Simple manometer – Piezometer – U tube manometer

manometer

It is a device in which the pressure of a fluid is measure. and in simple language, manometer is a pressure measuring instrument.

Simple manometer

A simple manometer is one which consists of a glass tube whose one end is connected to a point where pressure is to be measured and the other end remains open to atmosphere. So there are two types of manometer, one is (1) Piezometer and other is (2) U-tube manometer.

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Piezometer

A Piezometer is the simplest form of manometer which can be use for measuring moderate pressures of liquids. So it consists of a glass tube inserted in the wall of a vessel or of a pipe, containing liquid whose pressure is to be measured.

In the piezometer tube other end is open to the atmosphere. So The rise of liquid gives the pressure head at that point. Hence If at a point ‘A’ the height of liquid say water ‘h’ in piezometer tube. Then pressure at  A = αgh N/m²

Advantages of piezometer

• Simple construction 
• Not expensive
• Easy to take reading

Limitations of piezometer

• Not suitable to measure negative and high pressure.
• Brittle
• Parallel effect affect correct reading

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U tube manometer

Generally piezometers cannot be employed when large pressures in the lighter liquids are to be measured, Since this would require very long tubes, which cannot be handled conveniently.

So this U-tube manometer consists of a glass tube bent in U-shape, one end of which is connected to a point at which pressure is to be measured is to be measured and other end remains open to the atmosphere.

(1) For positive pressure

Let us consider ‘A’ be the point at which pressure is to be measure. and X-X is the datum line as shown in the fig (1)

Let h₁ = It is the height of the light liquid in the left limb above the datum line.

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

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h = Pressure in pipe, expressed in terms of head.

S₁ = Specific gravity of the light liquid.

S₂ = Specific gravity of the light liquid.

The pressure in the left limb and right limb above the datum line x-x = h+h₁ s₁. So in the right limb pressure head above x-x = h₂ s_2. Hence equating these two pressure, then we get h+h₁s₁ = h₂s₂.

(2) For negative pressure 

It has been shown in the figure (2) In the left limb pressure head above x-x = h+h₁s₁ + h₂s_2. So In the right limb pressure head above x-x = 0. Hence equating these equation, then we have h+h₁s₁ +h₂s₂ = 0.

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