McLeod gauge: Investigations of capillary depression phenomena and mercury vapor drag effect

This paper describes the accurate determination of pressure in the range 10−3–10−6 Torr (∠10−1–10−4 Pa) by a bakeable McLeod gauge using variable compression technique. A thorough investigation of two major aspects of pressure determination by a McLeod gauge, i.e., capillary depression phenomena and mercury vapor stream effect, has been made. The capillary depression phenomena in the wide bore reference arm tube has also been taken into account in determining the capillary depression correction in the compression capillary. The minimization of the error due to the vapor stream effect of mercury has been studied by cooling the cutoff point in the McLeod gauge at different temperatures, i.e., 0°, −15°, and −78 °C. The pressures thus determined are found to be accurate to ±2% or better.