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A vacuum gauge is used to measure pressures lower than the ambient atmospheric pressure, which is set as the zero point, in negative values (for instance, −1 bar or −760 mmHg equals total vacuum). Most gauges measure pressure relative to atmospheric pressure as the zero point, so this form of reading is simply referred to as "gauge pressure".
The Pirani gauge consists of a metal sensor wire (usually gold plated tungsten or platinum) suspended in a tube which is connected to the system whose vacuum is to be measured. The wire is usually coiled to make the gauge more compact. The connection is usually made either by a ground glass joint or a flanged metal connector, sealed with an o ...
A glass McLeod gauge, drained of mercury. A McLeod gauge is a scientific instrument used to measure very low pressures, down to 10 −6 Torr (0.133 mPa). It was invented in 1874 by Herbert McLeod (1841–1923). [1] McLeod gauges were once commonly found attached to equipment that operates under vacuum, such as a lyophilizer. Today, however ...
Further, the vacuum in the gauge eventually deteriorates due to slow diffusion of gases through the mercury, making the device inaccurate. [8] In 1938, Adolph Zimmerli (1886–1967) [9] invented a gauge that overcame the filling problems, at least for pressures below ambient pressure. [10] Zimmerli's gauge consists of three relatively wide columns.
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However, the inch of mercury is still used today in car performance modification to measure the amount of vacuum or pressure within the engine's intake manifold. This can be seen on "boost gauges (forced induction) or vacuum gauges (natural induction), which give a rough indication of the relative power being produced at any given time.