According to the results of key comparison CCM.P-K4 (2002) interferometric mercury U-tube manometers are the most accurate primary standards for low absolute pressure in the range 1 – 1000 Pa. In spite of well-known advantages of using oil as a manometric fluid (low density and vapour pressure), oil manometers do not find the wide-spread application till now. This is explained by the absence of a simple solution of their main problem – the suppression of free oil surface fluctuations (similar to the cat’s-eye type of floats in mercury manometers) and also by difficulties in the precise determination of the oil density due to high air-solubility.
The laser interferometric U-tube oil manometer, which is being developed at the D.I. Mendeleyev Institute for Metrology, has to be a new national standard of Russia for low absolute pressures up to 1kPa. Surface waves in oil are effectively suppressed with specially designed floats supporting the thin near-surface layer in manometer tubes [1].
A new approach to the research of capillary uncertainty is proposed. It is based on interferometric measurements of the oil surface curvature in manometer tubes. Results of this original research show, that there is no need to increase the tube diameters up to more than 40 mm for reducing the capillary uncertainty, as it has been done before usually. So, the device can be made compact enough to reduce mechanical and temperature instabilities of the interferometer.
The idea of planned experiment is suggested to correlate the oil density with the dissolved air quantity. It will permit to apply corrections for dissolved air and compressibility and thus to reduce the density uncertainty up to about (1–3)∙10-5.
1. Sadkovskaya I.V., Eichwald A.I. "Special floats for interferometric oil manometer", Abstracts of 16th International Vacuum Congress (2004, Venice, Italy), Posters. P. 1012.
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