Question

List any two direct methods of level measurement. Explain Hydrostatic method of level measurement. Give one advantage and one disadvantage of it.

Answer 1

➢ Direct methods of level measurement

Direct methods

1. Hook type

2. Sight glass type

3. Float type

4. Dip stick

➢ Hydrostatic method of level measurement.

➢ A liquid in a tank at rest exerts a force on the walls of the tank.

➢ This force in a liquid at rest, is known as "hydrostatic pressure", and is proportional to the depth (or height) of liquid in the tank.

➢ Hydrostatic pressure methods used for liquid level measurement are listed

below:

(i) Pressure gauge method

(ii) Air bellows

(iii) Air purge system

(iv) Liquid purge system

Air Purge Method (Bubbler Level Measurement)

Explanation: -

➢ It is consisting of a hollow tube which is inserted in the liquid of the tank.

➢ Two connection are made with the bubbler tube one to the pressure gauge and another to the regulated air supply, calibrated in terms of liquid level.

➢ A bubbler is connected in the series with air supply line which simply as a visual check to the flow of the supply of the air.

➢ A level recorder may be connected with the pressure gauge to keep continues record of liquid level as shown in fig.

➢ When there is no liquid in the tank or the liquid in the tank is below the bottom end of the bubbler tube and the pressure gauge indicates zero.

➢ In other words, if there is no back pressure because the air escapes to the atmosphere.

➢ As the liquid level in the tank increases, the air flow is restricted by the depth of liquid and the air pressure acting against liquid head appears as back pressure to the pressure gauge.

➢ This back pressure causes the pointer to move on a scale, calibrated in terms of liquid level.

➢ The full range of head pressure can be registered as level by keeping the air pressure fed to the tube the range of the device is determined by the length of the tube.

➢ Because air is continuously bubbling from the bottom of the tube, the tank liquid does not enter the bubbler tube and hence the tube is said to be purging

➢ The common purging fluid is air, but, if air reacts with the tank fluid or is absorbed, different gases are chosen depending on the liquid properties.

Advantage: -

1. The purge gas (compressed air) provides complete isolation from the measured liquid.

2. Minimum Maintenance

3. The instrument panel can be located up to several hundred feet from what is being measured.

4. They are very cost effective.

5. It is most suitable for measuring the corrosive or abrasive liquid.

6. Design and construction are very simple.

7. Pressure gauge can be placed above or below the tank level and can be kept as far away as 50 ft (12.7m) from the tank with the help of piping

➢ Disadvantage: -

1. Their calibration gets changed according to variations in product density.

2. Require compressed air.