Standard Unit of Pressure in Vacuum Physics


IN order to obtain a unit of pressure in terms of the height of a column in a liquid manometer, a unit which is also related decimally to absolute units of pressure, Dr. Florescu proposes to elevate the pressure due to a 0.75-mm. column of liquid of density 13.5951 gm./cm.3, g being 980.7455 cm./sec.2, to the status of a new unit, and call it the ‘vac’. But as Dr. Florescu admitted oPiginally, this pressure is effectively 103 dynes/cm.2, called internationally the millibar, and for the practical purposes of vacuum technology the pressure due to ¾ mm of a standard mercury column (density 13.5951 gm./cm.3, standard gravity 980.665 cm./sec.2) is one millibar (more closely it is 0.99992 mb.). Quite apart, therefore, from Dr. Volet's point, with which I entirely agree, I see no justification for introducing a new unit, the ‘vac’, the definition of which involves a new, arbitrary reference gravity, when the scale it would provide would in vacuum practice be indistinguishable from a scale of millibars.

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BIGG, P. Standard Unit of Pressure in Vacuum Physics. Nature 190, 523 (1961).

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