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INTRODUCTION

In our day to day life, we are meeting various applications of pressure. Have you ever noticed the flow of wind, the force of water coming out of the hose? All these are the different examples for different kinds of pressure. Now we may think what is pressure? In atmosphere, gases and fluids exert some force on earth’s surface these forces acting /unit area is the pressure.

Nearly all industrial processes use liquids and gases. Controlling those processes requires the measurement and control of liquid and gas pressure. Thus pressure measurement is one of the most important processes of measurements .If you are pealing an apple then the pressure is the key variable: if the knife is sharp, then the area of contact is small and you can peel with less force exerted on the blade .if an object weigh 100 pounds and rests on 1 sq inch of earth’s surface, It is exerting 100 pounds/sq inch .All objects on earth have weight and therefore exert pressure on earth that can be expressed in unit of weight in unit area.

Pressure can be defined as the amount of force applied or distributed over the surface and it can be measured as force per unit area

P=F/A

CLASSIFICATION OF PRESSURE SCALES

 

Atmospheric pressure

The atmosphere may be supposed to be divided into a number of horizontal layers parallel to the surface of the earth. Each layer bears the weight of all other layers above it and thus subjected a pressure due to their weight. This pressure is called as the atmospheric pressure. Evidently this pressure is greatest at the surface of the earth and decreased as move higher.

The normal atmospheric pressure at the sea level is: 1.03kg/cm2 or 10.3meters of H2O column or 76 cm of Hg column or 760 mm of Hg or 14.7 PSI.


Barometer

Barometer is an instrument used to measure the atmospheric pressure simple barometer consists of a long glass tube filled with mercury and sealed at one end and the placed in a pan of mercury.

The Hg in the tube settles down, leaving a vacuum above it. The height of the mercury column in the tube above the level of the mercury in the pan indicates the atmospheric pressure in inches of Hg.

Gauge pressure

It is the pressure, measured with the help of a pressure measuring instruments, in which the atmospheric pressure is taken as the datum or in other words, the atmospheric pressure on the gauge scale is marked as zero. Most liquid pressure gauges use atmospheric pressure (14.7 PSI) as the datum point. They indicate a pressure of 0 PSI at the surface of a liquid even though the pressure is actually 14.7 PSI or 1.03 Kg/cm2. Generally, the gauge pressure is above the atmospheric pressure.

 

Manometer

Monometer is the simplest measuring device used for measuring gauge pressures at very low range by balancing the pressure against the weight of the column of the liquid. It is generally used within a range of 2Kg/cm2. The action of all manometers depends on the effect of pressure exerted by the fluid at a depth.

U-tube manometer

It consists of a transparent glass tube constructed in the form of an elongated U and partially filled with a liquid, most commonly water or Hg.

Absolute pressure

It is the pressure equal to the algebraic sum of atmospheric pressure and gauge pressures.

Vacuum pressure

Gauges that indicate pressure below zero is called vacuum pressure. Absolute pressure gauges cannot indicate pressure below zero because this type of gauges, zero is perfect vacuum.

 

TYPES OF PRESSURE

Static pressure:

When a fluid is in equilibrium the pressure at a particular point is identical in all directions and independent of orientation. The air compresses to its minimum volume and exerts a force in all direction within the cylinder. This is the static pressure excreted by gas. As the weight increases the pressure within the cylinder increases.

Dynamic pressure:

It is the pressure above static pressure caused by the movement of fluids. The dynamic pressure can be produced by gravity or mechanically by the pump.

Differential Pressure:

It is the pressure difference between two related pressures. Measuring two related pressure and calculating the difference between the two measurements can determine the differential pressure. Differential pressure is frequently used to determine fluid flow rate.

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