Physical atmosphere

Physical unit
Unit name Physical atmosphere
Unit symbol ${\ displaystyle \ mathrm {atm}}$
Physical quantity (s) pressure
Formula symbol ${\ displaystyle p, p _ {\ mathrm {amb}}}$
dimension ${\ displaystyle {\ mathsf {M \; L ^ {- 1} \; T ^ {- 2}}}}$
In SI units ${\ displaystyle \ mathrm {1 \, atm = 101 \, 325 \; {\ frac {kg} {m \, s ^ ​​{2}}}}}$ ${\ displaystyle \ quad = \ mathrm {101 {,} 325 \; kPa}}$

The physical atmosphere is a non- SI -conforming unit of pressure . Since January 1, 1978, it is no longer a legal entity in Germany . The unit symbol is atm .

Historically, the unit has been defined so that the pressure of 1 atm is as high as the mean air pressure prevailing at sea ​​level . This is caused by the weight of the earth's atmosphere .

The pressure of the physical atmosphere is one of the standard conditions on which many processes and measured values ​​are based.

etymology

The word atmosphere is derived from ancient Greek ἀτμός atmós , German 'steam' , 'haze', 'breath' and σφαῖρα sphaira , German 'ball' ( Latinized sphära ). In the present context it refers to the gaseous envelope above the earth's surface . The pressure unit is based on the amount of normal pressure exerted by this shell.

definition

The standard atmospheric pressure was defined in 1954 as the unit of measurement "physical atmosphere":

${\ displaystyle 1 \, \ mathrm {atm} = 101 \, 325 \, \ mathrm {Pa} = 1 {,} 013 \, 25 \ \ mathrm {bar} \,}$.

The opposite is true

${\ displaystyle 1 \; \ mathrm {Pa} \ approx 9 {,} 8692 \ cdot 10 ^ {- 6} \; \ mathrm {atm} \ ,.}$

Previously, the standard was an atmospheric pressure of 760  Torr , that is, the pressure that holds a mercury column high in a mercury barometer . Since the pressure also depends on the gravitational acceleration and the density of the mercury , this definition was dependent on the measuring location and the temperature. With the definition of the units of the metric system in 1954 , the physical atmosphere became independent of temperature, location and weather conditions. The value of 101${\ displaystyle h = 760 \; \ mathrm {mm}}$${\ displaystyle p = \ rho \; g \; h}$${\ displaystyle g}$${\ displaystyle \ rho}$ 325 Pa was obtained using:

${\ displaystyle \ rho = \ mathrm {13 {,} 595 \, 1 \, {\ frac {g} {cm ^ {3}}}} \}$ (Density of mercury at 0 ° C)
${\ displaystyle g = 9 {,} 806 \, 65 \; \ mathrm {\ frac {m} {s ^ {2}}} \}$ ( Standard acceleration )

As a result, the unit " Torr " (also no longer legal today) was given a definition via the metric system:

${\ displaystyle \ mathrm {1 \; Torr = {\ frac {1 \; \ mathrm {atm}} {760}} = 133 {,} 322 \ ldots \; Pa}}$.

Individual evidence

1. ^ Resolution 4 of the 10th CGPM (1954). In: bipm.org. Bureau International des Poids et Mesures, accessed June 18, 2020 .
2. Peter Kurzweil: The Vieweg unit lexicon: formulas and terms from physics, chemistry and technology. Vieweg, 1999, p. 40 f
3. Hans U. v. Vogel: Chemists Calendar. Springer, 1956, p. 392
4. Le Système international d'unités . 7e édition, 1998 (the so-called "SI brochure"), chap. 4.2 Table 10, French and English
5. DIN 1314 printing - basic terms and units. 1977
6. DIN 1301, Part 3: Units - Conversion of non-SI units , 2018