Fluid Pressure and it's Measurement

 Fluid Pressure 

Whenever a liquid is contained in a vessel, it exerts force at all points at the sides of the container and also at the bottom. This force per unit area is called pressure. The direction of pressure is always at right angles to the surface. 

Pressure Head 

We all know that, the liquid exerts pressure on all sides of the container as well as at the bottom also. The height of the liquid up to which the pressure exerts from it's base is known as pressure head. 

Pascal's Law 

It states, " The intensity of pressure at any point in a fluid at rest, is the same in all directions". 

Atmospheric Pressure 

It has been established, since long, that the air possesses some weight. Subsequently, it was also thought that the air, due to its weight, must exert some pressure on the surface of the earth. Since the air is compressible, therefore its density is different at different heights. Thus due to these difficulties, the atmospheric pressure cannot be calculated. However, it is measured by the height of the column of liquid that it can support. 

It has been observed that at sea level, the pressure exerted by the column of air of 1 square meter cross-sectional area and of height equal to that of the atmosphere is 103 kN. Thus, we may say that the atmospheric pressure at the sea level is 103 kN/m (or 103 kPa). It can also be expressed as 10.3 meters of water, in terms of equivalent water column or 760 mm of mercury in terms of equivalent mercury column. 

Gauge Pressure 

It is the pressure, measured with the help of a pressure measuring instrument, in which the atmospheric pressure is taken as datum. Or in other words, the atmospheric pressure on the gauge scale is marked as zero. Generally, this pressure is above the atmospheric pressure. 

Absolute Pressure 

It is the pressure equal to the algebraic sum of atmospheric pressure and gauge pressures. It may be noted that if the gauge pressure is minus, the absolute pressure will be atmospheric pressure minus gauge pressure. A little consideration will show that, if the pressure intensity at a point is more than the local atmospheric pressure, the difference of these two pressure is called the positive gauge pressure. 
However, if the pressure intensity is less than the local atmospheric pressure, the difference of these two pressures is called the negative gauge pressure or vacuum pressure.  

Measurement of Fluid Pressure 

The principles, on which all the pressure measuring devices are based, are almost the same. However, for convenient sake, we may split up the same into the following two types : 

1. By balancing the liquid column. These are called tube gauges to measure the pressure. 
2. By balancing the liquid column with the help of spring or dead weight. These are called mechanical gauges to measure the pressure. 

Tube Gauges to measure Fluid Pressure 

The devices used for measuring the fluid pressure are : 

1. Piezometer Tube 
2. Manometer 

Piezometer Tube 

A piezometer tube is the simplest form of instrument used for measuring, moderate pressures. It consists of a tube, one end of which is connected to the pipeline in which the pressure is required to be found out. The other end is opened to the atmosphere, in which the liquid can rise freely without overflow. The height to which the liquid rises up gives the pressure head directly. 

It may be noted that piezometer tube is meant for measuring gauge pressure only as the surface of the liquid, in the tube, is exposed to the atmosphere. A piezometer tube is also not suitable for measuring negative pressure; as in such case the air will enter through the pipe. 

Manometer 

Frankly speaking, a manometer is an improved form of a piezometer tube. With the help of a manometer, we can measure comparatively high pressures and negative pressures also. Following are the few types of manometers : 

1. Simple Manometer 
2. Micromanometer 
3. Differential manometer, and 
4. Inverted differential manometer. 

Mechanical Gauges 

Whenever a very high fluid pressure is to be measured, a mechanical gauge is best suited for the purpose. A mechanical gauge is also used for the measurement of pressure in boilers or other pipes, where tube gauges cannot conveniently used. 

There are many types of gauges available in the market. But the principle, on which all these gauges work, is almost the same. Following three types of gauges are important from the subject point of view :  

1. Bourdon's tube pressure gauge. 
2. Diaphragm pressure gauge, and 
3. Dead weight pressure gauge. 

Bourdon's Tube Pressure Gauge 

The pressure, above or below the atmospheric pressure, may be easily measured with the help of a Bourdon's tube pressure gauge. A Bourdon's tube pressure gauge, in its simplest form, consists of an elliptical tube; bent into an arc of a circle, as shown in the figure, 

When the gauge tube is connected to the fluid, it's pressure flows into the tube. As a result, the Bourdon's tube increases pressure and tends to straighten itself. With the help of a simple pinion and sector arrangement, the elastic deformation of the Bourdon's tube rotates the pointer. This pointer moves over a calibrated scale, which directly gives the pressure. 

Diaphragm Pressure Gauge 

The pressure, above or below the atmospheric pressure, is also found out with the help of diaphragm pressure gauge. A diaphragm pressure gauge, in its simplest form, consists of a corrugated diaphragm as shown in the figure 

When the gauge is connected to the fluid, the fluid under pressure causes some deformation of the diaphragm. With the help of some pinion arrangement, the elastic deformation of the diaphragm rotates the pointer. This pointer moves over a calibrated scale, which directly gives the pressure. 

Dead Weight Pressure Gauge 

It is the most accurate pressure gauge, which is generally used for the calibration of the other pressure gauges in the laboratory. A dead weight pressure gauge, in its simplest form, consists of a piston and a cylinder of known area, which is connected to a fluid through a tube as shown in the figure. 

As shown in the figure, the pressure gauge, to be calibrated, is fitted on the small end of the tube. By changing the weight, on the piston, the pressure on the fluid is calculated and marked on the gauge at the respective points, indicated by the pointer. A small error due to frictional resistance to the motion of the piston may come into play.