Three and a Half Centuries Later - The Modern Art of Liquid-Column Manometry

After three and a half centuries the Torricellian mercury manometer remains the most accurate pressure standard. State-of-the-art manometers achieve parts-per-million total uncertainties near atmospheric pressures and imprecisions as low as 0,01 Pa. The total uncertainty is determined primarily by uncertainties in the measurement of the height of the mercury surfaces and the average mercury density. The latter is limited generally by the uncertainty of the average temperature of the mercury. The techniques used to locate the mercury surfaces and determine their heights not only determine the imprecision and accuracy of the height measurement, but can also have a significant effect on the maintenance of a stable and uniform mercury temperature. This review discusses the factors important in high-accuracy manometers with particular emphasis on surface detection and height measurement techniques. Specifically discussed are capacitance detection, white-light fringes, optical interferometry, with and without floats, and ultrasonic measurements.