Because pressure is commonly measured by its ability to displace a column of liquid
in a manometer, pressures are often expressed as a depth of a particular fluid (e.g., ce
ntimetres of water, millimetres of mercury or inches of mercury). The most common choi
ces are mercury (Hg) and water; water is nontoxic and readily available, while mercury
's high density allows a shorter column (and so a smaller manometer) to be used to measu
re a given pressure. The pressure exerted by a column of liquid of height h
and density ρ is given by the hydrostatic pressure equation p = ρgh, where g is t
he gravitational acceleration. Fluid density and local gravity can vary from one reading
to another depending on local factors, so the height of a fluid column does
not define pressure precisely. When millimetres of mercury or inches of mercury are quoted
today, these units are not based on a physical column of mercury; rather, they
have been
given precise definitions that can be expressed in terms of SI un
its.[citation needed] One millimetre of mercury is approximately equal to one t
orr. The water-based units still depend on the density of water, a measured, rather than de
fined, quantity. These manometric units are stil
l encountered in many fields. Blood pressureis measured in millimetres of mercury
in most of the world, and lung pressures in centimetres of water are still common.
Underwater divers use the metre sea water (msw or MSW) and foot sea water (fsw or FSW) uni
ts of pressure, and these are the standard units for pressure gauges used to measure pr
essure exposure in diving chambersand personal decompression computers.
A msw is defined as 0.1 bar, and is not the same as a li
near metre of depth, and 33.066 fsw = 1 atm.[4] Note
that the pressure conversion from msw to fsw is different from the length conversion
: 10 msw = 32.6336 fsw, while 10 m = 32.8083 ft
Gauge pressure is often given in units with 'g' appended
, e.g. 'kPag', 'barg' or 'psig', and units for measurements of abs
olute pressure are sometimes given a suffix of 'a', to avoid confusion, for exam
ple 'kPaa', 'psia'. However, the US National Institute of Standards and Technology recomm
ends that, to avoid confusion, any modifiers be instead applied to the quantity bei
ng measured rather than the unit of measure[5] For example, "pg = 100 p
si" rather than "p = 100 psig".
Differential pressure is expressed in units with 'd' ap
pended; this ty
pe of measurement is useful when considering sealing performance or w
hether a valve will open or close.
Presently or formerly popular pressure units include the following: