,4,6]. Each cycle lasted 100 s while the entire measurement campaign took 100 hours.…”

“…We have previously developed an innovative method for refractometry, termed gas-modulation refractometry (GAMOR), that allows for a significant reduction of the influence of drifts and fluctuations of the system [3][4][5]. This has opened up for the use of non-conventional cavity-spacer materials that otherwise were considered unsuitable due to a high thermal expansion.…”