A primary liquid column manometer (LCM) is under development at PTB for use in conducting low-pressure measurements up to 2 kPa in gauge and absolute pressure. To calculate pressure, all measured input quantities of the instrument, as there are liquid density, gravitational acceleration, and length, are traceable to the International System of Units (SI), thus making the LCM a primary pressure standard. The LCM is well suited to identifying small force-induced errors, particularly those of force-compensated pressure balances with non-rotating piston in their lower measurement range, and as such to disseminating the pascal, the SI unit of pressure.
This report focuses on the measurement of the height difference inside the instrument, where homodyne plane mirror interferometry is applied using the liquid’s free surface as the reflecting mirror for the laser beam.

From basics of Fabry-Perot (FP) resonator and roundtrip phase, a complete working equation for a FP cavity based optical pressure standard (OPS) is derived and presented which includes corrections of reflection phase-shift, diffraction and pressure-induced distortion. The correction from diffraction, i.e. Gouy phase, is negligible. To operate an OPS as a primary standard, two unknown parameters, i.e. mirror dispersion coefficient ϵ_α and pressure distortion coefficient d_m, in the working equation should be determined independently. Methods to determine ϵ_α and d_m are described and applied to an OPS developed at the National Institute of Metrology (NIM), China. Thermodynamic effect observed in the determination of d_m is also discussed.

Recently, ‘a new realization of the Pascal’ by using photon technology showed comparable results in terms of performance compared to the existing primary standard based on a mercury manometer. In this study, we describe KRISS optical vacuum standard system uses a 633 nm He-Ne laser, specially made by KRISS, as a light source, and a double channeled Fabry-Perot (FP) cavity made from ZERODUR. Hardware construction for KRISS optical pressure standard system has been completed. Currently, as the first step, it is aimed at calculating the pressure according to the frequency in the pressure range of 1 Pa to 10 kPa using nitrogen gas. Using the measured beating frequency, we determined the internal pressure in the cavity considering the refractive index virial coefficients (AR, BR and CR) and density virial coefficients (Ap, Bp and Cp), the Boltzmann constant kB, and the thermodynamic temperature. The uncertainty is currently in evaluation taking into account the uncertainty factor.

A novel optical pressure standard, based on a multi-reflection interferometric technique, has been recently developed. It is based on the measurement of the refractive index of a gas through an unbalanced homodyne interferometer (UINT) and is able to measure pressure with a relative standard uncertainty of 10 ppm at 100 kPa [1]. In this work, the interferometer was used to measure the molar polarizability of nitrogen, which resulted in agreement with recent previous determinations, paving the way for using photonic pressure standards as accurate and fast transfer standards of the pascal.

The development of pressure controllers has seen a great focus and their technical characteristics have improved over the last few decades, specifically in terms of their metrological capabilities. This advancement now makes them a suitable candidate to measure negative gauge pressure, either directly or in conjunction with other devices such as pressure balances. The Druck PACE in conjunction with CM3 (control module) has demonstrated performance characteristics in line with those required to complete negative gauge pressure calibrations. Three different calibration methods were developed within the Druck Ltd laboratory, from which two of them used a 200 kPa absolute pressure CM3, containing TERPS© (Trench Etched Resonant Pressure Sensor). The unit under test was a PACE1000 indicator built with a piezoresistive sensors with a pressure range from -100 to 100 kPa. The expanded uncertainty was evaluated for each method, as well as identifying the advantages and shortcomings of each method. As the calibrations were performed for gauge mode (not differential), the calibration range was -950 to -50 hPa.