Editing Calibration (section)

===The early calibration of pressure instruments===
[[Image:Utube.PNG|thumb|left|upright|100px|Direct reading design of a U-tube manometer]]One of the earliest pressure measurement devices was the [[Evangelista Torricelli#Suction pumps and the invention of the barometer|Mercury barometer, credited to Torricelli]] (1643),<ref name=tilford1992pressure>{{cite journal|last1=Tilford|first1=Charles R|title=Pressure and vacuum measurements|journal=Physical Methods of Chemistry|date=1992|pages=106–173|url=http://www.glb.nist.gov/calibrations/upload/pmc-2.pdf|access-date=28 November 2014|archive-url=https://web.archive.org/web/20141205044516/http://www.glb.nist.gov/calibrations/upload/pmc-2.pdf|archive-date=2014-12-05|url-status=dead}}</ref> which read atmospheric pressure using [[Mercury (element)|Mercury]]. Soon after, water-filled [[manometer]]s were designed. All these would have linear calibrations using gravimetric principles, where the difference in levels was proportional to pressure. The normal units of measure would be the convenient inches of mercury or water.

In the direct reading hydrostatic manometer design on the right,  applied pressure P<sub>a</sub> pushes the liquid down the right side of the manometer U-tube, while a length scale next to the tube measures the difference of levels. The resulting height difference "H" is a direct measurement of the pressure or vacuum with respect to [[atmospheric pressure]]. In the absence of differential pressure both levels would be equal, and this would be used as the zero point. 
  
The [[Industrial Revolution]] saw the adoption of "indirect" pressure measuring devices, which were more practical than the manometer.<ref name="FridmanSabak2011">{{cite book |last1=Fridman |first1=A. E. |last2=Sabak |first2=Andrew |last3=Makinen |first3=Paul |date=23 November 2011 |title=The Quality of Measurements: A Metrological Reference |publisher=Springer Science & Business Media |isbn=978-1-4614-1478-0 |pages=10–11 |url=https://books.google.com/books?id=kyX-1VzPokQC&pg=PA111}}</ref>
An example is in high pressure (up to 50 psi) steam engines, where mercury was used to reduce the scale length to about 60 inches, but such a manometer was expensive and prone to damage.<ref name=cusco1998guide>{{cite book|last1=Cuscó|first1=Laurence|title=Guide to the Measurement of Pressure and Vacuum|date=1998|publisher=The Institute of Measurement and Control|location=London|isbn=0-904457-29-X|page=5}}</ref> This stimulated the development of indirect reading instruments, of which the [[Bourdon tube]] invented by [[Eugène Bourdon]] is a notable example. 
{{multiple image
| footer    = Indirect reading design showing a Bourdon tube from the front (left) and the rear (right).
| width     = 100
| image1    = WPGaugeFace.jpg
| alt1      = Indirect reading design showing a Bourdon tube from the front
| image2    = WPPressGaugeMech.jpg
| alt2      = Indirect reading design showing a Bourdon tube from the rear
}}

In the front and back views of a Bourdon gauge on the right, applied pressure at the bottom fitting reduces the curl on the flattened pipe proportionally to pressure. This moves the free end of the tube which is linked to the pointer. The instrument would be calibrated against a manometer, which would be the calibration standard. For measurement of indirect quantities of pressure per unit area, the calibration uncertainty would be dependent on the density of the manometer fluid, and the means of measuring the height difference. From this other units such as pounds per square inch could be inferred and marked on the scale.

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