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Practical air guide

Pressure, temperature and relative humidity

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Pressure is the force of power on a defined surface area. The International System of units - SI unit N/m2 is called Pascal (Pa) therefore 1 N/m2 = 1Pa. As this is a very small unit of pressure in practice, pressure is generally represented in bar and psi.

1 bar = 105 Pa = 14.5 PSI
1 Pascal =1 Newton / 1 m²
1 PSI = 1 pound per in²


Normal atmospheric air pressure is stated as 14.7 PSI (1 Bar) at sea level. Generally used as a reference for pressure measurement, it is, however variable according to altitude. For tests and measurements, it is advisable to use absolute PSI or bar corresponding to absolute pressure
(When pressure is measured above atmospheric pressure it is referred to as gauge pressure)

Pabs = Patm + Prel
Pabs : absolute pressure
Prel : relative gauge pressure
Patm : normal atmospheric pressure

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The temperature of a substance is an indication of its hotness or coldness and is measured by some type of thermometer. Absolute temperature is generally given in C, although absolute is sometimes referred to as K or Kelvin
The International System of Units (SI) unit of temperature is Kelvin (K).
The following formula establishes the connection between Kelvin and degree Celsius (°C),

T= t + 273.15

T : absolute temperature (K)
t : temperature (°C)

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Relative humidity

The percentage of relative humidity is the relation between:

•  the quantity of water vapour present in a volume of air
•  the quantity of water corresponding to the saturation of this same volume of air (saturation causing condensation of excess water vapour)

The maximum quantity of water which can be absorbed in a volume of air increases with temperature.