The standard atmosphere (symbol: atm) is a unit of pressure defined as 101325 Pa (1.01325 bar). It is sometimes used as a reference or standard pressure. It is approximately equal to the atmospheric pressure at sea level.

History

It was originally defined as the pressure exerted by 760 mm of mercury at 0 °C and standard gravity (g = ).^[1] It was used as a reference condition for physical and chemical properties, and was implicit in the definition of the Centigrade (later Celsius) scale of temperature by defining 100 °C as being the boiling point of water at this pressure. In 1954, the 10th Conférence Générale des Poids et Mesures (CGPM) adopted standard atmosphere for general use and affirmed its definition of being precisely equal to  dynes per square centimetre .^[2] This defined both temperature and pressure independent of the properties of particular substance. In addition (the CGPM noted) there had been some misapprehension that it "led some physicists to believe that this definition of the standard atmosphere was valid only for accurate work in thermometry."^[2]

In chemistry and in various industries, the reference pressure referred to in "Standard Temperature and Pressure" (STP) was commonly 1 atm (101.325 kPa) but standards have since diverged; in 1982, the International Union of Pure and Applied Chemistry (IUPAC) recommended that for the purposes of specifying the physical properties of substances, "standard pressure" should be precisely 100 kPa (1 bar).^[3]

Pressure units and equivalencies

A pressure of 1 atm can also be stated as:

≡ bar

≡ pascals (Pa) or kilopascals (kPa)

≡ millibars (mbar or mb)

≡ torr (Torr)^{[n 1]}

≈ mmHg, 0 °C, subject to revision as more precise measurements of mercury's density become available^{[n 2][n 3]}

≈ inHg, 0 °C, subject to revision as more precise measurements of mercury's density become available^{[n 3]}

≈ kgf/cm²

≈ technical atmosphere

≈ cm H₂O, 4 °C^{[n 2]}

≈ in H₂O, 4 °C^{[n 2]}

≈ pounds-force per square inch (psi)

≈ pounds-force per square foot (psf)

= 1 ata (atmosphere absolute). The ata unit is used in place of atm to indicate that the pressure shown is the total ambient pressure, compared to vacuum, of the system being calculated or measured.^[4] For example, for underwater pressures, a pressure of 3.1 ata would mean that the 1 atm of the air above water is included in this value and the pressure due to water would total 2.1 atm.

Notes:

Other applications

Scuba divers and others use the word atmosphere and "atm" in relation to pressures that are relative to mean atmospheric pressure at sea level (1.013 bar).^[] For example, a partial pressure of oxygen is calibrated typically using air at sea level, so is expressed in units of atm.

The old European unit technical atmosphere (at) is equal to 1 kilogram-force per square centimetre (kgf/cm²),  Pa.^[]

See also

• Standard conditions for temperature and pressure
• Atmospheric pressure
• International Standard Atmosphere

References