The Liquid Gauge Family

The liquid gauge family is identified simply as vacuum gauges that have some liquid (usually mercury or a low-vapor-pressure diffusion pump oil) directly in contact with the vacuum. The amount of liquid movement is directly proportional to the force exerted on it, and the (measured) amount of movement is read as the vacuum. Because mercury has traditionally been used for vacuum measurement, the term millimeters of mercury is commonly used even with nonliquid gauges. 

Low-vapor-pressure oils can be substituted for some operations, but calibrations for density must be made so that their measurements can be interpreted as millimeters of mercury, torr, or Pa. Unfortunately, vacuum measurements can take a considerable amount of time when using oil because it takes a long time for a film of oil to settle from the walls of a manometer. 

On the other hand, a liquid trap is essential between any liquid vacuum gauge and the manifold itself. The simplest accident can cause hours of needless delay as the vacuum line is cleaned out. Normally a simple splash trap is sufficient (see Sec. 7.4.6). Vacuum systems that go below 1CT5 torr should consider to have a cold trap placed between their liquid vacuum gauges and vacuum systems. This placement will keep vapors from contaminating either side of the system. 

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