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Liquid Level Detection

Discussion in 'Electrical Engineering' started by Louis Bybee, Oct 16, 2003.

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  1. Louis Bybee

    Louis Bybee Guest

    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

    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.

    Remove the two fish in address to respond
  2. <link to sci.engr.control added>

    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 or similar)

    Why does it have to be non-contact? Displacer-type transmitters will likely
    be much more reliable in a foam situation.

  3. Radar cuts through foam. On the whole radar seems to be a superior

  4. 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.
  5. Louis Bybee

    Louis Bybee Guest

    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

    Thank you.

  6. ripper

    ripper Guest

    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.
  7. Louis Bybee

    Louis Bybee Guest

    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.

  8. Dick Alvarez

    Dick Alvarez Guest

    contact was a customer spec. As I understand it the owner remembered
    having problems with material collection on previous contact type
    sensors. ... The owner insisted on that particular sensor, and a
    solution be found that included the use of it.>>

    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.

    <<I was looking for answers for "the next time".>>

    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
  9. ripper

    ripper Guest

    There is another product you can use, it is called Metritape, but it is
    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.
  10. Louis Bybee

    Louis Bybee Guest

    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

    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.

  11. Hugh Neary

    Hugh Neary Guest

    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

  12. daestrom

    daestrom Guest

    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.

  13. Louis Bybee

    Louis Bybee Guest

    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.

    Remove the two fish in address to respond
  14. Louis Bybee

    Louis Bybee Guest

    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.

    Remove the two fish in address to respond
  15. Rowbotth

    Rowbotth Guest

    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.

  16. daestrom

    daestrom Guest

    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....

  17. B J Conner

    B J Conner Guest

    Suggest a gamma ray ray radiation system. Some of the better I deals you
    have had may seem more practical..
  18. rob

    rob Guest

    Gamma rays?
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