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Standard atmosphere (unit)
View on Wikipediafrom Wikipedia
| Atmosphere | |
|---|---|
| Unit of | Pressure |
| Symbol | atm |
| Conversions | |
| 1 atm in ... | ... is equal to ... |
| SI units | 101.325 kPa |
| US customary units | 14.69595 psi 29.92126 inHg |
| other metric units | 1.013250 bar 760 mmHg |
The standard atmosphere (symbol: atm) is a unit of pressure defined as 101325 Pa. It is sometimes used as a reference pressure or standard pressure. It is approximately equal to Earth's average atmospheric pressure at sea level.[1]
History
[edit]The standard atmosphere was originally defined as the pressure exerted by a 760 mm column of mercury at 0 °C (32 °F) and standard gravity (gn = 9.80665 m/s2).[2] It was used as a reference condition for physical and chemical properties, and the definition of the centigrade temperature scale set 100 °C as the boiling point of water at this pressure. In 1954, the 10th General Conference on Weights and Measures (CGPM) adopted standard atmosphere for general use and affirmed its definition of being precisely equal to 1013250 dynes per square centimetre (101325 Pa).[3] This defined pressure in a way that is independent of the properties of any particular substance. In addition, the CGPM noted that there had been some misapprehension that the previous definition (from the 9th CGPM) "led some physicists to believe that this definition of the standard atmosphere was valid only for accurate work in thermometry."[3]
In chemistry and in various industries, the reference pressure referred to in standard temperature and pressure was commonly 1 atm (101.325 kPa) prior to 1982, but standards have since diverged; in 1982, the International Union of Pure and Applied Chemistry recommended that for the purposes of specifying the physical properties of substances, standard pressure should be precisely 100 kPa (1 bar).[4]
Pressure units and equivalencies
[edit]| Pascal | Bar | Technical atmosphere | Standard atmosphere | Torr | Pound per square inch | |
|---|---|---|---|---|---|---|
| (Pa) | (bar) | (at) | (atm) | (Torr) | (psi) | |
| 1 Pa | — | 10−5 bar | 1.0197×10−5 at | 9.8692×10−6 atm | 7.5006×10−3 Torr | 0.000145037737730 lbf/in2 |
| 1 bar | 105 | — | = 1.0197 | = 0.98692 | = 750.06 | = 14.503773773022 |
| 1 at | 98066.5 | 0.980665 | — | 0.9678411053541 | 735.5592401 | 14.2233433071203 |
| 1 atm | ≡ 101325 | ≡ 1.01325 | 1.0332 | — | ≡ 760 | 14.6959487755142 |
| 1 Torr | 133.322368421 | 0.001333224 | 0.00135951 | 1/760 ≈ 0.001315789 | — | 0.019336775 |
| 1 psi | 6894.757293168 | 0.068947573 | 0.070306958 | 0.068045964 | 51.714932572 | — |
A pressure of 1 atm can also be stated as:
- ≈ 1.033 kgf/cm2
- ≈ 10.33 m H2O[5]
- ≈ 760 mmHg[6]
- ≈ 29.92 inHg[6]
- ≈ 406.782 in H2O[5]
- ≈ 2116.22 pounds-force per square foot (lbf/ft2)
The notation ata has been used to indicate an absolute pressure measured in either standard atmospheres (atm)[7][better source needed] or technical atmospheres (at).[8]
See also
[edit]References
[edit]- ^ "Water Pressures at Ocean Depths". NOAA Pacific Marine Environmental Laboratory. Retrieved 11 October 2022.
- ^ Resnick, Robert; Halliday, David (1960). Physics for Students of Science and Engineering Part 1. New York: Wiley. p. 364.
- ^ a b "BIPM - Resolution 4 of the 10th CGPM". www.bipm.org.
- ^ IUPAC.org, Gold Book, Standard Pressure
- ^ a b As a unit of measurement, the conventional metre of water (mH2O) is defined as an ideal column of water with density of 1000 kg/m3 under standard gravity gn of 9.80665 m/s2 i.e. 1 m × 1000 kg/m3 × 9.80665 m/s2 = 9806.65 Pa (though in practice the density of pure water is always less). 1 cmH2O = 0.01 mH2O and 1 inH2O = 0.0254 mH2O. BS 350:Part 1:1974 Conversion factors and tables, Part 1. Basis of tables. Conversion factors. British Standards Institution. 1974. p. 49.
- ^ a b As a unit of measurement, the conventional millimetre of mercury (mmHg) is defined as an ideal column of mercury with density of 13595.1 kg/m3 under standard gravity gn of 9.80665 m/s2 i.e. 0.001 m × 13595.1 kg/m3 × 9.80665 m/s2 ≈ 133.322 Pa. 1 inHg = 25.4 mmHg. BS 350:Part 1:1974 Conversion factors and tables, Part 1. Basis of tables. Conversion factors. British Standards Institution. 1974. p. 49.
- ^ "The Difference Between An ATM & An ATA". Scuba Diving & Other Fun Activities. March 2, 2008.
- ^ BS 350:Part 1:1974 Conversion factors and tables, Part 1. Basis of tables. Conversion factors. British Standards Institution. 1974. p. 50.
Standard atmosphere (unit)
View on Grokipediafrom Grokipedia
Definition and Properties
Precise Value
The standard atmosphere, denoted as atm, is a unit of pressure defined exactly as 101325 pascals (Pa). This precise value was established by Resolution 4 of the 10th General Conference on Weights and Measures (CGPM) in 1954, which adopted 1 atm as precisely 1,013,250 dynes per square centimeter, equivalent to 101325 Pa in SI units. Unlike earlier approximations, this definition fixes the standard atmosphere as a constant, non-variable quantity for international reference.[4] The standard atmosphere relates to the bar, another common pressure unit, as 1 atm = 1.01325 bar, with the bar itself defined exactly as 100000 Pa.[4] This equivalence underscores the standard atmosphere's role as a reference slightly above the bar, both expressed in modern pascal terms for precision in scientific and engineering contexts. The fixed nature of 1 atm = 101325 Pa ensures consistency across applications, distinct from variable physical measurements like historical mercury columns.Physical Basis
The standard atmosphere, as a unit of pressure, originates from the physical measurement of atmospheric pressure using a mercury barometer, where one standard atmosphere corresponds to the pressure that would be exerted by a column of mercury exactly 760 mm (0.76 m) in height at a temperature of 0°C under the influence of standard gravity, specified as .[5] This conventional representation ensures a reproducible reference based on observable physical properties of mercury and gravitational acceleration. In modern terms, the equivalence is exact such that 760 mmHg = 1 atm, with the millimetre of mercury (mmHg) defined using a fixed density of mercury at 0°C and the standard gravity to yield precisely 101325 Pa.[3] The underlying physical basis relies on the hydrostatic principle, which states that the pressure at the base of a fluid column is given by , where is the density of the fluid, is the acceleration due to gravity, and is the height of the column. For mercury at 0°C, the density is precisely 13595.1 kg/m³, reflecting its mass per unit volume under standardized conditions.[6] Substituting these values—, , and —yields the pressure equivalent to one standard atmosphere.[7] Standardization of temperature and gravity is essential for reproducibility, as variations in mercury's density due to thermal expansion or differences in local gravity would otherwise alter the measured pressure. By fixing the temperature at 0°C, where mercury's properties are well-characterized, and adopting the international standard value for gravity, this definition provides a consistent benchmark independent of geographic or environmental factors. This conceptual foundation corresponds to the modern exact value of 101325 Pa.Historical Development
Early Measurements
The concept of measuring atmospheric pressure emerged in the mid-17th century through pioneering experiments that quantified the force exerted by air. In 1643, Italian physicist Evangelista Torricelli invented the mercury barometer by filling a long glass tube with mercury, sealing it, and inverting it into a basin of mercury, which created a vacuum at the top and allowed the mercury to descend to a stable height.[8] He observed that this height stabilized at approximately 76 cm at sea level in Florence, attributing the support of the column to the weight of the overlying atmosphere rather than any inherent properties of the vacuum.[8] This device provided the first reliable means to gauge variations in air pressure, laying the groundwork for understanding atmospheric dynamics. Building on Torricelli's work, French mathematician Blaise Pascal conducted experiments around 1646–1647 to explore the nature of atmospheric pressure and the existence of a vacuum. In his treatise Experiences nouvelles touchant le vide published in 1647, Pascal detailed replications of Torricelli's barometer and further tests showing that the mercury column height decreased with elevation, confirming that air pressure diminishes as altitude increases due to the reduced weight of the air column above.[9] A key demonstration occurred in 1648 when his brother-in-law Florin Périer carried a barometer up the Puy de Dôme mountain in France, observing the mercury level drop by about 8 cm from base to summit, providing empirical evidence that atmospheric pressure is not uniform but varies with height.[9] In 1654, German engineer and inventor Otto von Guericke further illustrated the immense force of atmospheric pressure through his famous Magdeburg hemispheres experiment, conducted during a demonstration for Holy Roman Emperor Ferdinand III at the Diet of Regensburg. Guericke, who had invented the first functional air pump in 1650, joined two copper hemispheres to form a sealed sphere, evacuated the air inside using his pump, and then attempted to separate them with teams of horses—two teams of eight horses each failed to pull the hemispheres apart until air was readmitted, revealing the crushing power of external atmospheric pressure acting on the 50 cm diameter sphere.[10] This vivid public demonstration, later detailed in his 1672 book Experimenta Nova Magdeburgica de Vacui Spatio, underscored the tangible effects of air pressure and influenced subsequent studies in pneumatics and vacuum technology.[10] By the late 17th century, these experiments had led to rough approximations of sea-level atmospheric pressure as a standard reference, with Torricelli's observed mercury height of about 76 cm—equivalent to 760 mmHg—gaining acceptance among scientists as a baseline for normal conditions at sea level.[11] This value, though initially variable due to local weather and measurement inconsistencies, served as an informal benchmark in early meteorological and physical investigations, bridging empirical observations toward more precise definitions in later centuries.[11]Modern Standardization
By the 19th century, refinements in meteorological measurements had led to the widespread adoption of 760 mmHg as the standard value for atmospheric pressure at sea level, based on average observations using mercury barometers calibrated at 0 °C.[12] The 10th Conférence Générale des Poids et Mesures (CGPM) in 1954 established a precise, fixed definition of the standard atmosphere as exactly 101325 pascals (Pa), independent of mercury column variability or assumptions about standard gravity, for broad scientific and engineering use.[1] In 1982, the International Union of Pure and Applied Chemistry (IUPAC) recommended retaining the standard atmosphere (101325 Pa) as a non-SI unit for compatibility with existing literature and practices, while proposing 100 kPa (1 bar) as the preferred standard pressure for new thermodynamic data reporting.[13]Conversions and Equivalencies
Relation to SI Units
The standard atmosphere (atm) is a non-SI unit of pressure accepted for use with the International System of Units (SI), as recognized by the International Committee for Weights and Measures (CIPM) due to its practical importance in science and technology.[14] In formal SI contexts, however, pressure measurements must be converted to the SI derived unit, the pascal (Pa), which is defined as one newton per square meter (1 Pa = 1 N/m²).[2] The exact conversion factor is 1 atm = 101 325 Pa, ensuring precise interoperability between the two systems.[14] The atmosphere unit relates closely to the bar, another non-SI pressure unit accepted for use with the SI and defined as exactly 100 000 Pa (or 10⁵ Pa).[14] This makes 1 atm slightly greater than 1 bar, with 1 atm ≈ 1.013 25 bar, a difference arising from the historical definition of the atmosphere based on mean sea-level pressure.[2] Both units are commonly employed in engineering applications, but the bar is often preferred in metric-oriented systems for its alignment with powers of 10 in pascals, facilitating calculations in fields like fluid mechanics and instrumentation.[2] Although the SI strongly encourages the use of the pascal to promote uniformity, the standard atmosphere persists in legacy systems, reference standards, and specialized domains such as chemistry and meteorology, where its historical convenience outweighs the need for conversion in routine practice.[14] This ongoing use underscores the balance between standardization and entrenched conventions in technical fields.[15]Comparison with Other Units
The standard atmosphere (atm) unit, historically defined to align with mercury barometer readings, equates exactly to 760 torr, a unit named after Evangelista Torricelli and widely used in vacuum science.[16][17] Since the torr is defined as exactly equivalent to 1 millimeter of mercury (mmHg), 1 atm also equals 760 mmHg precisely, reflecting the original calibration of atmospheric pressure at sea level using a mercury column.[16][17] In engineering contexts, particularly in the United States where imperial units persist, the standard atmosphere converts to approximately 14.6959 pounds per square inch (psi), a measure based on force per unit area that facilitates tire pressures, hydraulic systems, and structural calculations.[16][18] Another common mercury-based unit is inches of mercury (inHg), where 1 atm equals 29.9213 inHg, often applied in altimetry and weather instrumentation.[16] The table below provides these key equivalencies for quick reference:| Unit | Equivalent to 1 atm |
|---|---|
| Torr | 760 (exact) |
| mmHg | 760 (exact) |
| psi | 14.6959 |
| inHg | 29.9213 |