Theambient pressure on an object is thepressure of the surrounding medium, such as agas orliquid, in contact with the object.[1]
Within the atmosphere, the ambient pressure decreases as elevation increases. By measuring ambientatmospheric pressure, a pilot may determinealtitude (seepitot-static system). Nearsea level, a change in ambient pressure of 1 millibar is taken to represent a change in height of 9 metres (30 ft).[citation needed]
The ambient pressure in water with a free surface is a combination of thehydrostatic pressure due to the weight of the water column and the atmospheric pressure on the free surface. This increases approximately linearly with depth. Since water is much denser than air, much greater changes in ambient pressure can be experienced under water. Each 10 metres (33 ft) of depth adds anotherbar to the ambient pressure.
Ambient-pressure diving isunderwater diving exposed to the water pressure at depth, rather than in a pressure-excludingatmospheric diving suit or asubmersible.
The concept is not limited to environments frequented by people. Almost any place in the universe will have an ambient pressure, from the hard vacuum of deep space to the interior of an exploding supernova. At extremely small scales the concept of pressure becomes irrelevant, and it is undefined at a gravitational singularity.[citation needed]
The SI unit of pressure is thepascal (Pa), which is a very small unit relative to atmospheric pressure on Earth, so kilopascals (kPa) are more commonly used in this context. The ambient atmospheric pressure at sea level is not constant: it varies with the weather, but averages around 100 kPa. In fields such asmeteorology and underwater diving, it is common to see ambient pressure expressed inbar or millibar. One bar is 100 kPa or approximately ambient pressure at sea level. Ambient pressure may in other circumstances be measured inpounds per square inch (psi) or instandard atmospheres (atm). The ambient pressure at sea level is approximately one atmosphere, which is equal to 1.01325 bars (14.6959 psi), which is close enough for bar and atm to be used interchangeably in many applications. In underwater diving the industry convention is to measure ambient pressure in terms of water column. The metric unit is themetre sea water which is defined as 1/10 bar.
Pressures are given in terms of the normal ambient pressure experienced by humans – standard atmospheric pressure at sea level on earth.
| Environment | Typical ambient pressure instandard atmospheres |
|---|---|
| Hardvacuum ofouter space | 0 atm |
| Surface ofMars, average | 0.006 atm[2] |
| Top ofMount Everest – 8,849 m (29,032 ft) | 0.333 atm[3] |
| Pressurized passenger aircraft cabin altitude 8,000 ft (2,400 m) | 0.76 atm[4] |
| Sea levelatmospheric pressure | 1 atm |
| Surface ofTitan | 1.45 atm |
| 10 m depth inseawater | 2 atm |
| 20 m depth in seawater | 3 atm |
| Recreational diving depth limit (40 m)[5] | 5 atm |
| Common technical diving depth limit (100 m)[6][7] | 11 atm |
| Experimental ambient pressure dive maximum (Maximum ambient pressure a human has survived)[8] | 54 atm |
| Surface ofVenus | 92 atm[9] |
| 1 km depth in seawater | 101 atm |
| Deepest point in theEarth's oceans[10] | 1100 atm |
| Centre of the Earth | 3.3 to 3.6 million atm[11] |
| Centre ofJupiter | 30 to 45 million atm[12] |
| Centre of thesun | 244 billion atm[13] |
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