pressure The result of the height differential in a liquid, either in the sense of the height to open surface from a submerged point (a reservoir outlet, for instance), or of the height of the surface facing a vacuum relative to a surface open to the atmosphere or other pressure, as in a barometer. The pressure of such a head depends on the liquid's inherent volumic mass, its temperature, and the gravitational acceleration that produces force from the head's mass; within such a context, the linear measure of the head (the height) provides the measure of its inherent pressure. For the reservoir, the head is controlled, the resulting force being available to drive turbines, etc. For the barometer, the head provides the balancing force to the extraneous pressure (of the atmosphere or such), hence a measure of it.
The traditional liquid in the barometer is mercury, for which the head is about 30 in or 760 mm for normal atmospheric pressure; standard conditions
[Gould F. A. Proc. Roy. Soc. London Ser. A Vol. 186, 195-200 (1946)] are a volumic mass of 13.595 1 × 103 kg m-3 at 0°C and the standard gravity of 9.806 65 m·s-2. Under such conditions, a head of 760 mm of mercury (29.921 26~ in) is defined as the standard atmosphere, identically a pressure of 101.325 kPa. The pressure is normally stated in terms of millimetres of mercury else inches of mercury, written also as mm of Hg or mmHg else in of Hg (Table 24). The mm of Hg, which is identically the torr, is the usual unit for blood-pressure readings. Water is an obvious alternative to mercury, but its markedly lesser volumic mass implies, for the same pressure range, an impractical column 13.6~ times as high as that for mercury. (The volumic mass of mercury is problematic to the level of precision shown here, probably because of variation in isotope proportions.
[Patterson J. B., Prowse D. B. Metrologia Vol. 21, 107-13 (1985)]
[Ambrose D. Metrologia Vol. 27, 233-47 (1990)])
| 1 mm of Hg | 1 in of Hg | 1 ft of H2O | |||
|---|---|---|---|---|---|
| 0.001 315 789 47~ | atmos | 0.033 421 052 6~ | atmos | 0.029 499 796 9~ | atmos |
| 1.333 223 68~ | mbar | 33.863 881 6~ | mbar | 29.890 669 3~ | mbar |
| 0.133 322 368~ | kPa | 3.386 388 16~ | kPa | 2.989 066 93~ | kPa |
| 1.0 | torr | 25.4 | torr | 22.419 845 69~ | torr |
| 0.039 370 087 0~ | in of Hg | 25.4 | mm of Hg | 22.419 845 7~ | mm of Hg |
| 0.044 603 340 0~ | ft of H2O | 1.132 925 | ft of H2O | 0.882 671 090~ | in of Hg |
| 0.019 336 774 7~ | lb-f/in2 | 0.491 154 077~ | lb-f/in2 | 0.433 527 504~ | lb-f/in2 |
Technically, one could have a head of gas, but this would lack a demarcatable surface and would have major variation of volumic mass. While a head of gas is of trivial importance as a tool, it is the essence of atmospheric pressure, which derives from the column of (progressively thinning) air above, reflected in actual pressure falling as elevation increases (see atmosphere for details). Atmospheric statements addressing surface conditions are usually expressed relative to nominal sea level, and resident barometers are usually adjusted correspondingly for their altitude.
