The conventional McLeod gauge has been modified and range 0.001-10mm H.G. resulting as an accuracy of ± 5% at scale point
The McLeod gauge consists of a bulb and capillary tube where their volume of mercury V1 is at atmospheric pressure P1. When vacuum is applied both are evacuated and when the gauge is rotated, this volume is compressed to a small portion in the capillary having volume V2 at corresponding pressure (i.e. vacuum) P2boyle's Law P1v1 = P2v2. The pressure is read by comparing the compressed level of mercury with the corresponding graduated scale parallel to the capillary tube.
