Transient heating in fixed length optical cavities for use as temperature and pressure standards

Abstract

Optical refractometry techniques enable realization of both pressure and temperature directly from properties of the gas. The NIST refractometer, a fixed length optical cavity (FLOC) has previously been evaluated for operation as pressure standard, and now in this paper, is evaluated for the feasibility of operation as a primary temperature standard as well. The challenge is that during operation, one cavity is filled with gas. Gas dynamics predicts that this will result in heating which in turn will affect the cavity temperature uniformity, impeding the ability to measure the gas temperature with sufficient accuracy to make the standard useful as a primary standard for temperature or pressure. Temperature uniformity across the refractometer must be less than 0.5 mK for measurements of the refractivity to be sufficiently accurate for the FLOC. This paper compares computer modeling to laboratory measurements, enabling us to validate the model to predict thermal behavior and to accurately determine the measurement uncertainty of the technique. The results presented in this paper show that temperature of the glass elements of the refractometer and ‘thermal-shell’ copper chamber are equivalent to within 0.5 mK after an equilibration time of 3000 s (when going from 1 kPa to 100 kPa). This finding enables measurements of the copper chamber to determine the gas temperature to within an uncertainty (k = 1) of 0.5 mK. Additionally, the NIST refractometer is evaluated for feasibility of operation as temperature standard.

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

“…The corresponding numbers when 200 s long cycles are employed were found to be even lower, 0.4 mK and 1.3 ppm, respectively. This indicates that the system is significantly less affected by pV-work than other systems, e.g., the NIST-FLOC system [6].…”

“…Until recently, the best characterized system was the FLOC system at NIST [6]. It was found that that system is adequate for assessment of pressure under the condition that it is given enough time to equilibrate after a gas filling (in the order of 3000 s to reach an uncertainty level of 0.5 mK).…”

“…Regarding the second concept, due to the small dimensions of the system, the gas volume of which is less than 5 cm 3 , when 100 kPa of gas are addressed, solely < 0.5 J of energy is let into the system. This is only 3% of the energy that is let into the system in the NIST FLOC system (172 cm 3 of gas, representing 17 J of energy) [6]. Moreover, since Invar has a three times larger volumetric heat capacity than glass (in fact, it has a more than three orders of magnitude larger volumetric heat capacity than the gas), it was concluded that the initial heating of the cavity walls will be at least two orders of magnitude smaller than that of the FLOC system.…”

Emerging technologies in the field of thermometry

“…Relative primary measurements, that use non-helium working gases, have been demonstrated with some trade-offs to accuracy [59]. Recently, Ricker et al [63] evaluated the potential feasibility of the fixed length optical cavity (FLOC) approach for thermometry. The FLOC approach, pioneered by the National Institute of Standards and Technology (NIST), USA is based upon a dual-cavity Fabry-Perot refractometer (see figure 7) that has been used as a pressure transfer standard, traceable to a mercury manometer [60] and was recently commercialized by MKS Instruments [64].…”

Analysis of the Outgassing in an Optical Pressure Standard

“…However, outgassing can adversely affect the gas purity. The OPS is typically designed with a small gas-filling volume, which allows to reduce the pV-work and shortens the time needed for thermal equilibrium [9], [10]. The outgassing effect is more prominent in a small volume and limits the low-pressure performance of the OPS.…”