Oils, Greases, and High Vacua

Abstract

IN the course of some work (which I hope shortly to publish) on the evaporative distillation of petroleum derivatives, I became aware of the possibility and advantages of using oil in place of mercury as working fluid in condensation pumps. I was distilling lubricating oil in an apparatus similar in principle to that used by Bronsted and Hevesy to separate the isotopes of mercury. The saturation pressure of the oil vapour could be deduced from the observed rate of distillation and the estimated molecular weight of the oil: in a particular case the saturation pressure was about one dyne/cm.2 at 118° C., that is, about the same as the saturation pressure of mercury at room temperature. No decomposition could be detected. Clearly, if this oil could be heated until its vapour pressure was, say, 100 dynes/cm.2, without decomposition, it could be used as working fluid in a condensation pump and might be expected to give a performance, without artificial cooling, comparable with the performance of a mercury condensation pump with a cold trap 100° C. below room temperature. I therefore prepared by fractionation a quantity of this oil and evacuated ionisation gauges (large and small thermionic valves), on oil condensation pumps. I have been unable to measure the lower limit of pressure reached by these pumps. 10-3 dynes/cm.2 has been reached without ovening the glasswork: when the glass was ovened, the ionisation current could not be detected with the instruments available—the pressure probably did not exceed 10-4 dynes/cm.2.