Armstrong Limit
Encyclopedia
The Armstrong limit, often called Armstrong's line, is the altitude
that produces an atmospheric pressure
so low (0.0618 atmosphere
) that water boils at the normal temperature of the human body: 37 °C (98.6 °F). It is named after Harry George Armstrong
, who founded the U.S. Air Force’s Department of Space Medicine in 1947 at Randolph Field, Texas.Along with Malcolm C. Grow
, Armstrong became one of the first two surgeons general for the U.S. Air Force
when the USAF split off from the Army Air Forces to become a separate branch of the U.S. military on September 18, 1947. Randolph Field was officially renamed Randolph Air Force Base
shortly thereafter on January 13, 1948. Armstrong was first to recognize that the phenomenon represented an absolute altitude beyond which humans could not survive in an unpressurized environment. The altitude is variously reported as being between 62,000–63,500 feet (18,900–19,350 meters or about 12 miles).
within the lungs—but not vascular blood
(blood within the circulatory system)—will boil away without a pressure suit
and no amount of breathable oxygen
delivered by any means will sustain life for more than a few minutes. The NASA
technical report Rapid (Explosive) Decompression Emergencies in Pressure-Suited Subjects, which discusses the brief accidental exposure of a human to near vacuum
notes the likely result of exposure to pressure below that associated with the Armstrong limit: "The subject later reported that ... his last conscious memory was of the water on his tongue
beginning to boil."
At the nominal body temperature of 37 °C (98.6 °F), water has a vapor pressure
of 47 millimetres of mercury (62.7 hPa); which is to say, at an ambient pressure of 47 mmHg, water’s boiling point is 37 °C. A pressure of 47 mmHg—the Armstrong limit—is onesixteenth that of the standard sea level atmospheric pressure of 760 millimeters of mercury (1013 hPa). Modern 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 formula shows that 47 mmHg is typically found at an altitude of 63100 feet (19,232.9 m).
Blood pressure is a gage pressure
, which means it is measured relative to ambient pressure and is therefore additive when determining the absolute pressure to be used in equations of state
(gas laws and the formulas relating pressure-dependent phase changes between liquid and gas). This is similar to a flat automobile tire. Even with zero gage pressure, a flat tire at 63,100 feet would still have an absolute pressure (pressure relative to a perfect vacuum) of 47 mmHg surrounding it—both inside and out. If one inflates the tire to non-zero gage pressure, this internal pressure is in addition to what the tire started with. Even for an individual with a diastolic
blood pressure on the low threshold of the normal range, 60 mmHg, blood pressure more than doubles the absolute pressure on the blood and is more than sufficient to prevent blood from outright boiling at 63,100 feet while the heart is still beating.
—inadequate oxygen causing confusion and eventual loss of consciousness. Air is 20.95% oxygen. At 50,000 feet breathing pure oxygen through a face mask, one is breathing the same partial pressure
of oxygen as one would experience with regular air at 15,655 feet above sea level.Partial pressure, in accordance with Dalton’s law
, denotes the extent to which a constituent gas contributes to a total pressure exerted by a mix of gases. The Armstrong limit (the pressure at which 37 °C water boils) is 47 mmHg. However, the Armstrong limit is not the threshold at which hypoxia first becomes a concern (although there is insufficient oxygen to support human life at the Armstrong limit). An experienced, well conditioned pilot is at significant risk of hypoxia when breathing pure oxygen at 50,000 feet where the pressure is 87.5 mmHg, which is 86 percent greater than the Armstrong limit’s 47 mmHg. This is equivalent to breathing mixed air (air comprising mostly nitrogen) at 15,655 feet. This correlation between 15,655 and 50,000 feet is as follows: Standard air pressure at 15,655 feet is 417.7 mmHg. Ordinary air is 20.9476 percent oxygen (U.S. Standard Atmosphere). Thus, the partial pressure of oxygen at 15,655 feet is given by the formula ; which is to say, oxygen contributes 87.5 mmHg of the 417.7 mmHg found at 15,655 feet on a standard day. This is the same oxygen partial pressure as is provided by breathing pure oxygen at 50,000 feet in an unpressurized cockpit where the total pressure and the oxygen partial pressure are one and the same: 87.5 mmHg. This is a particularly high altitude insofar as hypoxia-related risks go; it is over 1100 feet greater than the tallest mountain in the continental U.S., Mount Whitney
.
Commercial jetliners are required to pressurize their cabins to an equivalent altitude not greater than 8,000 feet (2,438 m). U.S. regulations on general aviation
aircraft (private pilots in small planes) require that the pilot—but not the passengers—be on supplemental oxygen if the plane spends more than a half hour at an altitude of at least 12,500 feet. General aviation pilots must be on supplemental oxygen if the plane spends any time at 14,000 feet or greater, and even the passengers must be on supplemental oxygen at 15,000 feet. Sky divers, who are at altitude only briefly before jumping, do not normally exceed 14,500 feet. Since 50,000 feet is the point at which breathing pure oxygen through an oxygen mask delivers the same oxygen partial pressure as is found with regular air at a hypoxia-inducing 15,655 feet, an altitude of 50,000 feet or higher requires increasing the pressure delivered into the lungs—as well as outside the lungs to avoid pulmonary barotrauma
(lungs expanding like a balloon and tearing); thus, the requirement for a pressure suit.
For modern military aircraft such as the United States’ F22 and F35, both of which have operational altitudes of 60,000 feet (18,200 m) or more, the pilot wears a “counter-pressure garment,” which is a Gsuit
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.
, set an altitude record
of 56850 feet (17,327.9 m) and wore a pressure suit in his open-cockpit Caproni Ca.161 biplane even though he was well below the altitude at which body-temperature water boils. Two years earlier, Francis Swain of the Royal Air Force reached 49967 feet (15,229.9 m) flying a Bristol Type 138
—also while wearing a pressure suit.
Altitude
Altitude or height is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often varies according to the context...
that produces an atmospheric pressure
Atmospheric pressure
Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...
so low (0.0618 atmosphere
Atmosphere (unit)
The standard atmosphere is an international reference pressure defined as 101325 Pa and formerly used as unit of pressure. For practical purposes it has been replaced by the bar which is 105 Pa...
) that water boils at the normal temperature of the human body: 37 °C (98.6 °F). It is named after Harry George Armstrong
Harry George Armstrong
Harry George Armstrong , known as "the father of space medicine", was a United States Marine, a member of the United States Army Air Forces, a Major General in the United States Air Force, a physician, and an airman. Armstrong served in the Marines during World War I and the Army and Air Force from...
, who founded the U.S. Air Force’s Department of Space Medicine in 1947 at Randolph Field, Texas.Along with Malcolm C. Grow
Malcolm C. Grow
Major General Malcolm C. Grow was the first Surgeon General of the United States Air Force from July 1, 1949 to November 30, 1949.-Biography:...
, Armstrong became one of the first two surgeons general for the U.S. Air Force
United States Air Force
The United States Air Force is the aerial warfare service branch of the United States Armed Forces and one of the American uniformed services. Initially part of the United States Army, the USAF was formed as a separate branch of the military on September 18, 1947 under the National Security Act of...
when the USAF split off from the Army Air Forces to become a separate branch of the U.S. military on September 18, 1947. Randolph Field was officially renamed Randolph Air Force Base
Randolph Air Force Base
Randolph Air Force Base is a United States Air Force base located east-northeast of San Antonio, Texas. The base is under the jurisdiction of the 902d Mission Support Group, Air Education and Training Command ....
shortly thereafter on January 13, 1948. Armstrong was first to recognize that the phenomenon represented an absolute altitude beyond which humans could not survive in an unpressurized environment. The altitude is variously reported as being between 62,000–63,500 feet (18,900–19,350 meters or about 12 miles).
Effect on bodily liquids
At or above the Armstrong limit, exposed bodily liquids such as saliva, tears, and the liquids wetting the alveoliPulmonary alveolus
An alveolus is an anatomical structure that has the form of a hollow cavity. Found in the lung parenchyma, the pulmonary alveoli are the dead ends of the respiratory tree, which outcrop from either alveolar sacs or alveolar ducts, which are both sites of gas exchange with the blood as well...
within the lungs—but not vascular blood
Vascular
Vascular in zoology and medicine means "related to blood vessels", which are part of the circulatory system. An organ or tissue that is vascularized is heavily endowed with blood vessels and thus richly supplied with blood....
(blood within the circulatory system)—will boil away without a pressure suit
Pressure suit
A pressure suit is a protective suit worn by high-altitude pilots who may fly at altitudes where the air pressure is too low for an unprotected person to survive, even breathing pure oxygen at positive pressure. Such suits may be either full-pressure or partial-pressure...
and no amount of breathable oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
delivered by any means will sustain life for more than a few minutes. The NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
technical report Rapid (Explosive) Decompression Emergencies in Pressure-Suited Subjects, which discusses the brief accidental exposure of a human to near vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
notes the likely result of exposure to pressure below that associated with the Armstrong limit: "The subject later reported that ... his last conscious memory was of the water on his tongue
Tongue
The tongue is a muscular hydrostat on the floors of the mouths of most vertebrates which manipulates food for mastication. It is the primary organ of taste , as much of the upper surface of the tongue is covered in papillae and taste buds. It is sensitive and kept moist by saliva, and is richly...
beginning to boil."
At the nominal body temperature of 37 °C (98.6 °F), water has a vapor pressure
Vapor pressure
Vapor pressure or equilibrium vapor pressure is the pressure of a vapor in thermodynamic equilibrium with its condensed phases in a closed system. All liquids have a tendency to evaporate, and some solids can sublimate into a gaseous form...
of 47 millimetres of mercury (62.7 hPa); which is to say, at an ambient pressure of 47 mmHg, water’s boiling point is 37 °C. A pressure of 47 mmHg—the Armstrong limit—is onesixteenth that of the standard sea level atmospheric pressure of 760 millimeters of mercury (1013 hPa). Modern 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 formula shows that 47 mmHg is typically found at an altitude of 63100 feet (19,232.9 m).
Blood pressure is a gage pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
, which means it is measured relative to ambient pressure and is therefore additive when determining the absolute pressure to be used in equations of state
Equation of state
In physics and thermodynamics, an equation of state is a relation between state variables. More specifically, an equation of state is a thermodynamic equation describing the state of matter under a given set of physical conditions...
(gas laws and the formulas relating pressure-dependent phase changes between liquid and gas). This is similar to a flat automobile tire. Even with zero gage pressure, a flat tire at 63,100 feet would still have an absolute pressure (pressure relative to a perfect vacuum) of 47 mmHg surrounding it—both inside and out. If one inflates the tire to non-zero gage pressure, this internal pressure is in addition to what the tire started with. Even for an individual with a diastolic
Diastole
Diastole is the period of time when the heart fills with blood after systole . Ventricular diastole is the period during which the ventricles are relaxing, while atrial diastole is the period during which the atria are relaxing...
blood pressure on the low threshold of the normal range, 60 mmHg, blood pressure more than doubles the absolute pressure on the blood and is more than sufficient to prevent blood from outright boiling at 63,100 feet while the heart is still beating.
Hypoxia below the Armstrong limit
The Armstrong limit does not delineate the altitude at which it first becomes necessary to wear a pressure suit. A pressure suit is customarily required at 50,000 feet (15,240 m) for a well conditioned and experienced pilot to safely operate an aircraft in unpressurized cabins. The prompt physiological reaction when breathing pure oxygen through a face mask in an unpressurized cockpit at altitudes greater than 50,000 feet above sea level is hypoxiaHypoxia (medical)
Hypoxia, or hypoxiation, is a pathological condition in which the body as a whole or a region of the body is deprived of adequate oxygen supply. Variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise...
—inadequate oxygen causing confusion and eventual loss of consciousness. Air is 20.95% oxygen. At 50,000 feet breathing pure oxygen through a face mask, one is breathing the same partial pressure
Partial pressure
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
of oxygen as one would experience with regular air at 15,655 feet above sea level.Partial pressure, in accordance with Dalton’s law
Dalton's law
In chemistry and physics, Dalton's law states that the total pressure exerted by a gaseous mixture is equal to the sum of the partial pressures of each individual component in a gas mixture...
, denotes the extent to which a constituent gas contributes to a total pressure exerted by a mix of gases. The Armstrong limit (the pressure at which 37 °C water boils) is 47 mmHg. However, the Armstrong limit is not the threshold at which hypoxia first becomes a concern (although there is insufficient oxygen to support human life at the Armstrong limit). An experienced, well conditioned pilot is at significant risk of hypoxia when breathing pure oxygen at 50,000 feet where the pressure is 87.5 mmHg, which is 86 percent greater than the Armstrong limit’s 47 mmHg. This is equivalent to breathing mixed air (air comprising mostly nitrogen) at 15,655 feet. This correlation between 15,655 and 50,000 feet is as follows: Standard air pressure at 15,655 feet is 417.7 mmHg. Ordinary air is 20.9476 percent oxygen (U.S. Standard Atmosphere). Thus, the partial pressure of oxygen at 15,655 feet is given by the formula ; which is to say, oxygen contributes 87.5 mmHg of the 417.7 mmHg found at 15,655 feet on a standard day. This is the same oxygen partial pressure as is provided by breathing pure oxygen at 50,000 feet in an unpressurized cockpit where the total pressure and the oxygen partial pressure are one and the same: 87.5 mmHg. This is a particularly high altitude insofar as hypoxia-related risks go; it is over 1100 feet greater than the tallest mountain in the continental U.S., Mount Whitney
Mount Whitney
Mount Whitney is the highest summit in the contiguous United States with an elevation of . It is on the boundary between California's Inyo and Tulare counties, west-northwest of the lowest point in North America at Badwater in Death Valley National Park...
.
Commercial jetliners are required to pressurize their cabins to an equivalent altitude not greater than 8,000 feet (2,438 m). U.S. regulations on general aviation
General aviation
General aviation is one of the two categories of civil aviation. It refers to all flights other than military and scheduled airline and regular cargo flights, both private and commercial. General aviation flights range from gliders and powered parachutes to large, non-scheduled cargo jet flights...
aircraft (private pilots in small planes) require that the pilot—but not the passengers—be on supplemental oxygen if the plane spends more than a half hour at an altitude of at least 12,500 feet. General aviation pilots must be on supplemental oxygen if the plane spends any time at 14,000 feet or greater, and even the passengers must be on supplemental oxygen at 15,000 feet. Sky divers, who are at altitude only briefly before jumping, do not normally exceed 14,500 feet. Since 50,000 feet is the point at which breathing pure oxygen through an oxygen mask delivers the same oxygen partial pressure as is found with regular air at a hypoxia-inducing 15,655 feet, an altitude of 50,000 feet or higher requires increasing the pressure delivered into the lungs—as well as outside the lungs to avoid pulmonary barotrauma
Barotrauma
Barotrauma is physical damage to body tissues caused by a difference in pressure between an air space inside or beside the body and the surrounding fluid...
(lungs expanding like a balloon and tearing); thus, the requirement for a pressure suit.
For modern military aircraft such as the United States’ F22 and F35, both of which have operational altitudes of 60,000 feet (18,200 m) or more, the pilot wears a “counter-pressure garment,” which is a Gsuit
G-suit
A G-suit, or the more accurately named anti-G suit, is worn by aviators and astronauts who are subject to high levels of acceleration force . It is designed to prevent a black-out and G-LOC caused by the blood pooling in the lower part of the body when under acceleration, thus depriving the...
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.
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 timedependent effects of hypoxia experienced at lower altitudes. Pressure suits had long been worn at altitudes well below the Armstrong limit to avoid hypoxia. In 1938, an Italian military officer, Mario PezziCaproni Ca.161
|-See also:...
, set an altitude record
Flight altitude record
These are the records set for going the highest in the atmosphere from the age of ballooning onward. Some records are certified by Fédération Aéronautique Internationale.-Fixed-wing aircraft:-Piston-driven propeller aeroplane:...
of 56850 feet (17,327.9 m) and wore a pressure suit in his open-cockpit Caproni Ca.161 biplane even though he was well below the altitude at which body-temperature water boils. Two years earlier, Francis Swain of the Royal Air Force reached 49967 feet (15,229.9 m) flying a Bristol Type 138
Bristol Type 138
|-See also:-References:NotesBibliography* Barnes, C.H. Bristol Aircraft since 1910. London: Putnam, 1964.* Thetford, Owen. Aircraft of the Royal Air Force 1918-57, 1st edition. London: Putnam, 1957....
—also while wearing a pressure suit.
External links
- US Naval Flight Surgeon's Manual. Chapter 1 is Physiology of Flight
- Ebullism at 1 Million Feet: Surviving Rapid/Explosive Decompression
- The Engineering ToolBox: “Air Pressure and Altitude above Sea Level”