Editing Barometer (section)

==Equation==
When atmospheric pressure is measured by a barometer, the pressure is also referred to as the "barometric pressure". Assume a barometer with a cross-sectional area ''A'', a height ''h'', filled with mercury from the bottom at Point B to the top at Point C. The pressure at the bottom of the barometer, Point B, is equal to the atmospheric pressure. The pressure at the very top, Point C, can be taken as zero because there is only mercury vapour above this point and its pressure is very low relative to the atmospheric pressure. Therefore, one can find the atmospheric pressure using the barometer and this equation:<ref>Cengal, Yunus A. and Boles, Michael A. (2014) ''Thermodynamics: An Engineering Approach''. McGraw-Hill Education. {{ISBN|978-0073398174}}</ref>{{clarify|this equation cannot work across the entire range of pressures shown in the table - needs clarification|date=June 2020}}

:P<sub>atm</sub> = ρgh

where ρ is the density of mercury, g is the gravitational acceleration, and h is the height of the mercury column above the free surface area. The physical dimensions (length of tube and cross-sectional area of the tube) of the barometer itself have no effect on the height of the fluid column in the tube.

In thermodynamic calculations, a commonly used pressure unit is the "standard atmosphere". This is the pressure resulting from a column of mercury of 760&nbsp;mm in height at 0&nbsp;°C. For the density of mercury, use ρ<sub>Hg</sub> = 13,595&nbsp;kg/m<sup>3</sup> and for gravitational acceleration use g = 9.807&nbsp;m/s<sup>2</sup>.

If water were used (instead of mercury) to meet the standard atmospheric pressure, a water column of roughly 10.3&nbsp;m (33.8&nbsp;ft) would be needed.

Standard [[atmospheric pressure]] as a function of elevation:

Note: 1 [[torr]] = 133.3 Pa = 0.03937 [[inch of mercury|inHg]]

{| class="wikitable" style=text-align:right;
|-
! P<sub>atm</sub> / kPa !! Altitude <br /> (m) !! !! P<sub>atm</sub> / inHg!! Altitude <br /> (ft)
|-
| 101.325 || (Sea Level) 0 || || 29.92  || (Sea Level) 0
|-
| 97.71 || 305 || || 28.86  || '''1,000'''
|-
|94.21 || 610 || ||27.82  || '''2,000'''
|-
| 89.88 || '''1,000''' || || 26.55 || 3,281
|-
| 84.31 || 1,524 || || 24.90  || '''5,000'''
|-
| 79.50 || '''2,000''' || || 23.48  || 6,562
|-
| 69.68 || 3,048 || || 20.58  || '''10,000'''
|-
| 54.05 || '''5,000''' || || 15.96|| 16,404
|-
| 46.56 || 6,096 || || 13.75  || '''20,000'''
|-
| 37.65 ||  7,620 || || 11.12  || '''25,000'''
|-
| 32.77 ||  8,848 ||* || 9.68  || 29,029*
|-
| 26.44 || '''10,000''' || || 7.81  || 32,808
|-
| 11.65 || 15,240 || || 3.44  || '''50,000'''
|-
| 5.53 || '''20,000''' || || 1.63  || 65,617
|-
|colspan=5 | <nowiki>*</nowiki> Elevation of [[Mount Everest]], the highest point on earth
|}