A different way of measuring pressure
New method for top-notch capacitive pressure measurement
PTBnews 1.2020
25.02.2020
Left: Conventional pressure measurement with a pressure balance according to pPB = Fg/Aeff (PB: pressure balance; g: gravitational force; Aeff: effective surface of a piston/cylinder system). Right: The new electrical approach: the relative change in capacitance C(T) caused by the measuring gas at a known temperature T, which is determined by means of a calibrated resistance thermometer R(T), can be directly linked to the gas pressure. The dielectric constant and the interaction of the gas particles enter into the required ab initio calculations: pab-initio(C,T, Gasab-initio)
Pressure is the result of a force acting vertically onto a surface. Today, the most accurate pressure measurements are still working according to this principle, where the gas pressure under a piston of a certain area is adjusted via the mass stack i.e. via the weight force. At the same time, it has long been endeavored to develop further high-precision methods. As early as 1998, Mike Moldover of the US American metrology institute NIST had voiced his idea of measuring pressure via an electrical (capacitance) measurement using theoretical calculations of the gas properties of helium. In the following years, however, implementing this groundbreaking thought would prove to be a real challenge. Both the precision capacitance measurement and the highly stable capacitors needed for this purpose, as well as the theoretical calculations using solely natural constants (ab initio calculations) were not yet possible with the required accuracy. Moreover, there was no accurate possibility to compare them with conventional pressure balances.
Each of the experimental obstacles has been removed at PTB over the last decade. Due to activities carried out within the scope of the new definition of the base unit kelvin, conventional pressure measurements both with pressure balances and via capacitance measurements were raised to an unprecedented level worldwide. Thanks to the latest theoretical calculations achieved by diverse research groups across the globe, it has now become possible to measure a pressure of 7 million pascals (i.e. 70 times normal pressure) with a relative uncertainty of less than 5 · 10–6. This uncertainty has been confirmed by comparison with a conventional pressure balance. It was the first comparison on an equal footing between mechanical and electrical pressure measurements.
Thus, a second method is now available to calibrate pressure with high accuracy. The method itself and the direct comparison with the conventional pressure standard offer, for one thing, the possibility to verify theoretical calculations of helium – an important model system in atomic physics. For another, they also allow other gases to be measured and thus, both theory and gas metrology to be further developed.
Contact
Christof Gaiser
Department 7.4
Temperature
Phone: +49 30 3481-7349
christof.gaiser@ptb.de
Scientific publication
C. Gaiser, B. Fellmuth, W. Sabuga: Primary gas-pressure standard from electrical measurements and thermophysical ab initio calculations. Nature Physics 16, 177–180 (2019)