Using Water Instead of Air in a Bubbler System


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I have run into a old bubbler system at a Pump Station that measures liquid level. Instead of air going through a constant velocity regulator and then a bubbler tube, they are using water.

Has anyone ever ran into a bubbler type system that is using water instead of air? I don't see how this can work or give any sort of real reading?
1. Liptak describes an 'aspirated' system with water-air injection as example e in the diagram at the link below

Instrument Engineers Handbook, Liptak, Vol 1, 4th Ed. pages 421-422:
The purge supply pressure should be at least 10 PSI (69 kPa) higher than the highest hydrostatic pressure to be gauged. The purge flow rate is kept small and relatively constant at about 1 SCFH (500 cm 3/min), so there will be no significant pressure drop in the dip tube. Usually, the purge media is air or inert gas, although liquids can also be used.

System E can be considered in remote locations where gas purge media is not available and a bubbler is desired instead of a liquid purge for level detection. Here, water is jetted across a gap while air is aspirated into the stream and compressed. The air-water mixture enters the dip tube, where the small amount of water runs down the inside of the bubbler tube while the pressure of the escaping air is detected as a measure of level. Such a setup would be in service only when the operator wanted to make a level reading, so the water would not flow into the vessel continuously.
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Does the setup you saw include some element that aspirates air into a water flow stream?
They are measuring the level in a wet well in a sewage pump station.

>What are they measuring the level of that a continuous
>supply of additional water does not appreciably affect the
There are many pressure and differential pressure applications when a purge flow is necessary to keep the impulse lines free of the process which can plug the impulse lines. I would suggest that this is what they are doing while at the same time measuring the backpressure at the end of the "bubbler" tube.

good luck