# Technical atmosphere

Physical unit
Unit name Technical atmosphere
Unit symbol ${\ displaystyle \ mathrm {at}}$ Physical quantity (s) pressure
Formula symbol ${\ displaystyle p}$ dimension ${\ displaystyle {\ mathsf {K \; L ^ {- 2}}}}$ system Technical measurement system
In SI units ${\ displaystyle \ mathrm {1 \; at = 98 \, 066 {,} 5 \; {\ frac {kg} {m \, s ^ ​​{2}}}}}$ ${\ displaystyle \ mathrm {{} = 98 {,} 066 \, 5 \; kPa}}$ Derived from Meters of water column

The technical atmosphere is a non- SI -conforming unit of pressure . Since January 1, 1978, it is no longer permitted in Germany for specifying the pressure. The unit symbol is at .

## 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 size of the normal pressure exerted by this shell, but deviates by around 3% from the physical atmosphere in order to connect to the unit of force, the kilopond .

## definition

The technical atmosphere was originally defined as the magnitude of the pressure of ten  meters of water column :

1 at = 10  mWS = 1  kp / cm 2

Since this variable depends on temperature, acceleration due to gravity and water purity, it was then standardized with the SI pressure unit Pascal :

${\ displaystyle \ mathrm {1 \; at = 98 \. 066 {,} 5 \; Pa = 9 {,} 806 \, 65 \; N / cm ^ {2} = 0 {,} 980 \, 665 \ ;bar} }$ ${\ displaystyle \ mathrm {1 \; Pa \ approx 1 {,} 01972 \ cdot 10 ^ {- 5} \; at}}$ For example, a pressure of exactly 1 at is created if the density of the pressure-generating liquid is exactly 1 g / cm 3 , the acceleration due to gravity is exactly the same as the standard fall acceleration 9.80665 m / s 2 and the height is exactly 10 m. ${\ displaystyle p = \ rho \; g \; h}$ ${\ displaystyle \ rho}$ ${\ displaystyle g}$ ${\ displaystyle h}$ ## Designations

In contrast to the units of the international system of units (SI), it was customary in the technical atmosphere to indicate in the unit symbol what type of pressure specification it was in each case. An index a, ü or u was then added to the unit symbol:

• ata: absolute pressure in at, counted from pressure in a vacuum
• atü: overpressure in at, counted from the pressure of the surrounding air
• atu: negative pressure in at, counted from the pressure of the surrounding air

## atü

The old unit atmospheres (for " At mosphären- Ü overpressure") was found for. B. on the tire pressure inflators at gas stations. Since the car tire can only carry the vehicle if the tire has a higher pressure in relation to the ambient pressure (≈ 1 at), tire pressure inflation devices have measured the pressure difference to the ambient pressure accordingly.

## Individual evidence

1. ^ Paul Dobrinski, Gunter Krakau, Anselm Vogel: Physics for engineers . Springer, 2003, ISBN 3-519-46501-9 , pp. 690 ( limited preview in Google Book search).
2. DIN 1301, Part 3: Units - Conversion of non-SI units , 2018