Anybody know of a proximity sensor that will not be submersible that will detect the rise of a liquid, and not of the foam the liquid creates? It is for a gallon filling machine. I already have a sensor but I'm having problems with the foam. Doesn't let me fill it to the top.
Thanks,
Ignacio
From the brief description, I assume that it's okay for some foam to overflow as long as the bulk liquid does not. Home brewing?
Perhaps, pair an air jet with the sensor probe, shaped and directed so that the area seen by the sensor is foam-free? May not need to be much more than a muffin fan, a funnel, some tubing, and a couple of cable ties.
There's probably a good DSP solution, depending on your current sensor type, that looks for the deepest hard return above the bottom of the container.
-- Rich Webb Norfolk, VA
Strain gauge to measure the weight of the tank?
Vladimir Vassilevsky DSP and Mixed Signal Design Consultant
A float?
Good Luck! Rich
On a sunny day (Wed, 30 Sep 2009 12:41:10 -0700) it happened Rich Grise wrote in :
He could use a speaker and microphone, and cause acoustic feedback. the frequency then depends on the depth of the 'pipe'. Or ping the vessel, look for resonance, same story.
If cost is not much of a factor, radar is used in some applications.
A float wouldnt work, since the cavity of the gallon is but an 1.5 in, and liquid needs to be coming in that single opening, plus I want the user to just slide the gallon in without hindrances. Weight was a possible option, but then I guess i need some springs and design the table where the gallon stands (need to look into that). I was avoiding timing since I thought it was not needed and was more complicated since I'm filling 8 gallons at a time, from a single tank, and didn't know flow currents, etc, but well that man end up being the solution, time it from the beginning of flow to fill.
Yes foam can overflow no problem.
Otherwise I thought a sensor less sensitive to foam will just solve my problem straight, IF there is one... Cost is not tooooo much of a problem..
The air jet solution i had thought of, might be the solution too, will try that after i've found what sensors I have to work with.
Hey thanks everybody. Keep it coming if you have any more ideas/ or sensors you know..
The resonance won't work, because the frequency depends on the mass density of the oscillating medium, and foam is a _lot_ denser than air.
The Helmholtz resonance (which is what you get when you blow across a beer bottle) is not a transmission line resonance like an organ pipe, it's a mass-spring resonance--the air in the neck is the mass, and the rest of the air is the spring. The formula is approximate, of course, but it works really well.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
There are two solutions that are kinda easy: one is a bubbler (just a tube with a tiny air pump); if the end of the tube dips into the liquid, the bubbles generated cause a bit of sound that can be detected with a microphone.
The other is a capacitive probe; if the liquid level is in a nonconductive jug, you put a pair of collars around the neck and call those a capacitor; an oscillator (74HC14 is fine) will make a frequency that depends on the capacitance, and if the bore fills with liquid, the capacitance goes up and the frequency goes down. Detecting the frequency change (maybe an NE567 or CD4046 or similar phase-locked loop is the easiest way) completes the task.
It sounds hard, but those capacitive sensors are, I hear, common chemistry-lab appliances, and you could even use a hardware-store stud finder for the purpose (it's really the same function inside).
Yes, I was peripherally involved with a proposal a while back for a tanker ship fill and trim control system and, IIRC, radar-based tank level indicators were quite popular.
Ultrasonics may work for the barrel-filler, although there might need to be more examination of the return than just finding the peak amplitude. But the fill flowrate, even if not constant, should at least be able to set a gate around an expected surface return.
-- Rich Webb Norfolk, VA
The ones I know of used for pipelines, use a radioactive source on one side of the pipe or column and a detector on the other.
They effectively measure the density. Foam has a far lower density than the liquid.
There is also a variant that uses the known dielectric constant of the liquid and puts capacitor plates on either side, and looks for capacitance changes. This works OK in really clean environments but not so well in the pipeline business.
The pipeline people calibrate the radioactive attenuation so well that they can put different grades of crude through the same pipeline at different times, and automatic switching equipment at the receiving end automatically knows which tank to send it into based on arriving density. The pipelines are long enough that arriving density can lag sent density by hours. This lets them use a single pipeline to move many different grades of crude and other petrochemicals.
There is a kind of depth gauge for use in liquids, which is like a really stretchy balloon long with capacitor plates on either side. Pressure from the liquid collapses the balloon, the plates get closer, capacitance goes up. To a very good approximation the resulting capacitance is proportional to the collapsed length. Similar sensors are used in reel-to-reel high speed tape drive servomechanisms to measure position of a tape loop in a vacuum column.
Tim.
On a sunny day (Wed, 30 Sep 2009 16:18:08 -0400) it happened Phil Hobbs wrote in :
That is the air, but if you ping, or maybe I should say 'tap' the vessel, then you get a tone, or specific sound if you will, even if it was filled with vacuum for the rest of it. Like tapping a hollow head versus a filled one (for example). The spectrum you get (maybe you could do a fft), could be analysed, and calibrated one time against some known fluid levels. Then the micro could drive a nice LCD display, and send data via for example a serial link.
On a sunny day (Wed, 30 Sep 2009 16:05:51 -0400) it happened Spehro Pefhany wrote in :
The radar is the one that is foam sensitive.
Atmel capacitive proximity sensors....should do the job, and cheap.
=A0
can't you move the filling pipe with a sensor down mounted on it a few inches down during the filling? valve on - pipe down, valve off - pipe up
-Lasse
=A0
What are you using now for the sensor? I recall some level sensors that measured the capacitance between two concentric tubes. C depends on the dielectric constant of the liquid and the height within the tube. If the fluid is conducting you'd have to put an insulator around the tubes.... that sounds problematic.
George H.
"you put a pair of collars around the neck and call those
Ahh, put the capacitor on the outside. Not sure I get the geometery though. Are you suggesting two rings one above the other? Why not put the plates on each side of the neck?
George H.
You could insert a second tube into the jug, and measure the level in that tube, usually by pressure. The foam doesn't cause pressurization...
Charlie
Google for "anti foam filling" for better ideas that a second tube.
The OP did not fill us in with the apparatus, just that its a "gallon filling machine".
More information was asked for in an earlier message, but the OP must have moved on.
don
Had to do this for a milk tanker once; milk is electrically conductive, and so is milk foam.
A tolerably precise conductivity sensor did the job. I set up an AC bridge, using a centre-tapped bifilar-wound transformer for two arms of the bridge, and the conductivity sensor and fixed resistor for the other two.
The transformer was part of a Baxandall Class-D oscillator running at around 10kHz, which meant two pairs of RM6 ferrite cores, and I used a couple of CMOS transmission gates to demodulate the output.
Worked fine.
-- Bill Sloman, Nijmegen
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