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Fluid Level Sensors

  1. Nov 17, 2009 #1
    Hi All,

    Wondering if someone can give me some pointers on some fluid level sensors or switches.

    He is a short run down on what I want to achieve.

    What we want to do is create a system where we can keep a fluid level (dye based Ink) a constant level inside a chamber.

    We are confined to a very small space so these need to be small so we can not use any type of mechanical float valve as these are all to big. The chambers are about
    10mm wide 30mm Long and 30mm Deep.

    The plan is to install an electronic valve on external tank that will trigger to open when the fluid level drops inside the chamber

    I am sure there is quite a range of sensors that I can use but rather than start testing whatever I can find I thought I would ask the question here first.

    Cheers!!
    Paul
     
  2. jcsd
  3. Nov 17, 2009 #2

    vk6kro

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    There is a way of doing this that occupies little space and can't contaminate your solutions.

    You get a gas pressure switch and attach a flexible plastic tube to it.
    Then place the other end of the tube just under the top of the fluid, pointing downwards.

    Just the little bit of extra pressure in the trapped air is enough to trigger the switch.
    Mine only need to be less than a quarter of an inch under the solution.
    When the liquid level drops, the switch goes back to its normal condition.
    So, you could use this to pump more solution into the tank.

    I have a couple of these switches and they are marked "World Magnetics, Traverse City, MI, 49684". Mine are model PSF 102 but they are a few years old, so that may not be current.
    Try this web site:
    http://www.designflexswitches.com/
    They even offer free samples.
     
  4. Nov 17, 2009 #3
    Hi,

    Thanks for the reply..

    Using a pressure type switch is possible but I would prefer to try an avoid a sealed chamber
    if I could, we were doing something before which relied on the chambers being sealed but it proved difficult as the plastic chambers are made from a poly plastic and are difficult to glue/seal, also the level of ink inside the chamber has to be only 5-6mm from the base of the chamber which is quite low

    What we are wanting to do is not a one of product, we manufacture these systems and would like to produce them in a way where we can accurately keep a constant level of ink in the chamber


    Paul
     
  5. Nov 17, 2009 #4

    dlgoff

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    I don't believe vk6kro meant you have to have a sealed container. One could bubble air and use a pressure switch set to the range to do the pump triggering.
     
  6. Nov 17, 2009 #5

    vk6kro

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    The only sealed chamber is in the plastic tube which goes from the pressure sensor to the liquid.
    This has a small quantity of air in it and the rising liquid in the mouth of the tube seals it off and compresses the air in the tube.
    This is what operates the switch.
     
  7. Nov 17, 2009 #6
    Hi,

    Yes, I have just read the details from the link that vk6kro posted and I see that it is the pressure change within the fluid it self rather than the entire chamber.

    How close would we need to install the switch to the chamber? would it need to be sitting on top of the chamber or can it or can we run it externally away from the chamber?


    Paul
     
  8. Nov 17, 2009 #7

    vk6kro

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    It does not use air bubbles. If the tube is fixed to the inside of the container and the fluid rises in the container, it compresses the air in the tube.

    These switches are so sensitive that only a 1% compression in volume of the air in the tube is enough to trigger the switch.

    I used about a foot of plastic hose but if you used a longer hose, the switch would still work. However, the end of the hose would be deeper in the ink before it triggered the switch.

    The ones I have have a normally closed switch, so this could directly switch on a pump and the pump would switch off when the ink level was high enough.

    If your ink was solvent based, you would need to talk to the maker about compatibility with the plastic of the switch, although it would be some distance from the switch.
     
  9. Nov 17, 2009 #8
    OK so far so good, 1 foot would be more than enough length out of the chamber in fact we could probably get away with about 6 inches..

    However another thought just crossed my mind, the chamber also moves back on forth during the print process which would interfere with the pressure as well would it not?

    I guess we could set up a couple of sensors so the pumps are only trigger wild the chambers are stationary or in a particular spot
     
  10. Nov 17, 2009 #9

    mgb_phys

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    Another common system for industrial inkjets is a floating bead with an embedded magnet and a magnetic sensor on the outside of the cartridge
     
  11. Nov 17, 2009 #10
    Yes, I have looked at magnetic sensors but as we are very limited for space it was not possible. The Cartridge has three internal chambers with the sizes mentioned in my first post.
    I have external access to two of the chambers but not the third and as the chambers are very close together I thought we may get interference between the magents
     
  12. Nov 17, 2009 #11

    vk6kro

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    If the ink sloshed around, the ink could move away from the end of the tube and cause the pump to turn on. So, yes it would be worth waiting until the tank stopped moving before filling the tank. Or, you could put a shield around the tube (but open top and bottom) so that waves moving across the ink did not reach the tube.

    Hi, MGB. Note the floating bead method.
     
  13. Nov 17, 2009 #12
    Guys a while back we put a little thought into using waterproof ultrasonic sensors as well but thought these would not be accurate enough, basically by measuring the distance between the top of the chamber and the top of the ink level. Has anyone had success with these in this type of environment?
     
  14. Nov 18, 2009 #13
    I've never heard of it done on this small a scale but could you use two parallel conductors and measure resistance or capacitance as the ink level fell.
     
  15. Nov 18, 2009 #14
    That would be the simplest method and the certainly the cheapest.

    I am no expert with a multi-meter by any means but there is a level of resistance in the ink, it is a dye based ink so I would expect there would be a fair percentage of water in it anyway

    I stumbled across this last night which might be a starting point for us to test with:
    http://www.quasarelectronics.com/smart-kit/1080-liquid-level-sensor-or-rain-alarm.htm

    What else would be needed to test this using the resistance to trigger the level?
     
  16. Nov 18, 2009 #15
    Here is a capacitive liquid level sensor that is very sensitive. Originally designed to measure fluid level in syringe. The dielectric constant of water is about 80. The sensor in this circuit uses an ac signal, so may be insensitive to conductance of ink.
    http://www.discovercircuits.com/DJ-Circuits/Capmeter1C.htm
    Bob S
     
  17. Nov 18, 2009 #16

    dlgoff

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    I'm thinking that if you have all that sloshing to deal with while the tank is moving, you are going to see it with any technique. So I would stick with the pressure switch and use a switch (relay contacts perhaps) to open the pump circuit while the tank is moving.
     
  18. Nov 18, 2009 #17
    Yes, a pressure switch would be most accurate but the issue with the switches is the space they require, each cartridge has three chambers which will require a 3mm tube coming out of each chamber, bare in mind as the cartridge moves back and forth these hoses (along with the ink feed tubes that are already there) will need to move with it which may cause excessive drag.

    It is a tricky one I know, previously we have done them using sealed chambers and relied on vacuum created by the ink level lowering in the chamber but it is proving to be difficult to keep the chambers perfectly sealed as even the slightest air leak will cause it to stop drawing ink.

    No matter which method we go with we have to use a trigger where it can only measure when the cartridge is stationary so this would not be a problem

    Bob S, Thanks for the link I will take a look at that, the capacity of a syringe would be similar to the size of our chambers...
     
  19. Nov 18, 2009 #18

    vk6kro

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    You obviously have to consider all options, but it would be worth getting one of those free samples on the way to you so you can test the concept. As a manufacturer, you should have no trouble getting switches as samples. Get on the phone and explain what you are trying to do.

    Keep in mind that conductivity and capacitive methods will require electronics which would have to be developed and then housed and given regulated power sources.
    Ultrasonic and optical methods would require even more electronics.
    A pressure switch can directly switch a small pump or a relay.
    Accuracy was very good. Probably within 1 mm.

    I would mount the switches on the moving ink containers and bring wires out from there. I used plastic tubing as used for aerating fish tanks, but I'm not sure how it would cope with being flexed repeatedly.
     
  20. Nov 19, 2009 #19

    dlgoff

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    The way I look at it is; tubing is cheap. I've seen many applications using small moving tube that last for years.
     
  21. Nov 19, 2009 #20

    vk6kro

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    Agreed. There is always silicone tubing if this was going to flex the tubing for hours at a time.

    The important thing would be to stop any kinking of the tube that might cause it to rupture eventually.

    Probably the ink doesn't need to be checked every second. Maybe every 10 minutes or so, the whole thing could be brought to a halt and a pressure reading taken to see if the ink needs topping up.
     
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