Units of pressure, types of pressure
Among many process variables
pressure is that parameter which is
critical for safe and optimum operation in hydraulic equipment, separating
processes (such as filtration, distillation, etc.), vacuum processing, etc. Using
pressure measurements we are able to measure level of liquids in tanks, or
flowrate of liquids or gases in the pipes. In order to keep pressure within
desired range we need to be able to accurately measure this process variable,
and then control it. It is not easy to use instrumentation for pressure
measurements without knowledge of a fundamental concept of pressure.
Pressure is equal to the
force divided by the area over which it is applied. In the International System
(SI) the unit for pressure is called Pascal
(Pa) and is equal to the force of one 1,N divided by an area of one 1,m2:1,Pa
= 1, N/m2. There are other units, which are not within SI, namely:
1 bar = 105 Pa =
0.1 MPa;
1
atm = 101325 Pa - (standard or physical atmosphere);
1
kgf/cm2 = 98066.5 Pa = 0.0980665 MPa - (technical atmosphere);
1 lbf/in2 = 1 psi = 6894.76 Pa = 0.00689476
MPa;
1 mm Hg = 133.322 Pa - (760 mm Hg = 101324.7 Pa » 101325
Pa).
The advantage of the Pascal is that it does not depend on the
gravitational acceleration. It means that this unit is the same in places with
various values of gravitational acceleration. Even on other planets it does not
change.
There are various types of
pressures. Figure 1 gives
illustration of terms used in pressure measurements (From book Van Wylen G.J.,
Sonntag R.E. "Fundamentals of Classical Thermodynamics", Sec. Ed.). Absolute pressure in a system is equal
to the total pressure of a liquid or a gas which acts on the walls of this
system. The difference between absolute and atmospheric pressure is called gage or
manometric pressure and is read by
ordinary pressure gauge:
Pg = Pabs - Patm. (1)
If Pabs < Patm, then the difference between
atmospheric pressure and absolute pressure is called vacuumetric pressure and is read by ordinary vacuum gauge:
Pvac = Patm - Pabs. (2)
Figure
1.
Illustration of terms used in pressure measurements.
Instrumentation for pressure
measurements may be classified regarding to the operational principle used or
type of pressure to be measured.
If we consider operational principles employed, then process instrumentation for pressure measurements may be categorised as follows:
• liquid filled
pressure instrumentation: “U”-tube manometers, well manometers, bell-type
manometers, liquid barometer, absolute pressure manometer;
•
elastic-element mechanical pressure gages: Bourdon tube pressure
gages, bellows-type pressure gages, diaphragm-type pressure gages;
• dead-weight
pressure gages;
• electrical-type
pressure gages: piezoelectric pressure gages, capacitance pressure gages, strain-type
pressure gages.
Here is the classification
of instrumentation for pressure measurements with respect to the type of the
measured pressure:
• pressure gages, for measurements of
pressures above atmospheric pressure;
• vacuum pressure gages, for measurements of
pressures below atmospheric pressure;
• vacuum manometers, for
measurements of both pressures above and below atmospheric pressure;
• barometers, for measurement of
atmospheric pressure;
•
differential pressure and vacuum
gages, for measurements of difference of pressures.
Article Source:: Dr. Alexander Badalyan, University of South Australia
