“Bubbler systems are ideal for level measurement of open channel run off systems or duct and tank situations where debris, foam, steam, or surface turbulence makes standard methods of level measurement impractical.”1
This technique of level detection is primarily employed for vessels which work below atmospheric pressure. A typical bubbler type level measurement device is shown in the figure below.
This arrangement basically comprises of :
- A pipe i.e. a dip tube: It usually has its open end mounted in proximity to the vessel base.
- A purge gas: It is normally an air supply but an inert gas can also be used specifically in areas which are susceptible to contamination or an oxidative reaction with the medium. “As gas flows down to the dip tube’s outlet, the pressure in the tube rises until it overcomes the hydrostatic pressure produced by the liquid level at the outlet. That pressure equals the process fluid’s density multiplied by its depth from the end of the dip tube to the surface.”2
- A pressure transmitter or transducer: It is connected to the dip tube for the purpose of monitoring the pressure.
- A differential pressure regulator: It is mainly needed to create a constant flow of gas necessary for avoiding variations in calibration.
Key features of bubbler level sensors include:
- “Instrument air lines should be trace heated if there is a frost risk.”3
- A bubbler level measurement system should always be calibrated at highest temperature. By doing this, one can prevent the occurrence of overfills.
- The diameter of the dip pipe should be moderately large i.e. around 2 inches since it avoids loss of pressure.
- Where pressurized tanks are used, two sets of dip pipes are required for level detection.
- The purge gas provided in the system must be adequately clean and dry.
Following are the major advantages offered by bubbler level measurement technique:
- Their design and construction is very simple.
- They are very cost effective.
Despite above mentioned advantages, the usage of bubbler level sensors include few disadvantages too. They are:
- Their calibration gets changed according to variations in product density.
- These sensors require timely cleaning and maintenance since the tip of the pipe can gather some of the process material which gets solidified and blocked into the hole.
- They are not considered appropriate for use in non-vented vessels.
- They don’t provide much accurate results for level measurement in case of corrosive or slurry-type applications.