How to Read a McLeod Gauge

There are two different methods of reading a McLeod gauge One uses a linear scale, whereas the other uses a square scale. Because the square scale is primarily used, I will limit my explanation to its use. Refer to Fig. 7.42. 

A liquid or cold trap should be A trap placed between the McLeod 

Once the mercury in Capillary A is the same height as the inside top of Capillary B, reading the pressure is done by simply drawing a visual line from h2 (the top of the mercury in Capillary B) over to the reading scale and reading your vacuum. In Fig. 7.42, the pressure is 0.04 pHg, 4 x 10 5 mm Hg, or 4 x 10 5 torr. 

After reading the pressure, rotate the plug of the three-way stopcock 180° to the 2 position to draw the mercury back into the storage bulb. Once the mercury is back in the storage bulb, turn the three-way stopcock 90° to a closed position. Once a vacuum reading has been made, turn the two-way stopcock connecting the McLeod gauge and the vacuum system 90° to a closed position. 

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