# What is an atmosphere of pressure?

The atmosphere of pressure is qualitative unit of measure which is defined as the pressure exerted by the earth's atmosphere at sea level. The symbol is often used ATMis equivalent to 101325 Pa (pascals) and, although 1 atm is often referred to as the normal pressure value, the atm is not a unit of the International System of Units.

## History and uses

The pressure atmosphere was first adopted in 1954 at the tenth General Conference on Poids and Measures (General Conference on Weights and Measures). Its value was established at 101325 Pa and its general use was recommended as reference value from which to calculate physicochemical properties of a system.

In several disciplines and industries, mainly related to chemistry, the concept "under standard pressure and temperature conditions" was widely used to refer to values ​​of 1 atm and 25 ºC, or 1 atm and 20 ºC, also referred to as "standard conditions". normal temperature and pressure.

Atmospheric pressure can be defined as the force per unit area that the air in the atmosphere exerts on the earth's surface at sea level. This pressure is the weight of the vertical column of air above a certain point, and due to the variable density of the air and the changing atmospheric conditions, its calculation is very complicated, although its measurement is very simple, even with such rudimentary instruments. like the first mercury barometers of the 17th century, but obviously not a fixed value across the planet or over time but it is a qualitative and variable measure.

The standard state it is actually an arbitrary state defined by the researcher, although the use of standardized reference values ​​in the scientific field has its clear advantages; It allows, for example, to compare data from different investigations in a direct way. The most widely used standards today are those established by the IUPAC (International Union of Pure and Applied Chemistry), What are they:

1. Standard conditions of temperature and pressure: 0 ºC (273.15 K) and 105 Pa (100 kPa, 1 bar). They are usually referred to by the acronym STP, from the English Standard conditions for temperature and pressure. Before 1982, the IUPAC recommended 273.15 K and 1 atm (1.01325 × 105 Pa).
2. Normal conditions of temperature and pressure: the qualitative conditions corresponding to the "laboratory" conditions in which the measurements are made. If they are different from the standard conditions, they should be mentioned next to the results. In the IUPAC Gold Book, a "normal value" is one measured at 101325 Pa, that is, at 1 atm, but it does not mention a "normal" temperature value.

Thus, the current use of atmospheres as a unit of pressure is not considered standard and would not be the most recommended for reference measurements, although it would be a close approximation to the use of standard conditions defined in pascals and degrees centigrade, both units adopted in The international system of units.

In some fields, however, the atm unit is still widely used. For example, scuba divers often calibrate their equipment against sea level air pressure using atm units.

Another very similar unit, although much less used throughout history, is the technical atmosphereabbreviated at and that is equivalent to 98.0665 kPa.

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