Pressure (symbol: p) is a measure of force per unit area.
The SI unit for pressure is the pascal (Pa), equal to one newton per square metre (N·m-2 or kg·s-2·m-1).
Non-SI measures (still in use in some parts of the world) include the pound-force per square inch (PSI) and the bar.
In the United States air pressure is still measured in inches of mercury (as in the mercury barometer). Some meteorologists prefer the hectopascal (hPa) for atmospheric air pressure, because it gives the same numbers as the older millibar (mbar).
''where p equals pressure, F equals force, and A equals area.
Pressure is sometimes measured not as an absolute pressure, but as the excess of that pressure above atmospheric pressure, sometimes called gauge pressure.
An example of this is the air pressure in a tire of a car, which might be said to be "thirty PSI", but is actually thirty PSI above atmospheric pressure.
In technical work, this would be written as "30 PSIG" or, more commonly, "30 psig".
"Pressure is a scalar quantity, but teachers and authors do
not appear to believe this in their hearts."
The standard atmosphere (atm) is a curious unit of pressure, defined to six figures of precision to approximate a reality that varies constantly from place to place and moment to moment. It is approximately equal to typical air pressures at sea level and defined to be
1 atm = 101 325 Pa = 101.325 kPa = 1013.25 hPa. k = kilo and h = hecto.
Obsolete manometric units of pressure such as inches or millimeters of mercury are based on the pressure exerted by the weight of some "standard" fluid under some "standard" gravity.
They are effectively attempts to define a unit for expressing the readings of a manometer.
Manometric pressure units should no longer be used for scientific or engineering purposes, due to the lack of repeatability inherent in their definitions.
Static pressure is the pressure due to the density and depth of a fluid. Stagnation pressure is the pressure due to the velocity of a fluid, and is defined to include static pressure. In addition, there can be differences in pressure due to differences in the elevation (height) of the fluid.
The pressure of a moving fluid can be measured using a Pitot probe, or one of its varations such as a Kiel probe or Cobra probe, connected to a manometer. Depending on where the inlet holes are located on the probe, it can measure static pressure or stagnation pressure.
The force density f (= ∂F/∂V) is equal to the gradient of the pressure: ; if it concerns the gravitational force, the force density is the specific weight.
The cgs unit of pressure is barye (ba). It is equal to 1 dyn·cm-2.