a liquid with a low vapor pressure at room temperature; it is one of the vacuum materials. It is used chiefly as a pressure fluid in oil-diffusion pumps, as a sealant for mechanical pumps, as a lubricating material for the rubbing parts of vacuum devices, and for filling liquid vacuum meters. As a pressure fluid for oil-diffusion vacuum pumps, vacuum oil must have the lowest possible vapor pressure and the highest possible thermal stability at the operating temperature of the pump, as well as being chemically inert with respect to atmospheric oxygen and the gases being evacuated.
Vacuum oil is obtained by vacuum distillation of natural and synthetic fluids; a distinction is made among mineral, órgano silicon, and other oils according to their chemical composition. Mineral and organosilicon vacuum oils are the most widely used in vacuum technology. In the USSR, VM-1, VM-2, and VM-5 mineral vacuum oils are used in high-vacuum oil-diffusion pumps with a maximum vacuum, without cooled traps, not less than 2.7 × 10-4, 4.7 × 10-4, and 6.6 × 10-6 newtons per sq m (N/m2), or 2 × 10-6, 3.5 × 10-6, and 5 × 10-8 mm Hg, respectively; VM-4 and VM-6 are used in mechanical (preevacuation) pumps with vapor pressures of 5.3 × 10-3-1.3 × 10-4 N/m2 (4 × 10-5-1 × 10-6 mm Hg) and not higher than 1.3 × 10-4 N/m2 (1 × 10-6mm Hg) respectively; and G oil is used in oil-diffusion booster pumps with a limiting vacuum of 10-2 N/m2 (10-4 mm Hg). Mineral vacuum oils such as Wakuojl 220 (Poland), Apiezone B (England), and May van 20 (USA), are used abroad. In the USSR and abroad, the following organosilicon oils are used in high-vacuum oil-diffusion pumps: VKZh-94A and VKZh-94-B (USSR), mixtures of ethyl polysiloxanes with limiting vacuums not lower than 2.7 × 10-4 N/m2 (2 × 10-6 mm Hg) and 1.3 × 10-3 N/m2(1 × 10-5 mm Hg) respectively; the methylphenyl polysiloxane mixture PFMS-2/5L (USSR), with a limiting vacuum not lower than 2.7 × 10-4 N/m2 (2 × 10-6 mm Hg); and the silicones DS-702, DS-703, DS-704, and DS-705 (USA), with limiting vacuums trom 2.7 × 10-4 to 6.6 × 10-7 N/m2 (from 2 × 10-6 to 5 × 10-9 mm Hg). Other vacuum oils include polyphenyl ethers 5F4E (USSR) and Convalex 10 (USA), with a limiting vacuum from 2.7 × 10-7 to 8 × 10-7 N/m2 (from 2 × 10-9 to 6 × 10-9 mm Hg).
E. N. MARTINSON