What is manometer? types,uses and advantages
(Usage of image is allowed, attribution needed)

Hello and welcome back to The mechanical post !
Today lets learn what is manometer, its types, applications, advantages and disadvantages, starting off with the definition of manometer.                                                                                                                                                                        
Manometer is a type of pressure measuring device which measures the pressure of a fluid by balancing the levels of fluid in the 2 columns of its U-shaped tube. 

Types of manometer.

Manometer's are broadly classified as follows :                                   
  •  Simple manometers 
  1. Piezometer
  2. U-tube manometer
  3. Single column manometer ( micrometer )         
  • Differential manometers. ( This section will be covered in part2)
  1. U-tube differential manometer.
  2. Inverted U-tube differential manometer.
Before starting let us get acquainted by the terms gauge pressure and absolute pressure

Gauge pressure:-

When the pressure is measured above the normal atmospheric pressure as reference it is called as positive gauge pressure

Absolute pressure:-


When the pressure is measured from absolute zero or complete vacuum it is known as absolute pressure. 

Lets start with simple manometers.

Piezometer tube

 It is one most simplest type of manometer. It consists of glass tube of around 12mm diameter in the shape of letter "L". One end of the tube is connected to the source whose pressure is to be measured (for e.g.- a pipeline) while the other end is kept open to the atmosphere as shown below:

Piezometer tube
Piezometer tube (Usage of image is allowed, attribution needed)

Working :- When a pressurized liquid flows through the pipeline, it enters the piezometer. Due to the pressure, the liquid level starts to rise. At a certain point, when the pressure is balanced on both the sides the liquid comes to rest. This rise in the level of the liquid in the tube is denoted by "h". 

For pressure at point X 

P = w*h

Where, 
P = pressure of the liquid in N/m^2.
w = specific weight of liquid in N/m^3.
h = length of rise of liquid level in meters.

Advantages of Piezomoter tube

The following are the advantages of piezometer tube
  • Simple in construction.
  • Inexpensive.
  • Easily replaceable.
  • Suitable for low and medium gauge pressure.

Disadvantages of Piezomoter tube

The following are the disadvantages of piezometer tube
  • It can measure gauge pressure only.
  • It cannot be used for high pressure measured.
  • Only pressure of liquids can be measured.                                                                            

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Simple U-tube manometer

As the name suggest it is a U-shaped glass tube of diameter 10 to 12mm, whose one end is connected to the source whose pressure is to be measured, while the other end is open to the atmosphere as show in the figure

U - tube manometer
U tube manometer (Usage of image is allowed, attribution needed)

The U-tube manometer is filled with a liquid whose specific gravity is more then the specific gravity of the liquid whose pressure is to be measured. This heavy liquid filled in the manometer is known as manometeric liquid. Usually, mercury ( Hg ) is used as a manometric fluid.

Working:-  When the pressurized fluid flows through the pipeline, it enters the U-tube manometer. This liquid exerts pressure on the manometric fluid. This causes the manometric fluid rise in the right limb. However, as the manometric liquid is heavier it rises slowly as compared to the liquid in the pipeline.

Let us consider, 
h1 = height of liquid in left limb above the datum X-X
h2 = height of liquid in right limb above datum X-X
h = pressure heat at point A in terms of meters of water
s1 = specific gravity of the liquid in pipeline
s2 = specific gravity of heat liquid

According to pascal's law, pressure in left limb must be equal to pressure in left limb.

Pressure in left limb above X-X  =  Pressure in right limb above X-X.

h + s1*h1 = s2*h2.
h = ( s2*h2 - s1*h1)m of water.

Thus, we get the pressure of the liquid flowing in the pipeline in terms of pressure head in terms of meters of water.

To measure negative pressure,

Pressure in the left limb above X-X = Pressure in the right limb above X-X

h + s1*h1 + s2*h2 = 0 ( since there is no liquid )


Advantages of U- tube manometer :

The following are the advantages of U tube manometer.
  • Simple in construction.
  • Inexpensive in cost and working.
  • Accurate readings.
  • Pressure readings can be easily calculated.

Disadvantages of U tube manometer :

The following are the disadvantages of U tube manometer.
  • U tube manometer cannot be used for high pressure measurement as there is a possibility of the liquid overflowing from the tube.
  • As the right limb of the U tube manometer is exposed to the atmosphere there is a chance of foreign particles to enter the manometer.
  • It cannot detect small changes in pressure.


Vertical single column manometer or micromanometer

It is a modified form of U-tube manometer. A reservoir having a larger cross-sectional area is connected to one area of the left limb of the manometer as shown in the figure. Due to large cross-sectional area of the reservoir, for any variation in pressure, the change in the liquid level in the reservoir will be very small and is negligible. Therefore the pressure is given by the height of th liquid in the right limb.

Vertical single column manometer
Micromanometer (Usage of image is allowed, attribution needed)

As shown in the figure vertical single column manometer connected to a pipe containing light liquid under higher pressure. The pressure of liquid in a pipe forces the manometric liquid downwards in the reservoir. Due to large area of reservoir, the fall of manometric liquid level will be very small. This downward movement of manometric liquid in the reservoir will cause a rise of manometric liquid in the right limb.
Let, 
h1 = Height of light liquid in the left limb above the datum AB.

h2 = Height of heavy liquid in right limb above the datum AB.

h = Pressure head of light liquid in a pipe.

s1 and s2 = Specific gravity of light liquid in a pipe and heavy liquid in      right limb.

🔺h = Fall of heavy liquid level in the reservoir.

The pressure at the point 'X' in a pipe is given by,

h = a*h2/A (s2 - s1 ) + s2*h2 - s1*h1.

here , a = Cross-sectional area of the right limb.
           A = cross -sectional area of reservoir. 

But, area A is too large as compared to area 'a'. Hence the ratio a/A is negligible.


Advantages of miromanometer :

The following are the advantages if micromanometer.
  • High sensitivity
  • Increased accuracy in pressure measurement.
  • Can measure low pressure with high accuracy.

Disadvantages of micromanometer :

The following are the disadvantages of micromanometer.
  • It is expensive as compared to U-tube manometer. 
  • Although it can measure pressure with high accuracy but it loses against Bourdon tube gauge in pressure measurement.

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