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Practical air guide

Pressure, temperature and relative humidity

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Pressure is the balance of power on a defined surface area. Pascal (Pa) represents the pressure unit of the International System of Units (SI), but in industrial practice, pressure is represented in pounds per square inch (PSI).

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


Normal atmospheric air pressure is stated as 14.7 PSI 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.

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


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The International System of Units (SI) unit of temperature is Kelvin (K).
The following formula establishes the connection between Kelvin, degree Celsius (°C), and degree Fahrenheit (°F), the units more commonly used:

T= t + 273.15
t= (tf-32) x 5/9

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

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Relative humidity

The percentage of relative humidity is the relation between:

• the quantity of water vapor 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 vapor)

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