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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".
It is also given as inches of water gauge (iwg or in.w.g.), inches water column (inch wc, in. WC, " wc, etc. or just wc or WC), inAq, Aq, or inH 2 O. The units are conventionally used for measurement of certain pressure differentials such as small pressure differences across an orifice, or in a pipeline or shaft, [ 1 ] or before and after a ...
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Thirdly, they make gauges with scales that read in many different pressure units (the article only mentions two). Fourthly, I believe there are other manufacturers who make very similar gauges but don't use the trademarked Magnehelic brand name. Alex Holden 08:36, 24 March 2019 (UTC) I'm tagging this article as needing an expert.
1906 — Marcello Pirani – Pirani gauge (to measure pressures in vacuum systems) 1915 — J.C. Stevens — Chart recorder (first chart recorder for environmental monitoring) 1924 — Irving Langmuir — Langmuir probe (to measure plasma parameters) 1930 — Samuel Ruben invented the thermistor
The gauge may be used for pressures between 0.5 Torr to 1×10 −4 Torr. Below 5×10 −4 Torr, a Pirani gauge has only one significant digit of resolution. The thermal conductivity and heat capacity of the gas affects the readout from the meter, and therefore the apparatus may need calibrating before accurate readings are obtainable.
He also recommends having both feet on the ground and yours eyes closed during the reading. Regarding ideal numbers, the famous M.D. says: "Your blood pressure is supposed to be under 140 over 90 ...
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.