Answer / carl_ellis

Question: How to calculate flow of air bubbler type level transmiter? According hight and density?

I love this question because it deals with the understanding of how a bubbler works - flow is not the issue, pressure is.

The calculated value for a bubbler is NOT flow. The critical, calculated value for a bubbler is pressure, because the air/gas pressure in the bubbler tube is a direct representation of the head pressure of the liquid column; which in turn, can be interpreted as the level of the liquid column.

Measurements of head pressure are always done in units of water column (wc), inches of water column or mm of water column, because water column is universal everywhere in the world.

The column of liquid might not be water, it might be some chemical or solution whose specific gravity (density) is not the same as water. So to determine the level, the head pressure measurement of the air/gas pressure in the bubbler dip tube needs to be corrected by the specific gravity. Level = measurement (wc)/specific gravity

The flow rate of air/gas is typically just enough to create a constant stream of bubbles, proving that the air/gas column in the tube has sufficient pressure to overcome the liquid column head pressure. Flow rates regulated by a constant flow, differential relay range from 0.5-2.0 CFH (15 L/hr to 55 L/hr).

Carl Ellis
Measure First

Answer / madani

Suppose level to be measured is 02 meters from the datum line.Fill the vessel with service liquid of 02 meters
height and adjust the purge rotameter flow until Transmitter output reads 20 ma DC.

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