Pressure

Pressure (symbol: p) is a measure of force per unit area.

$p={\frac {F}{A}}$

where

p is the pressure

F is the force

A is the area

Often F is taken to be the of the magnitude of the mean vector force normal to the surface of area A upon which it exerts; the "surface" not necessarily being a that of a body, but for example the cross sectional area of a conduit.

"Pressure is a scalar quantity, but teachers and authors do not appear to believe this in their hearts." (McClelland, 1987)

SI units

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).

In the above equation:

p is the pressure, measured in pascals

F is the force, measured in newtons

A is the area, measured in square metres

Other Units

Non-SI measures (still in use in some parts of the world) include the pound-force per square inch (PSI) and the bar.

The cgs unit of pressure is barye (ba). It is equal to 1 dyn·cm^-2. Attempts to convert PSI to cgs units sometimes result in pressure being expressed in "g·cm^-2", but this usage is incorrect and should be avoided. (Grams are a unit of mass, not force.)

In the United States air pressure is still measured in inHg — 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).

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".

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.

A list of some non-SI units

• atmospheres
• bars
• centimeters of mercury
• centimeters of water
• feet of water
• inches of water
• inches of mercury
• dynes/sq.centimeter (barye)
• kg-force/sq.centimeter
• kg-force/sq.meter
• kilonewton/sq.meter
• kips/sq.inch
• meganewtons/sq.metre
• metres of water
• millibars
• millimeters of mercury
• millimeters of water
• newtons/sq.centimeter
• newtons/sq.meter
• newtons/sq.millimeter
• pounds-force/sq.foot
• pounds-force/sq.inch (PSI)
• poundals/sq.foot
• tons(UK)-force/sq.foot
• tons(UK)-force/sq.inch
• tons(US)-force/sq.foot
• tons(US)-force/sq.inch
• tonnes-force/sq.cm
• tonnes-force/sq.metre

Static Pressure

Static pressure is the pressure due to the density and depth of a fluid.

$p={\rho }{g}{h}\,$

where

p is the pressure, measured in pascals

ρ is the density of the fluid, measured in kilograms per cubic metre

g is acceleration due to gravity, measured in metre per second squared

h is the height of fluid above the point being measured, measured in metres

Stagnation pressure

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.

Force density

The force density f (= ∂F/∂V) is equal to the gradient of the pressure: $\mathbf {f} =\nabla \mathbf {P}$; if it concerns the gravitational force, the force density is the specific weight.

Conversion between pressure units

Pressure units and conversion factors

	Pascal	bar	N/mm2	kp/m2	kp/cm2 (=1 at)	atm	torr
1 Pa (N/m2)=	1	10-5	10-6	0.102	0.102×10-4	0.987×10-5	0.0075
1 bar (daN/cm2) =	105	1	0.1	10,200	1.02	0.987	750
1 N/mm2 =	106	10	1	1.02×105	10.2	9.87	7,500
1 kp/m2 =	9.81	9.81×10-5	9.81×10-6	1	10-4	0.968×10-4	0.0736
1 kp/cm2 (1 at) =	98,100	0.981	0.0981	10,000	1	0.968	736
1 atm (760 torr) =	101,325	1.013	0.1013	10,330	1.033	1	760
1 torr (mmHg) =	133	0.00133	1.33×10-4	13.6	0.00132	0.00132	1

Human body

In the human body, a baroreceptor is a pressure sensor. Arterial baroreceptors are located in each of the two internal carotid sinuses (carotid arteries, neck arteries) and in the aortic arch. They are part of a system, including the carotid sinus reflex, that regulates arterial blood pressure.

Since sound is nothing more than cyclic pressure waves, our ear drums act as dynamic pressure sensors in a similar way as a microphone. Obviously, they also sense changes in static pressure, hence ear "popping" with changes in altitude.

See also

• Partial pressure
• Kinetic theory#Pressure
• Atmospheric pressure
• Sound pressure
• Microphone
• Timeline of temperature and pressure measurement technology
• Conversion of units
• Blood pressure
• Category:Units of pressure

External links

• Calculations and conversions of all pressure units
• Conversions of English and American pressure units to metric units
• An Exercise in Air Pressure

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Pressure can also be psychological, political, etc.; see also peer pressure.