Armstrong limit
The Armstrong limit or Armstrong's line is a measure of altitude above which atmospheric pressure is sufficiently low that water boils at the normal temperature of the human body. Exposure to pressure below this limit results in a rapid loss of consciousness, followed by a series of changes to cardiovascular and neurological functions, and eventually death, unless pressure is restored within 60–90 seconds.[1] Because of this, airplanes usually fly below the Armstrong limit. On Earth, the limit is around Template:Cvt above sea level,[1][2] above which atmospheric air pressure drops below 0.0618 atm (6.3 kPa, 47 mmHg, or about 1 psi). The U.S. Standard Atmospheric model sets the Armstrong limit at an altitude of Script error: No such module "convert".. The Armstrong limit is often used as the lower limit of near-space.
The term is named after United States Air Force General Harry George Armstrong, who was the first to recognize this phenomenon.[3]
Effect on body fluids
| Location | Pressure | ||
|---|---|---|---|
| kPa | psi | atm | |
| Olympus Mons summit | Template:Decimal-align | ||
| Mars average | Template:Decimal-align | ||
| Hellas Planitia bottom | Template:Decimal-align | ||
| Armstrong limit | Template:Decimal-align | ||
| Mount Everest summit[4] | Template:Decimal-align | ||
| Earth sea level | Template:Decimal-align | ||
| Dead Sea level[5] | Template:Decimal-align | ||
| Surface of Venus[6] | Template:Decimal-align | ||
At or above the Armstrong limit, exposed body fluids such as saliva, tears, urine, and the liquids wetting the alveoli within the lungs—but not vascular blood (blood within the circulatory system)—will boil away if the subject does not wear a full-body pressure suit. A test subject at the NASA Johnson Space Center accidentally exposed to near vacuum in 1965 "reported that ... his last conscious memory was of the saliva on his tongue beginning to boil."[7]
At the nominal body temperature of Template:Cvt, water has a vapour pressure of Script error: No such module "convert".; which is to say, at an ambient pressure of Script error: No such module "convert"., the boiling point of water is Template:Cvt. A pressure of 6.3 kPa—the Armstrong limit—is about 1/16 of the standard sea-level atmospheric pressure of Script error: No such module "convert".. At higher altitudes water vapour from ebullism will add to the decompression bubbles of nitrogen gas and cause the body tissues to swell up, though the tissues and the skin are strong enough not to burst under the internal pressure of vapourised water. Formulas for calculating the standard pressure at a given altitude vary—as do the precise pressures one will actually measure at a given altitude on a given day—but a common formulaScript error: No such module "Unsubst". shows that 6.3 kPa is typically found at an altitude of Script error: No such module "convert"..
Hypoxia below the Armstrong limit
Well below the Armstrong limit, humans typically require supplemental oxygen in order to avoid hypoxia. For most people, this is typically needed at altitudes above 4,500 m (15,000 ft). Commercial jetliners are required to maintain cabin pressurization at an equivalent effective cabin altitude of not greater than Script error: No such module "convert".. U.S. regulations on general aviation aircraft (non-airline, non-government flights) require that the minimum required flight crew, but not the passengers, be on supplemental oxygen if the plane spends more than half an hour at an equivalent effective cabin altitude above Script error: No such module "convert".. The minimum required flight crew must be on supplemental oxygen if the plane spends any time above an equivalent effective cabin altitude of Script error: No such module "convert"., and even the passengers must be provided with supplemental oxygen above an equivalent effective cabin altitude of Script error: No such module "convert"..[8] Skydivers, who are at altitude only briefly before jumping, do not normally exceed Script error: No such module "convert"..[9]
Historical significance
The Armstrong limit describes the altitude associated with an objective, precisely defined natural phenomenon: the vapor pressure of body-temperature water. In the late 1940s, it represented a new fundamental, hard limit to altitude that went beyond the somewhat subjective observations of human physiology and the timeTemplate:Nbhyphdependent effects of hypoxia experienced at lower altitudes. Pressure suits had long been worn at altitudes well below the Armstrong limit to avoid hypoxia. In 1936, Francis Swain of the Royal Air Force reached Script error: No such module "convert". flying a Bristol Type 138 while wearing a pressure suit.[10] Two years later Italian military officer Mario Pezzi set an altitude record of Script error: No such module "convert"., wearing a pressure suit in his Caproni Ca.161bis biplane even though he was well below the altitude at which body-temperature water boils.
A pressure suit is normally required at around Script error: No such module "convert". for a well conditioned and experienced pilot to safely operate an aircraft in unpressurized cabins.[11] In an unpressurized cockpit at altitudes greater than Script error: No such module "convert". above sea level, the physiological reaction, even when breathing pure oxygen, is hypoxia—inadequate oxygen level causing confusion and eventual loss of consciousness. Air contains 20.95% oxygen. At Script error: No such module "convert"., breathing pure oxygen through an unsealed face mask, one is breathing the same partial pressure of oxygen as one would experience with regular air at around Script error: No such module "convert". above sea levelScript error: No such module "Unsubst".. At higher altitudes, oxygen must be delivered through a sealed mask with increased pressure, to maintain a physiologically adequate partial pressure of oxygen. If the user does not wear a pressure suit or a counter-pressure garment that restricts the movement of their chest, the high-pressure air can cause damage to the lungs.
For modern military aircraft such as the United States' [[Lockheed Martin F-22 Raptor|FTemplate:Nbhyph22]] and [[Lockheed Martin F-35 Lightning II|FTemplate:Nbhyph35]], both of which have operational altitudes of Script error: No such module "convert". or more, the pilot wears a "counter-pressure garment", which is a [[G-suit|gTemplate:Nbhyphsuit]] with high-altitude capabilities. In the event the cockpit loses pressure, the oxygen system switches to a positive-pressure mode to deliver above-ambient-pressure oxygen to a specially sealing mask as well as to proportionally inflate the counter-pressure garment. The garment counters the outward expansion of the pilot's chest to prevent pulmonary barotrauma until the pilot can descend to a safe altitude.[12]
See also
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- Effects of high altitude on humans
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References
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External links
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