Liquid Level Detection

I recently finished the startup on a Waste Water Reclaim System. Most of the
system came up rather uneventfully with the exception of the gray water
storage tank. It had a tendency to form froth on the liquid surface.
Accurate liquid level detection in this tank is important for stable system
performance.
An ultrasonic level detector was factory installed.
It became apparent rather quickly that the sensor was detecting the froth,
and the resulting false reported liquid level in the tank was responsible
for frequent cycling, and system shut down.
I solved the issue in the short term by installing a large cylinder in the
tank, centered under the sensor, with holes in the lowest portion. That
allowed system startup, and tuning, to be completed.
My question is the availability of a non-contact sensor that will detect
true liquid level through foam, or froth?
Thank you.
Louis--
*********************************************
Remove the two fish in address to respond
Reply to
Louis Bybee
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If the ultrasonic sensor is any good, you should be able to adjust it to work through foam. If it's a cheapie preset one, then you need to buy a real one (like
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or similar)
Why does it have to be non-contact? Displacer-type transmitters will likely be much more reliable in a foam situation.
Cameron:-)
Reply to
Cameron Dorrough
Radar cuts through foam. On the whole radar seems to be a superior solution.
Walter.
responsible
Reply to
Walter Driedger
How about a separate standpipe piped in from the bottom of the tank to a location alongside. The standpipe would only have water displaced from the bottom, no foam.
Louis Bybee wrote:
Reply to
**THE-RFI-EMI-GUY**
responsible
Non-contact was a customer spec. As I understand it the owner remembered having problems with material collection on previous contact type sensors.
I agree with you, and suggested a pressure transmitter with an air bleed, but the owner wasn't receptive to anything except an ultrasonic (they were in use in other areas of the plant (in non-foaming environments) without trouble).
Thank you.
Louis
Reply to
Louis Bybee
If the water is relatively clean you could use submersible pressure transmitters, alternatively a DP cell. If the water has suspended solids and you have compressed air to hand, you could use the good old bubble tube and DP cell. Also Milltronics make the radar type reportedly good for foam, but I haven't tried them so cant comment.
responsible
Reply to
ripper
That is essentially what I created. I used a large piece of PVC pipe installed vertically in the tank with holes drilled in the very bottom, and the top of the pipe was above the liquid/foam level.
Even with the water level never dropping below the holes in the pipe there was enough turbulence in the water that some foam accumulated inside the pipe, and had to be cleaned out once a month.
The owner insisted on that particular sensor, and a solution be found that included the use of it.
Now knowing the limitations of an ultrasonic sensor in a foam environment, I was looking for answers for "the next time".
Thank you.
Louis
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Reply to
Louis Bybee
Louis Bybee wrote Unless a customer really knows what he is doing, some times it is necessary, and quite ethical, to educate the customer. That generally is one of the functions of an engineer. A good customer appreciates it, and it builds good-will for you. This is not my field of electrical engineering, but I got involved in liquid level controls in our Boy Scout camps. I used mostly Warrick Controls electromechanical and electronic conductivity sensors and controls for water wells and tanks and for sewage pumping. Probably you can choose the sensitivity to detect the liquid but not to the foam. Warrick makes a variety of controls, electrodes, electrode fittings, etc. Unless the foam is really exotic, it should be possible to select electrodes that will be resistant to the foam's effects. For example, you might want to use standard rod-type electrodes that are coated with Teflon or equivalent almost to the lower end. You probably would not want to use simple wire-suspended shielded electrodes, as they could become clogged with foam. Warrick has a Web page. You may need to study it a bit to see your choices on sensitivity and electrodes etc. Then contact their representative. I used Calpacific Equipment Company, in Berkeley, California. A Google search shows many representatives. Other manufacturers make similar equipment, but I am less familiar with them than with Warrick. More elaborate systems, like ultrasonic, radar, etc., have their places. But where a simple system will do the job, generally it reduces cost and increases reliability. Beyond simple float-switches, conductivity sensors are pretty simple. It was a long time ago that I worked on such things in Scout camps, so I an not current on such equipment. (It was a long time ago, because the equipment still is there and functioning.) Dick Alvarez alvarez at alumni dot caltech dot edu
Reply to
Dick Alvarez
I would set down with the owner, tell him that the ultrasonic is reading the foam level and ask him what to do. If he says something like "you are the expert", tell him that the expert already suggested a solution which he rejected, so now it's his responsibility to either listen to the expert, hire another expert, or solve the problem himself.
If keeping this job is important to you, write up a report with all known solutions with costs, advantages, and disadvantages, and let him pick one. Or hire me and let *me* tell him that his preferred solution won't work .
Reply to
Guy Macon
There is another product you can use, it is called Metritape, but it is expensive. Basically it is a resistor encaplsulated in a tube. The head of liquid compresses the tube thus varying the resistance. Do a search for Metritape.
Reply to
ripper
Thanks to all who responded.
I have received some excellent suggestions, and sources, that will allow me to offer reasonable solutions the next time this comes up.
The building owner refused to even consider other alternatives. Even with multiple people advising that the sensor he requested was considerably less than desirable for the application. Even the company that built the recovery system advised in writing that they wouldn't be responsible for the results. With our litigious society today I'm surprised they even consented to build it.
The system has been running successfully (with monthly cleaning of the well pipe) now for about six months. I can't believe how bull-headed the owner was! He had his mind made up, and wasn't about to be confused with any facts, or undesirable results! :-]
Thank you.
Louis
Reply to
Louis Bybee
Is the foam a uniform blanket, or is it isolated patches that float on the surface. What is the chemical producing the foam?
If the foam is not too heavy or of uniform thickness and density it should be possible to tweak the ultrasonic. Failing this a chemical de-foaming agent or modification to your process would have to be considered.
HN
Reply to
Hugh Neary
Do you have access to some low-pressure air? One system that is pretty immune to dirty water and contamination is to bubble air down through a tube to near the tank bottom. Use a fine needle-valve to control the flow rate to a small, steady amount. The pressure in the tube downstream of the needle valve will be proportional to the liquid level. Then a pressure switch or transducer can give you the level.
The inside of the tube is rarely wetted (only on a loss of air), so it doesn't clog.
daestrom
Reply to
daestrom
The foam is non-uniform in level, and consistency. It is formed primarily from soaps, and other related compounds. This is an effluent tank with a rather high throughput so chemical treatment specifically for defoaming is undesirable from many levels.
Others have suggested solutions that appear to be very workable.
Thank you.
Louis-- ********************************************* Remove the two fish in address to respond
Reply to
Louis Bybee
responsible
One of the sensor types I suggested was a pressure transducer with purge air (along the lines of a bubble tube). I have used these in the past where corrosive chemical level was measured.
The customer didn't want any external level measuring devices such as, sight glasses - still wells - etc., on the tank. He insisted on specifying sensors, and left others to make the misapplied ones work.
This owner was a real pain. His requests were rarely based on rational data. I can only imagine the pleasure of working for him on a regular basis.
Thank you.
Louis-- ********************************************* Remove the two fish in address to respond
Reply to
Louis Bybee
If you can live with a range where the detector operates - for instance, the tank may be 50 feet deep, but if you can live with a detection system that could be installed in the top 10 to 20 feet - you might be able to do something with a radioactive device. These can be programmed to detect the level of the substance you are looking to control, and they would definitely ignore foam.
I know Ronan Engineering used to make these - as far as I know they are still in business. Seems like an anwer.
HR.
Reply to
Rowbotth
Vega make radar based sensors just for this purpose. The antena horn on these used to be large (40cm long) but is now less than 1/2 that length and the price was about $US2500 and the price has now halved.
Reply to
The Enlightenment
Yeah, I know just the type. Had similar customers myself. That's when you look at your quarterly income statement and decide just how badly you need the work ;-) Hate it when they hire you for your expertise, then ignore your advice. The worst ones are those that then criticize the results and conveniently 'forget' that half the design was their idea.
Oh well, you can only do so much....
daestrom
Reply to
daestrom
Suggest a gamma ray ray radiation system. Some of the better I deals you have had may seem more practical..
Reply to
B J Conner
I agree, use either radar or a bubbler. I have had good success with bubblers in river water intake systems where there is trash and such floating on the surface. Of course, if non-contact is a must, radar is likely the way to go.
TZ
Reply to
Zman

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