Need a proximitiy sensor only detect liquid not foam! Any suggestions? Recommendations?

The positive displacement types were not that small.

-- Les Cargill

For the gas pump at your local gas station, yes, but for the high flow dispensers at fuel depots that fill tanker trucks they mostly or entirely use turbine meters, which are just as accurate as the smaller positive displacement types.

"> The capacitive sensor will not always reject foam IF the liquid is

Ahh, good point.

Bill's AC conductivity measurment sound like fun.

George H.

Under static conditions pressure is dependent on head height _only_, not the shape or size of the container. The overall mass of the liquid depends on the container shape, but not the pressure.

Yes, that is the main problem with them, they are very large and expensive, with many very precise surfaces subject to wear. Well suited to low flow rates like the ~10 GPM gas pump though.

I inspected a vapor recovery system at a large refinery fuel depot 10+ years ago, when such systems were fairly new, and one of their engineers took me through the filling terminal where they fill ~10,000 gallon tankers in only a few minutes, over 10 at a time. The turbine meters were not much bigger than the 4" pipe they go in, only one moving part, very long life with little or no maintenance, and accurate enough for custody transfer, at much lower cost for these high flow rates.

Even more off topic, the vapor recovery system was somewhat interesting. Vapor is recovered from the car tank to the station tank and station tank to the tank truck, with two pipe vapor recovery nozzles at each stage replacing liquid with vapor in the supply tank. The depot tank farms have no vapor space (floating lids) so it is then captured in a pair of large activated carbon absorbers which toggle between use and regeneration when saturated. The regenerating absorber is heated with hot air flow, driving off the vapors to a refrigerated direct coolant contact condenser, where the heated air plus fuel vapor passes through a spray of cold ethylene glycol and water mix. The condensed fuel is separated and pumped back to the premium tank, several hundred gallons a day, and the air with residual vapor goes to the active absorber.

The antifreexe mix level kept going down slowly, requiring a 50 gallon top off every few months, despite a complete lack of detectable leaks. Turns out they were selling it as premium; it was not completely insuluble in gasoline. I expect the addition of ethanol to fuel here in the US required them to change the condenser to a heat exchanger type with no coolant to fuel contact.

Most of the time the erratic pressure sensed is far less than the final volume and as it gets to the top, this subsides due to the liquid suppressing it with its distance of travel.

Most of the time automatic fillers have their pressure adjusted to over come this problem.

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A pressure device used to sense when it is 1 inch submerged would be pretty shape insensitive.

Use 3 or more load cells at the far corners of a platform that the filling container sits on. We use systems like this at work. All load cells on the deck are summed.

Brings to mind an idea long-simmering in my head... measuring the sump level in my salt-water aquarium.

Problems:

Salt water

Salt "mist"

Continual waves due to splashing skimmer filter

BUT:

I only need a low/high indication, but fail-safe sensing and valving... water on the great room floor is not an option ;-) ...Jim Thompson

How about something like a cap-sense chevron stuck to the outside of the tank? OTOH, any "automatic" system will, at some point, dump water on your great room floor, most likely when you aren't home and SWMBO is. Murphy is that way.

A gezillion years ago I designed a capacitance sensor for water level in automobile radiators... inserted in the upper hose.

In the aquarium case I could stick on a capacitance sensor on the "Plexiglas" sump case.

Fail safe might not be too difficult, R-O generation is pretty low pressure. ...Jim Thompson

Cap-sense is trivial to do with today's gizmos. Cheap too.

Probably a better idea. It wouldn't be as conspicuous. Might give false readings if the siphon breaks though.

Fail-safe will, and not be.

Run I/R beams, one for low level, one for high, through the aquarium to detectors on the opposite side, blocking the beams with an I/R opaque float in a clear tube inserted in the tank.

Ed

How about measuring the refraction similarly?

Install a webcam to observe tank, outsource to China or India to observe and report tank level to you. Cheapest solution!

I don't know. To do that, I would think you'd need lenses to get a narrow emitted beamwidth, and would want a known angle of incidence other than 90 degrees. Then you'd have to move the detector around to the maximum response point. I'm not sure how you could do it with a fixed detector/emitter pair and a movable float. Maybe Phil Hobbs will chime in.

Ed

Measuring the reflection off the free surface is one method, though the waves may make that a little tough. Of course sensing the waves themselves might be one option--LED in a black tube on one side, phototransistor in a black tube on the other, look for few-hertz AC modulation of the light.

Cheers

Phil Hobbs

Well, that's certainly "outside the tank" thinking. ;-)

A lens on the receiver should work to pick up the LED. I was thinking of the refractive index difference between water and air, looking through the side of the tank. Waves would create an error but I'd think it much like that a float would give. It should average and even be useful for a fine measurement.