Level Measurement

Bubbler method level measurement principle and installation

What is air purge or bubbler method?

When it comes to monitoring liquid level, density, or interface level in an open tank, the bubble tube concept of hydrostatic measurement is a method that is both practical and inexpensive to use. It is especially appropriate for installations falling within the following categories:

  • It is possible for the liquid from the process to crystallize in the transmitter impulse lines.
  • The temperature of the process is higher than the temperature limit of the flange-mounted transmitter.
  • There are no side connections on the process tank for a flange-mounted transmitter.
  • Because the liquid used in the process is caustic, it must be kept away from the transmitter at all times.
  • Measuring the tank level below the surface, such as the level of an underground pit

Open tank with one bubble tube

Bubbler method level measurement principle and installation 1

What is the principle of liquid level measurement in bubbler system?

Principle of Operation

  • A tube that is partially submerged in liquid receives air through a restrictor. The lower portion of the tube is fixedly spaced above the tank’s bottom (see Figure). 
  • The hydrostatic head on the tube is removed by the air supply, and the extra pressure is seen as tiny bubbles erupting from the tube’s bottom.  
  • As a result, the back pressure in the tube is an indicator of the pressure exerted on its bottom by the liquid level. 
  • Since the tube’s location is fixed, any variation in this back pressure results from a change in the liquid’s level.
  • The transmitter’s back pressure is attached to its high pressure side, while its low pressure side is vented. (open to atmosphere)
  • In this way, the transmitter’s reading of differential pressure might be interpreted as an indication of liquid level.
  • Any change in the transmitter differential pressure must be caused by a change in the liquid density or interface level if the level of the liquid remains constant (continuous overflow). 
  • Thus, measurements of density and interface level are also possible. These measurements may still be taken even if the tank level changes by connecting two tubes of various lengths to the transmitter’s opposing sides.
  • The bubbles can be given through connectors at the side of the tank if it is not possible to insert bubble tubes entirely into the tank.  For instance, in the event that the tank is equipped with a mixer and/or baffles, in the event that the liquid is corrosive, etc.


Calculating the Length Difference in Dimension “H” Using a Set of Tubes

When measuring density, the accuracy of the measurement will increase with the size of the length difference (dimension “H”) between the tubes. Dimension “H” is the intended level measurement range for interface measurements.

Physical restrictions are imposed by the tank’s height and the volume of sludge at its base. A minimum of 75 mm (3 inch) of clear liquid should be present below the bottom tube.

What are the main components of a bubbler system?

Piping Parts List


10×1 mm or 0.25 inch OD; or 10×1 mm or 0.375 inch OD; Pipe: DN 8 or 1/4 inch; or DN 10 or 3/8 inch

Bubble tubes:

Pipe sizes for bubble tubes are DN 8 or 1/4 inch, or DN 10 or 3/8 inch.


Bubbler method level measurement principle and installation 2

A suitable glass rotameter with a valve is required for flow regulation.


Bubbler method level measurement principle and installation 3

1/4 NPT snubbers should be installed at both ends of the process line to decrease or eliminate any undesirable pressure pulsations.

Air pressure regulator:

A regulator for air and gas pressure that is used to control the pressure of purge air or gas.

Open tank with two bubble tube

Bubbler method level measurement principle and installation 4

Installation procedure

  • The appropriate bubble tube and transmitter pipe is chosen.
  • There should be no sediment and a minimum of 75 mm (3 inches) of clear liquid below the tube’s bottom. A minimum of 75 mm (3 inches) of liquid should also be present above the upper tube in two-tube setups.
  • The bubble tube assembly should be placed in a representative liquid region with little liquid turbulence.
  • The bubble tube assembly has to be firmly secured in place.
Bubbler method level measurement principle and installation 5
  • The bottom of bubble tubes should be notched so that air emerges continuously as little bubbles rather than sporadically as giant bubbles, which might cause inaccuracies. For further information on this notching, see above Figure . (Side-connection installations are exempt.)
  • Air quality, including temperature and moisture content, must be suitable for the process liquid.
  • An inert gas (such as nitrogen) can be utilised if air interacts with the process liquid.
  • To reduce the usage of air, a differential pressure regulator may be appropriate
  • It is not recommended to utilise the rotameter needle valve for tight shutoff. Installing a hand valve upstream will allow for service or total shutoff of the purge medium as needed.
  • Install a pressure regulator downstream of the cut off valve if the purge supply pressure is significantly variable or exceeds the maximum rating of the rotameter or differential regulator.

What is the operation of bubbler system?


  • Controlling the flow of air is one of the functions of a restrictor. A needle valve located at the bottom of the rotameter is used to do this.
  • Adjust the air flow using the needle valve to get the best possible size of the little bubbles.
  • In the event that a DP feedback regulator is utilized, the needle valve on the regulator will be utilised to control the flow of air.
  • When taking measurements, you should always make sure that the open ends of the tubes are covered with liquid from the tank.
  • When performing measurements at the interface, the maximum level must be lower than the open end of the upper tube, and the minimum level must be higher than the open end of the lower tube.
  • In the event that the range has been changed, the transmitter will need to be recalibrated to the new range. modifying the “H” dimension could be necessary for the new range.
  • It is not recommended to utilise the needle valve on the rotameter as a system air shutdown; rather, you should use the upstream valve at the air supply to the system.
  • It is essential that the liquid level not drop below the bottom of the bubble tube (or the tank connection).
  • Check for leaks in all of the connections in the bubbler system installation.

Sundareswaran Iyalunaidu

With over 24 years of dedicated experience, I am a seasoned professional specializing in the commissioning, maintenance, and installation of Electrical, Instrumentation and Control systems. My expertise extends across a spectrum of industries, including Power stations, Oil and Gas, Aluminium, Utilities, Steel and Continuous process industries. Tweet me @sundareshinfohe

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