Parallel plate capacitor

  1. Hi evry1

    I am trying to make a parallel plate capacitor , which am using to make a capacitive proximity sensor.

    Question 1

    I thought of making 2 plexiglass plates of dimension ( 90 X 50 X2.5 mm). An aluminium mesh is sandwiched between them. Now a positive voltage is supplied to aluminum mesh and the plexiglass plates is surrounded by another aluminium mesh such that the mesh between the plexiglass and this mesh is perpendicular to each other. This surrounded mesh is connected to ground.

    Will such a combination work??

    Question 2
    Another option was to join 2 aluminium plates together with a piece of plexiglass as dilectric in between them.

    How can i find the value of capacitance in both cases??

    Thanks for any ideas
  2. jcsd
  3. dlgoff

    dlgoff 3,148
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  4. vk6kro

    vk6kro 4,058
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    I like the first option.

    Capacitors with parallel plates are fairly immune to the surrounding environment.

    Mounting the plates at right angles like that would make it very sensitive to the surroundings.

    (And that may be a problem, because it would be very likely to pick up mains hum and other types of electrical interference.)

    Measuring the capacitance depends on what you have.
    I would guess that the capacitance would be about 20 pF. Probably less if anything.

    Putting DC on it probably won't help, but if you had an oscillator giving about 100 KHz out and put this in series with a resistor and the capacitor to ground ... then take this to an opamp with high impedance input, you just might get enough change in amplitude to detect something approaching the plates.

    Or, you could put two small capacitors in series and this capacitor to ground between them.
    The output from the series capacitors would depend a lot on the capacitor to ground, so you might be able to detect this.


    I have attached a drawing of these methods, but I don't want to imply that this is a simple project or that the drawings are complete plans. They might give you some food for thought, though.
  5. One thing of which am not clear is how is the electric field when the plates are perpendicular to each other?

    Is there any kind of isolating sheet, to cover that area so that it may not pick up other interferences?
  6. vk6kro

    vk6kro 4,058
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    This is pure speculation, but I would guess that the electric field would be like this:

    electric field.PNG

    The important part is that the field would also extend beyond the area shown and the dielectric constant of whatever was in that field would affect the total capacitance.

    Mounting the plates at right angles seems like an unusually good idea.

    It does leave the detector vulnerable to interference but I suspect that the use of a high frequency oscillator as a signal source will help with this. Lower frequencies can be filtered out.
  7. Suppose if the two parallel plates are placed such that ( L X B = 55mm X 25mm) ; d = 97mm and air in between
    C = ( k * epsidon0 *A )/d gives --------0.125pF.

    Is that value shows that it is a bad setup?
    When water comes in between the plates, the k = 80 and the capacitance value = 10.04pF.

    So is such a setup good or bad??
  8. Are you trying to measure the presence of water, or a water level? I have seen capacitive water level measuring systems where the capacitance is part of an oscillator circuit that changes frequency as the water level rises. Is this distilled water, de-ionized water, fresh water?
    Bob S
  9. i am measuring the presence of water. I am attaching the two plates on the upper and lower parts of a pipe (inside). Water comin through is rain water.
    I was trying to use an oscillator as vk6kro mentioned in the 3rd post of this thread.
  10. One version I recall was an oscillator where the capacitor was part of the resonant circuit so the oscillator changed frequency when the water was present. The capacitor I think had a long outside conducting grounded tube, and a coaxial inner conductor. The oscillator output went into an updating monovibrator (one-shot), which constantly updated when there was no water, but when the oscillator frequency was too low, the one-shot constantly timed out.

    [edit] Here is a possible capacitace-based liquid level monitoring ckt:
    One important feature (not pointed out) is that the signal on the capicitance probe is ac, so there is no electrolysis.
    Bob S
    Last edited: Nov 13, 2009
  11. Hey Bob S

    I have alread seen that liquid level capacitor. But in my case i there are other materials that are coming along. so i need to detect both of them ( water and materials). That is why i thought of implementing a capacitive proximity sensor.
  12. AT the moment i am trying 2 versions of the solution.

    Solution 1:

    Two aluminium plates mounted on the top and bottom sides of a pipe with a distance of 10cm between them and area of plates = 0.0025 m2.

    I have calculated a graph to show the variation of capacitance on different levels of water. If water is not completely filling the gap between the plates, then the total capacitance is the sum of 2 capacitances in series.
    C tot = epsidon * A /[(d1/k1)+(d2/k2)]
    where d1 = level of water ; k1= dielectric of water = 80 ; d2 = corresponding height of air;
    k2= dielectric of air = 1.

    See the [​IMG]

    The values of capacitance changes very negligibly for up to 80mm of water level. Does this mean that such a parallel plate capacitor will not produce good output.( even if we use a high frequency oscillator and high input impedance in the circuit???)

    Solution 2:
    A mesh of aluminium sandwiched between two plexiglass materials and another aluminium mesh perpendicular to this mesh surrounding them.

    With the solution 2, one problem is the fixation of such a capacitor in the pipe. That is why it has been given the second place.

    Is there any better way to modify the solution 1 to get good results???
  13. vk6kro

    vk6kro 4,058
    Science Advisor

    You would have to know that the water was very pure before you could calculate using its dielectric constant.
    If it was tap water, it would have enough conductivity that you could regard the top of the water as the bottom plate of the capacitor.

    I wondered if you could take advantage of the water conductivity like this:

    Water level.PNG

    You have plates of metal along the outside of the container at the sides.

    If the water is fully conducting, there will be capacitance from the water to the plates through the walls of the pipe.
    So, it would be like two capacitors in series, however the actual capacitance could be quite high depending on the length of the plates. The capacitance should be directly proportional to the height of the water.
  14. The pipe is circular in nature. so how is it possible to have as you mentioned in your picture?
    Here is a picture of what it looks like.
    The plates could be either outside or inside.In the picture it is shown inside.

    Attached Files:

  15. vk6kro

    vk6kro 4,058
    Science Advisor

    You could still use it with a circular pipe. The capacitance would depend on the thickness of the plastic pipe, its dielectric constant and the length of the plates and the height of the water.

    This diagram shows it looking along the pipe.

    Water level 2.PNG

    The plates would have to be outside the pipe, but this should be an advantage.

    What is this for?
  16. vk6kro

    vk6kro 4,058
    Science Advisor

    I think you could get better capacitance by putting the two outside plates in parallel and making a contact with the water.
    Another advantage of this would be that you could use it if the water in the pipe was grounded electrically.

    Water level 3.PNG

    Probably, the outside plates could just be a metal pipe which fitted over the plastic one.

    You could make a hole in the top of the plastic pipe and have a connection from there to a stainless steel plate in the bottom of the pipe. This would be to avoid leakage problems at the hole.
  17. Hi vk6kro

    Do you mean to mount the plates in a curved way? from your drawing i understand as if you have placed the plates along the curvature of the pipe.

    Or did you actually mean like this ?? (diagram below)
    both Aluminium plates on the sides connected to + and the middle aluminium plate to ground???

    If the plates are mounted along the curvature, then i guess the C is not same as calculating for the parallel plates, right?

    This setup is needed to detect the presence of water or any other objects inside the pipe.
    Last edited: Nov 19, 2009
  18. Born2bwire

    Born2bwire 1,776
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    Gold Member

    Obviously vk6kro can answer this more clearly but in essence he is talking about a two "plate" capacitor. One plate is a metal sleeve surrounding the pipe, the second is a strip of conductor that runs along on the inside of the pipe. The pipe itself will separate the two "plates" and provide a small amount of dielectric.

    I was thinking of a similar idea, running a metal sleeve and then having a central rod, but that was before I realized you wanted to run the pipe horizontally, not vertically.

    So in your diagram, the plates outside would be shorted together and curved to fit the exterior. The inner plate stays the same. You can connect the inner plate to ground, which as v6kro mentions, probably will set the water to ground as well since it will probably be contaminated enough to be conductive.

    I don't know of a closed form for doing this configuration but you can do a numerical simulation easily enough. The main question would be how you estimate the water but giving it a slight conductivity is probably realistic.
  19. vk6kro

    vk6kro 4,058
    Science Advisor

    The curve would have to follow the shape of the pipe.

    We are assuming the water is conductive, so it fills the pipe to some height making contact with the inside of the pipe.
    Then we want just the thickness of the plastic pipe to be the dielectric. So the outer conductor has to be as close to the outside surface of the pipe as possible.

    This way, the capacitance should be quite large and measuring it should be easier too.

    You can get very thin copper sheet from craft stores.

    You could have an oscillator like one of the following
    555 and schmitt Osc.JPG
    which would generate a square wave whose frequency depended inversely on the depth of water in the pipe. These oscillators have one side of the capacitor grounded, which may be necessary in this case if the water is grounded.
  20. Sorry for my lack of knowledge.. I didnt understand it completely.
    If two copper plates are curved to fit on the outer surface of the pipe and a stainless steel plate is placed inside in the middle of the pipe,how is the capacitance value calculated? i mean capacitance without the presence of water? When water comes, it changes the overall capacitance of the oscillator, right?

    Pipe has a thickness of 4 mm; DImension of copper plates = 130 X 65 (L X B). DIlectric constant of pipe = around 4; height of water varies from 1 mm to 100 mm. lets say 50mm for instance.
    Last edited: Nov 19, 2009
  21. Born2bwire

    Born2bwire 1,776
    Science Advisor
    Gold Member

    No easy way. If we had two full cylinders within a cylinder then you could do it using closed form equations. Essentually you are just doing a Poisson equation, though with the inhomogeneity of the dielectric pipe adds further complications. You would set the inner strip to be ground and assume a constant potential across the surface of the outer cylinder. Then it is simply finding the charge distribution on the surfaces. Integrate the charge distribution across the surface of a plate to find the total charge, divide by the voltage you chose to set the plates at, and Bob's your uncle (or other suitable relative) and you have capacitance.

    The dielectric pipe adds a bit of trouble because it will be polarized by the fields and adds additional boundary conditions. Really, I think the best way to tackle this would be to do a finite element simulation. You could easily then model the dielectric of the pipe and you can also solve for increasing water levels too. You may be able to solve for it in closed form without any water, it would be rather difficult though. If there wasn't a dielectric pipe, then I would suggest a moment method computational solver. That would not be too difficult to write up. Fortunately, you could just assume that you have an infinite pipe so it is a 2D problem and you would get the capacitance per unit length.

    Or you could just build it and take measurements with different levels of water and different levels of contaminants in the water to change the conductivity. But where's the fun in that?

    Oh, what a fun little problem.

    EDIT: Don't forget to insulate the outer plates on the outside of the pipe. If the pipe is buried and you do not insulate it then the outer plates get pulled to earth ground which may be the same as the ground you pull down on the inner conductor.
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