# Pressure Measuring Devices

## What are Pressure Measuring Devices?

Pressure measuring devices are used for measurement of the fluid pressure. Fluid pressure means, the pressure exerted by –

1. – liquids on its boundaries.
2. – gases on its boundaries.

The pressure at a point on a surface in contact with a liquid is the thrust acting normally per unit area around that point. If a total force or thrust ( F ) of a liquid acts normally over a flat surface of area ( A ) then, the fluid pressure is given by –

\text {Pressure (P)} = \left [ \frac {\text {Normal force (F)}}{\text {Area of surface (A)}} \right ]

Various types of fluid pressure measuring devices in use are –

1. Torricelli’s simple barometer.
2. Air Pressure gauges.
3. Piezometer.
4. Manometer.

### 1. Torricelli’s Simple Barometer

The simple barometer is used for measuring of atmospheric air pressure. It is \text {1 m} long glass tube closed at one end. It is filled with clean and dry mercury. The open end of the tube is closed by thumb and inverted into a dish of mercury as shown in figure. When thumb is removed, the mercury level in the tube falls down a little and comes to at rest at a vertical height of \text {76 cm} above the mercury level in the dish.

Therefore, atmospheric air pressure ( P_a ) is equivalent to \text {76 cm} of mercury column.

We know that, Atmospheric pressure ( P_a ) = h \rho g , and density of mercury is \rho = 13.6 \times 10^3 \text {kg-m}^{-3}

\therefore \quad P_a = 0.76 \times 13.6 \times 10^3 \times 9.8 = 1.013 \times 10^5 \text {pascal} = 1.013 \text {bar}

A simple barometer is shown in figure. It is also known as Torricelli’s Barometer.

### 2. Air Pressure Gauge

Pressure gauges are used to measure fluid pressures above or below the atmospheric pressure. It is of two types –

1. Bourdon’s tube pressure gauge. It uses elliptical Bourdon’s tube for sensing pressure.
2. Diaphragm pressure gauge. It uses corrugated diaphragm for sensing pressure.

A Bourdon’s tube pressure gauge is consists of an elliptical cross section tube bent into an arc of a circle as shown in figure. This bent tube is called Bourdon’s tube. A corrugated diaphragm is used in Diaphragm pressure gauge in place of Bourdon’s tube.

When the pressure gauge is connected in the pressure line, the fluid under pressure flows into the tube / diaphragm which tends to straighten itself. Since the tube / diaphragm is encased in a circular cover, it tends to become circular instead of straight. With the help of a simple pinion and sector arrangement, the elastic deformation of the tube / diaphragm rotates the pointer. This pointer moves over a calibrated scale, which directly gives the reading of fluid pressure in line.

### 3. Piezometer Tube

A piezometer tube is used for –

1. Measuring of moderate liquid pressures flowing in pipe line.
2. Direct measurement of gauge pressure of liquid flowing in the pipe line in terms of pressure head.

Piezometer is consists of a narrow ( L ) shaped glass tube. Smaller limb is directly inserted in the pipe line through which liquid flows as shown in figure. End of the vertical and longer limb of tube is open to atmosphere.

Care should be taken that, the piezometer tube is not projected inside the pipe internal surface. Inserted end of the tube should be smooth and flush with the inside surface of the pipe.

Due to pressure of the liquid in the pipeline, liquid rises in the vertical limb of piezometer. The height of rising of liquid gives the reading ( h ) of pressure head of flowing liquid.

TO BE NOTED –

Since, the limb of piezometer tube is open to atmosphere, it is not suitable for measuring negative pressure or vacuum as in such a case the air will enter in the pipe through the tube.

### 4. Manometer

A manometer is an improved form of a piezometer tube to measure high liquid pressures and negative pressures too.

Manometers are of following types –

1. Simple U-Tube manometer – It is used to measure high pressure of lighter liquids. It is also used for measuring of negative pressure. A simple manometer is consists of a glass U-Tube. One leg of tube is attached to the gauge point of pressure measurement. The other end remains open to the atmosphere.
2. Differential manometer. – It is used for measurement of difference of pressures of lighter liquids flowing in pipe lines. Mercury is used as gauge liquid. Two ends of U-Tube are connected to the two points of same pipe line or different pipe lines, whose difference of pressure is required to be measured.
3. Inverted differential manometer – It is used for measuring difference of low pressures with high accuracy. It is also used for measurement of difference of pressures of heavier liquids flowing in pipe lines. A lighter liquid is used as gauge liquid. Two ends of U-Tube are connected to the two points of same pipe line or different pipe lines, whose difference of pressure is required to be measured.
4. Micro manometer or sensitive manometer – This is used for precise measuring of very low pressures. Hence, it is also called a sensitive manometer. Here, one limb of U-Tube is made much larger in cross section than that of the other limb to increase the sensitivity.