Compression vacuum gauges according to McLeod

According to the type of scale division, a distinction is made between two forms of compression vacuum gauges those with a linear scale (see Fig. 3.7) and those with a square-law scale (see Fig. 3.8). In the case of the compression vacuum gauges of the McLeod linear-scale type, the ratio of the enclosed residual volume Vc to the total volume V must be knovm for each height of the mercury level in the measurement capillary this ratio is shown on the scale provided with the instrument. In the case of compression vacuum gauges with a square-law scale, the total volume and the capillary diameter d must be known. 

Nowadays a shortened McLeod type compression vacuum gauge according to Kammerer is used to measure the partial final pressure of mechanically compressing pumps. Through the high degree of 

Principle of measurement with compression vacuum gauges 

If h is the difference in the mercury level between the measurement capillary and the reference capillary (measured in mm), then it follows from the Boyle-Mariotte law  

These relationships remain unchanged if the difference in level is read off a scale with mbar division. The pressure is then obtained in mbar  

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