Selection and characterization of liquids for a low pressure interferometric liquid column manometer

At PTB, the development of a new primary interferometric liquid column manometer for measurement of absolute and gauge pressures in the range up to 2 kPa is an ongoing project. For measurements in this pressure range, liquids with a low density are advantageous because of a better pressure resolution determined through differential column height measurements. In addition, low vapor pressure is a precondition for an accurate measurement of absolute pressure. The choice of liquid is crucial for the accuracy of the new instrument. This paper discusses the effect of liquid properties such as vapor pressure, density, viscosity, surface tension and gas absorption capacity on the performance of the micromanometer. Seven liquids - all oils used in vacuum applications (vacuum oils) - representing different classes of chemical compounds were analyzed in respect to their viscosity, density, thermal expansion and compressibility. Detailed investigations were performed on three liquids out of seven to determine their density versus gas saturation, ab- and desorption kinetics and surface tension, as well as the wettability of the micromanometer's materials. Results of the measurements and recommendations for the use of the liquids are presented.