A McLeod manometer with prescribed volumes for use as a standard instrument

Abstract

The McLeod manometer is one of the few instruments with which it is possible to measure the actual values of low gas pressures (5–10−5mm Hg) without changing the composition of the gas. The McLeod gauge is therefore indispensable for calibrating the faster indicating instruments which only register relative pressure units, e.g. ionization gauges, heat-conductivity manometers, etc.Since it is very cumbersome to make and calibrate a McLeod gauge that can be used as a standard, it is worthwhile to improve the construction in such a way that a calibration can be omitted. In connexion with this problem a critical study was made of the formulae used to calculate the gas pressure from the readings of the McLeod manometer, which lead to expressions for the relative systematic and accidental errors limiting the measuring accuracy.By using a very accurate set of jigs in making the capillaries and the transition volumes at the connexion between capillaries of different diameters and by addition of a small correction volume to the compression bulb it was possible to construct a McLeod gauge indicating the actual gas pressures in the range of 3.5–10−5mm Hg on one convenient set of scales. The volume of the compression bulb and the volume and diameters of the capillaries were calculated in such a way that full centimetres on the various scales correspond to 10−3, 10−2 and 10−1mm Hg respectively. The zero lines of the different sub-ranges of the instrument lie at whole centimetres from each other and at multiples of half-centimetres from the top of the compression capillary.