Measurement of Pressure Exerted by the Gas

The pressure exerted by a gas can be measured by a device called Manometer
. Two types of Manometers are commonly used. These are Open End Manometer and Closed End Manometer.

1.  OPEN END MANOMETER

It consists of a U-shaped glass tube partially filled with mercury. One limb of the tube is shorter than the other. The longer limb is open while shorter limb is connected to glass bulb containing a gas whose pressure is to be determined. The mercury inside of longer limb is under atmosphere pressure while as mercury on the side of shorter limb is under the pressure of gas. There are three possibilities;

 CASE 1:

If Hg level in the two limbs is same, then

                Gas pressure   =   Atmosphere pressure.

CASE 2:

                If Hg level in longer limb is higher, then

                Gas pressure = Atmosphere pressure + h, (Where ‘h’ is difference in two levels).

CASE 3:

If Hg level in shorter limb is higher, then

Gas pressure = Atmosphere pressure – h, (Where ‘h’ is difference in two levels).

Open limb manometer is used to find the pressure of gases of all type i.e. whose gas pressure is greater than atmospheric pressure or less than atmospheric pressure.

2. CLOSED-END MANOMETER

The closed end manometer is similar to open–end Manometer except that the longer limb is closed. The space above the mercury level in longer limb is perfect vacuum. The gas present in the bulb exerts pressure on mercury level in shorter limb. As a result, it moves down and level of mercury in longer limb rises. The gas pressure is equal to the difference in the levels of mercury in two limbs.   

Thus, Gas pressure   =   Difference in mercury level in two limbs.