Barometer and Manometer | Working principle , Applications and Differences

Barometer and Manometer both are pressure measuring devices .

Barometer  

Fig. 1 Barometer

It is a instrument that basically used to find out the atmospheric pressure . In principle , any liquid can be used to fill the Barometer, but we use mercury because its high density makes possible an instrument of reasonable size because if we will use other liquid of low density then size of instrument will be so large . With the help of liquid mercury we can predict the weather by tracking changes in atmospheric pressure from the movement of cold and warm weather systems .

Fig. 2 Barometer simple diagram

In fig. 2

p1 = p2

here , p1 represent the atmospheric pressure (p0)

and p2 = ρgh 

here , ρ = Density of mercury  

So , Atmospheric pressure (p0)  =ρgh

Thus , the mercury barometer reads the atmospheric pressure  directly from the height of the mercury column .

For example  , if the height of mercury in a barometer is 76 cm , then atmospheric pressure  will be

p0 =ρgH = ( 13.6×103 ) ( 9.8 ) ( 0.760 ) = 1.01× 105 N/m2

Manometers

Fig. 3 Manometer

The manometers are simple instruments that used for determining the gauge pressure of fluids . There are two types of manometers as in the following

1. Simple manometer

Fig. 4 Simple manometer

It consists of a vertical glass tube, the lower end of which is attached to the point where the pressure is to be measured and the upper end is left open to the atmosphere. It works on a very simple principle .The height of the liquid in the tube balances the pressure in the container to which it is connected .If h is the height of the liquid in the tube, then gauge pressure at point A in the vessel is PA = ρgh where ρ is the density of the liquid .

Limitations of a simple manometer are as follows :

– It cannot measure pressure in a gasses system .

– It can measure positive gauge pressures only in liquids .

– It cannot measure very large pressures because for this , it may require very long tube .

2. U-tube manometer 

An U-tube manometer consists of two tubes connected at the bottom to form a U-shaped tube . It can measure positive and negative gauge pressures in liquids as well as in gases . It works on Pascal’s law. The pressure at a point in a vessel is obtained by the manometric equation . 

Fig. 5 U -tube manometer

For Fig. 5 , let’s write the pressure equation for manometer :

We start from a point where pressure is to be obtained and move towards the free end through the tube. As we move down in the tube pressure increases, as we move up in the tube pressure decreases and as we move horizontally pressure does not change in accordance with Pascal’s law.
In above figure , pressure at point 1 is equal to Pgas . We take point 1 and 2 as reference points because from point 1 to point 2 tube is filled with mercury and drop in height is equal  to rise in height . So, net change in pressure will be zero . So pressure  at point 1 and 2 will be same . If we will go from  point 2 to free end then pressure will be decrease by ρgh .

pressure equal for manometer is ,

 pgas – ρgh = patm = po

 pgas – po = rgh

pgauge = ρgh

Difference between Barometer and Manometer

1. Barometers have one open end and a closed end with a vacuum . Manometers can be open on both ends or closed on one end 
2 . Barometers generally uses for measuring atmospheric pressure ,  while  manometers uses for  measure more precise pressures of other gases or Gauge pressure of gases .
3 . Barometer is little  simple then the manometer , it’s actually a close- end manometer

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