DEFINITION OF PRESSURE
It is the ratio between a force and the surface on which it acts; it is
dimensionally expressed in force units per surface units.
P(Pa) = F(N) / S(m2)
ATMOSPHERIC PRESSURE
It is the pressure exerted on a surface of 1 cm2 at sea level, at
a temperature of 20 °C and with a relative humidity of 65%; it is equivalent
to a column of water of 10,33 m or to 760 mmHg.
ABSOLUTE PRESSURE
It is the pressure of a fluid respect to the absolute vacuum and it is
obtainable adding the atmospheric pressure to the gauge one.
GAUGE PRESSURE
It is the differential pressure of a fluid above and below the atmospheric
pressure normally read on the pressure gauges.
UPSTREAM PRESSURE
Pressure of the compressed air at the inlet of the pneumatic component.
DOWNSTREAM PRESSURE
Pressure of the compressed air at the outlet of the pneumatic component.
DIFFERENTIAL PRESSURE ( DP)
It is the difference between upstream and downstream pressure.
BOYLE-MARIOTTE’S LAW
The volume of a closed quantity of gas with constant temperature is
inversely proportional to the absolute pressure, thus means that for a given
quantity of gas the product between the absolute pressure and the volume is
a constant value:
P1 · V1 = P2 · V2 = Constant.
GAY - LUSSAC’ LAW
The volume of a quantity of gas with constant pressure is proportional to
its temperature:
V1 / V2 = T1 / T2
or even, with constant volume, the pressure of a quantity of gas is
proportional to its temperature.
P1 / P2 = T1 / T2
Then we obtain the General Equation of Gases:
P · V = n · R · T
where:
P = pressure (atm)
V = volume (Nl)
n = gram molecules of gas contained in the volume (mol)
R = perfect gas constant (0,0821 Nl · atm · K-1 · mol-1)
T = absolute temperature in Kelvin (273 K = 0 °C)
NORMAL CONDITIONS OF AIR
In the design of industrial pneumatic circuits are employed measures that
refer to the "Normal conditions of air".
As "normal cubic meter of air" (1Nm3) we refer to 1 m3
of air at a temperature of 273 K (0 °C) and at a pressure of 1,0013 bar (pressure
of the normal air at the sea level): 1 Nm3 = 1000 Nl. |
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