The Manometer 
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The Manometer : it's based on the pressure difference between Pa and the pressure of enclosed gases, it's used to :

1- Measure the pressure of an enclosed gas .
2- Detect variation in atmospheric pressure .

It's formed of uniform glass U-tube containing a suitable liquid,  one end is connected to the enclosed gas while other end is open to Pa .

The difference between the two levels of the liquid is measured   Delta P = rhg

if Pgas>Pa : level in free branch is higher by "h" Pgas = Pa + rhg   "h" is called +ve column .

If Pgas<Pa : level in free branch is lower by "h" Pgas = Pa - rhg    " h" is called -ve column 

 

 

 
NB : 
Mercury is preferred in case of high pressure difference, to produce reasonable height of liquid column due to it's high density.

Water or oil are preferred in case of low pressure difference, to produce remarkable length of liquid column due to their low densities .

Work done to transfer a liquid through a cylindrical tube =    Force * Displacement
W = F * S = (Delta PA) * S = Delta P * V = Pressure difference * volume = rhgV