### Units of pressure, types of pressure

*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,m

^{2}:1,Pa = 1, N/m

^{2}. There are other units, which are not within SI, namely:

^{5}Pa = 0.1 MPa;

^{2}= 98066.5 Pa = 0.0980665 MPa - (technical atmosphere);

^{2}= 1 psi = 6894.76 Pa = 0.00689476 MPa;

*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.

**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:

**P**

_{g}= P

_{abs }- P

_{atm}

**. (1)**

_{abs }<

_{ }P

_{atm}, then the difference between atmospheric pressure and absolute pressure is called

*vacuumetric pressure*and is read by ordinary vacuum gauge:

**P**

_{vac}= P

_{atm}- P

_{abs}

**. (2)**

**Figure 1.**Illustration of terms used in pressure measurements.

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.

**•**

*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

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