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

In-Situ Liquid Density Measurement for Liquid Column Manometry

“…The solid density standard (Figure 1), created for the use in the interferometric liquid column manometer, is a nominally 100 cm³ hollow sphere made from Ti6Al4V -a widely used and well machinable alpha-beta phase titanium alloyoffering a low volumetric thermal expansion coefficient and low density of around 4430 kg/m³. To build a solid density standard of around 830 kg/m³ -the density value of the Edwards 45 liquid used inside the LCM [2] the standard was brought in a thin-walled hollow spherical shape, where all the mass is located next to the outer circumference of the spherical body.…”

“…Against that, vacuum oils have low liquid densities -which increases the sensitivity of the pressure measurement by a factor of 10 or more, and a low vapour pressure -which makes the measurement of very small absolute pressures possible. But however, they suffer from high thermal volumetric expansion coefficients, gas absorption [2], [6] and possible long-term instability of their density. All these effects would be considered if the liquid density would be known, which means continuously measured.…”

Progress in development of an interferometric oil manometer

“…Therefore, in the PTB approach, vacuum oils were considered as the best candidates as a manometric liquid inside the new interferometric oil manometer. Based on the results of the study of their properties [11], an Alpha-olefin, namely Edwards 45, usually used for vacuum pumps, was chosen. When being used in the manometer, a thermal control by a thermostatic bath is needed due to the oil's thermal expansion coefficient of around 0.7 ppm/mK (1 ppm ≙ 10 -6 ).…”

“…In order to achieve a relative uncertainty of a few ppm for the liquid density, in spite of the effect of the gas absorption and other instability effects, the idea came up to measure the density, for the first time in a liquid column manometer, in-situ, using the temperature-of-flotation method [11].…”

“…The practical realisation of this method is possible because, in the case of the chosen vacuum oil, the volumetric thermal expansion coefficient of the oil [11] is by about factor 29 higher than that of the solid density standard, so that the oil density reacts much more sensitively to temperature changes.…”