What is Air Purge Method ?


Air Purge Method (Bubbler Level Measurement)

Air purge method is used for level measurement. It is also known as bubbler method. This is one of the most popular method for hydrostatic liquid level measuring system. which is suitable for any type of liquid level. The Bubbler System is an inexpensive but accurate means of measuring the fluid level in open or vented containers, especially those in harsh environments such as cooling tower sumps, swimming pools, reservoirs.

The system consists of a source of compressed air, air flow restrictor, sensing tube and pressure transmitter. The only component of the Bubbler System that is exposed to the elements is the sensing tube.


Air purge method

It is consist of a hollow tube which is inserted in the liquid of the tank. Two connection are made wit 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, slightly above the maximum head in the tank. 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.


  • can be used to measure the level of the wet well to control the intake pumps.
  • can be a retrofit replacement for ultrasonic level transmitters.
  • To Measure Specific Gravity.
  • To measure tank level.


  • The purge gas (compressed air) provides complete isolation from the measured liquid.
  • Minimal Maintenance when an Auto Blow Down system for the dip tube is purchased and integrated with the Level Bubbler.
  • The instrument panel can be located up to several hundred feet from what is being measured.
  • They are very cost effective.
  • It is most suitable for measuring the corrosive or abrasive liquid.
  • Design and construction is very simple.
  • 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.


  • They are not considered appropriate for use in non-vented vessels.
  • Their calibration gets changed according to variations in product density.
  • Require compressed air.


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