Editing Pressure measurement (section)

====McLeod gauge====
[[File:McLeod gauge.jpg|thumb|upright|A McLeod gauge, drained of mercury]]

A [[McLeod gauge]] isolates a sample of gas and compresses it in a modified mercury manometer until the pressure is a few [[Millimeter of mercury|millimetres of mercury]]. The technique is very slow and unsuited to continual monitoring, but is capable of good accuracy. Unlike other manometer gauges, the McLeod gauge reading is dependent on the composition of the gas, since the interpretation relies on the sample compressing as an [[ideal gas]]. Due to the compression process, the McLeod gauge completely ignores partial pressures from non-ideal vapors that condense, such as pump oils, mercury, and even water if compressed enough.

{{block indent | em = 1.5 | text =  '''Useful range''': from around 10<sup>−4</sup>&nbsp;Torr<ref>{{cite web |url=http://www.tau.ac.il/~phchlab/experiments/vacuum/Techniques_of_high_vacuum/Vacuum5.html |title=Techniques of High Vacuum |archive-url=https://web.archive.org/web/20060504124236/http://www.tau.ac.il/~phchlab/experiments/vacuum/Techniques_of_high_vacuum/Vacuum5.html |website=[[Tel Aviv University]] |date=2006-05-04 |archive-date=2006-05-04}}</ref>  (roughly 10<sup>−2</sup>&nbsp;Pa) to vacuums as high as 10<sup>−6</sup>&nbsp;Torr (0.1&nbsp;mPa),}}

0.1&nbsp;mPa is the lowest direct measurement of pressure that is possible with current technology. Other vacuum gauges can measure lower pressures, but only indirectly by measurement of other pressure-dependent properties. These indirect measurements must be calibrated to SI units by a direct measurement, most commonly a McLeod gauge.<ref>{{Cite book | first1=Thomas G. | last1=Beckwith | first2=Roy D. | last2=Marangoni | first3=John H. | last3=Lienhard V | name-list-style=amp | date=1993 | title=Mechanical Measurements | edition=Fifth | publisher=[[Addison-Wesley]] | location=Reading, MA | isbn=0-201-56947-7 | pages=591–595 | chapter=Measurement of Low Pressures }}</ref>