Manometers are used to measure pressure exerted by fluids.It works on the principle of Pascal’s law.
Pascal’s law states;
In a closed system containing fluid at rest,change in pressure is transferred undiminished through out the fluid.
Manometer uses a column of liquid to measure pressure (generally relative to the atmospheric pressure).The instrument is attached to the system whose pressure is to measured. If the pressure is greater than ,the height of the liquid column increases.
With ref. to the image above,the idea is to measure the pressure exerted by the gas denoted by P.The important points to be considered while measurement:
- In static fluids,pressure varies only with altitude and not horizontally!
- Pressure of fluids increase with increase in altitude and decrease with decrease in altitude.
- The variation of pressure with respect with to altitude is given by rhogh ,where rho is the of the measuring fluid.
In the U-tube manometer,pressure P is exerted by gas on the left leg of the manometer and the right leg of the manometer is open to atmosphere.
The difference between atmospheric pressure and gas pressure P is indicated by height difference ‘h’. This is referred to as gauge pressure which is measured in reference to atmospheric pressure.