Ambient pressure

The ambient pressure on an object is the pressure of the surrounding medium, such as a gas or liquid, in contact with the object.^[1]

A laboratory studying ambient pressure at Oregon State University

Atmosphere

Within the atmosphere, the ambient pressure decreases as elevation increases. By measuring ambient atmospheric pressure, a pilot may determine altitude (see pitot-static system). Near sea level, a change in ambient pressure of 1 millibar is taken to represent a change in height of 9 metres (30 ft).^{[citation needed]}

Underwater

The ambient pressure in water with a free surface is a combination of the hydrostatic 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 another bar to the ambient pressure.

Ambient-pressure diving is underwater diving exposed to the water pressure at depth, rather than in a pressure-excluding atmospheric diving suit or a submersible.

Other environments

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]}

Units of pressure

Further information: Pressure § Units

The SI unit of pressure is the pascal (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 as meteorology and underwater diving, it is common to see ambient pressure expressed in bar or millibar. One bar is 100 kPa or approximately ambient pressure at sea level. Ambient pressure may in other circumstances be measured in pounds per square inch (psi) or in standard 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 the metre sea water which is defined as 1/10 bar.

Examples of ambient pressure in various environments

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 in standard atmospheres
Hard vacuum of outer space	0 atm
Surface of Mars, average	0.006 atm [2]
Top of Mount 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 level atmospheric pressure	1 atm
Surface of Titan	1.45 atm
10 m depth in seawater	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 of Venus	92 atm [9]
1 km depth in seawater	101 atm
Deepest point in the Earth's oceans[10]	1100 atm
Centre of the Earth	3.3 to 3.6 million atm[11]
Centre of Jupiter	30 to 45 million atm[12]
Centre of the sun	244 billion atm [13]

References

1. McGraw-Hill Dictionary of Scientific and Technical Terms Copyright © 2003 by McGraw-Hill Companies, Inc. "Sci-Tech Dictionary ambient pressure on Answers.com". McGraw-Hill Companies, Inc.{{cite web}}: CS1 maint: numeric names: authors list (link)
2. Bolonkin, Alexander A. (2009). Artificial Environments on Mars. Berlin Heidelberg: Springer. pp. 599–625. ISBN 978-3-642-03629-3.
3. "Online high altitude oxygen calculator". altitude.org. Archived from the original on 29 July 2012. Retrieved 15 August 2007.
4. K. Baillie and A. Simpson. "Altitude oxygen calculator". Retrieved 2013-11-27. - Online interactive altitude oxygen calculator
5. Brylske, A. (2006). Encyclopedia of Recreational Diving, 3rd edition. United States: PADI. ISBN 1-878663-01-1.
6. "Trimix Diver - IANTD - World Headquarter". Archived from the original on 2013-09-03. Retrieved 2013-06-16.
7. "TDI Advanced Trimix Diver".
8. Comex S.A. HYDRA 8 and HYDRA 10 test projects Archived August 4, 2009, at the Wayback Machine
9. "Venus: Facts & Figures". NASA. Archived from the original on 2006-09-29. Retrieved 2007-04-12.
10. "Scientists map Mariana Trench, deepest known section of ocean in the world". The Telegraph. 7 December 2011. Archived from the original on 8 December 2011. Retrieved 24 September 2013.
11. David. R. Lide, ed. (2006–2007). CRC Handbook of Chemistry and Physics (87th ed.). pp. j14–13. Archived from the original on 2017-07-24. Retrieved 2013-11-27.
12. Elkins-Tanton, Linda T. (2006). Jupiter and Saturn. New York: Chelsea House. ISBN 0-8160-5196-8.
13. Williams, David R. (September 1, 2004). "Sun Fact Sheet". NASA. Retrieved 12 January 2015.

Further reading

• Underwater diving portal

• Damon, Edward G; Gaylord, Charles S; Hicks, William; Yelverton, John T; Richmond, Donald R; White, Clayton S. "The effects of ambient pressure on tolerance of mammals to air blast". Albuquerque: Lovelace Foundation for Medical Education and Research. Archived from the original on May 22, 2011. Retrieved 5 January 2010.
• Frost, David Lawrence; Sturtevant, Bradford (1986). "Effects of ambient pressure on the instability of a liquid boiling explosively at the superheat limit". Journal of Heat Transfer. 108 (2). Caltech Library: 418–24. doi:10.1115/1.3246940. Archived from the original on December 12, 2008. Retrieved 5 January 2010.