atmosphere (unit)

atmosphere (unit)
Name	atmosphere
Quantity	Pressure
Units1	SI base units
Units1 expr	kilogram per metre per second squared (kg·m−1·s−2)
Units2	Derived
Units2 expr	101325 pascals

atmosphere (unit)

The atmosphere (symbol: atm) is a unit of pressure defined as 101325 pascals and used historically and presently in standards, engineering, meteorology and chemistry. It appears in international standards, national metrology institutes and scientific literature and is referenced alongside SI units such as the Pascal, historic units like the bar (unit), and practical measures used by organizations including the National Institute of Standards and Technology, the International Bureau of Weights and Measures, and the International Organization for Standardization. The unit connects work in laboratories, naval architecture, aerospace programs such as NASA, and industrial codes maintained by bodies like the American Society of Mechanical Engineers and the International Electrotechnical Commission.

Definition and symbol

The atmosphere is defined as exactly 101325 pascals (101325 Pa), with the accepted symbol "atm", a designation used in publications from the International System of Units discussions at the International Bureau of Weights and Measures and cited by organizations such as the National Physical Laboratory (UK), the Physikalisch-Technische Bundesanstalt, and the International Organization for Standardization. The unit is conventionally used alongside the Pascal, the bar (unit), and older measures like the torr, and appears in technical standards from the American National Standards Institute and manuals produced by the United States Navy and Bureau of Mines. The symbol "atm" is distinct from similar abbreviations used by entities such as the American Meteorological Society, the Royal Society, and publishers including Springer Nature.

Historical development and standards

The atmosphere as a standardized unit arose in the 19th century during efforts by scientists affiliated with institutions such as the Royal Society, the French Academy of Sciences, the Prussian Academy of Sciences, and figures linked to the Metre Convention to unify pressure measurement across laboratories like the Royal Observatory, Greenwich and the Observatoire de Paris. Early pressure work by researchers connected to the Royal Institution, the University of Cambridge, and the École Polytechnique employed mercury manometers calibrated against column heights measured in laboratories such as the Kew Observatory; later codification occurred with international agreements influenced by the Metre Convention and implemented through the International Bureau of Weights and Measures and national metrology institutes like the National Research Council (Canada). Adoption and refinement were reflected in standards produced by the International Organization for Standardization, technical committees of the International Electrotechnical Commission, and engineering societies including the American Society of Mechanical Engineers.

Relation to other pressure units

The atmosphere relates directly to the Pascal (1 atm = 101325 Pa), to the bar (unit), the torr (based on the millimeter of mercury), and to units used in naval and aeronautical practice such as the psi (pound-force per square inch) and units found in texts from Society of Automotive Engineers publications. Comparative values are used in standards from the International Organization for Standardization and conversion tables in reference works from institutions like the National Institute of Standards and Technology and the Cambridge University Press catalog. Historical comparisons also reference the technical atmosphere and local measures standardized by national bodies including the Bureau International des Poids et Mesures and the United States National Bureau of Standards.

Usage in science and industry

The atmosphere unit is commonly used in chemical thermodynamics, physical chemistry and gas law expressions found in texts from publishers such as Wiley-VCH, Elsevier, and Oxford University Press and in laboratory standards from the American Chemical Society. Industrial practice references atm in process engineering, petrochemical standards overseen by the American Petroleum Institute, diving tables developed by institutions like the Professional Association of Diving Instructors, and aerospace engineering work by NASA and the European Space Agency. It remains present in meteorological summaries by agencies such as the National Weather Service, oceanographic studies from the Woods Hole Oceanographic Institution, and in safety regulations published by the Occupational Safety and Health Administration.

Measurement and conversions

Measurement of pressure in atm uses manometers, deadweight testers and electronic sensors calibrated to traceable standards maintained by the National Institute of Standards and Technology, the Physikalisch-Technische Bundesanstalt, and the International Bureau of Weights and Measures. Conversion factors to other units (1 atm = 101325 Pa = 1.01325 bar ≈ 760 torr ≈ 14.6959 psi) are tabulated in technical documents from the International Organization for Standardization, handbooks published by CRC Press, and metrology guides from the European Association of National Metrology Institutes. Laboratories performing precise work often reference calibration certificates issued by national metrology institutes such as the National Research Council (Canada) or the National Physical Laboratory (UK).

Variants and derived units

Related forms include the standard atmosphere sometimes denoted "atm" for at-sea level standardization used in aviation and maritime practice, the technical atmosphere (at) historically defined differently, and pressure measures derived for specialized fields such as the bar, millibar (used by the World Meteorological Organization), and the torr used in vacuum science at facilities like the CERN. Derived units and dimensionless ratios appear in engineering standards from the American Society of Mechanical Engineers, chemical safety data sheets from the European Chemicals Agency, and instrumentation specifications by manufacturers like Honeywell and Siemens.

Category:Units of pressure