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Thursday, September 27, 2012

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)

terms used in pressure measurements.

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


Basics of Instrumentation & Control

To view more posts on BASICS : CLICK HERE


To view more posts on PRESSURE : CLICK HERE


To view more posts on FLOW : CLICK HERE


To view more posts on LEVEL : CLICK HERE


To view more posts on TEMPERATURE : CLICK HERE

Analytical Instrumentation

To view more posts on Analytical Instrumentation : CLICK HERE
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