Coolant Level Sensor Circuit

Discussion in 'The Projects Forum' started by davidmills2002, Oct 12, 2012.

  1. davidmills2002

    Thread Starter New Member

    Oct 12, 2012
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    Hi,

    I have a coolant tank for a piece of equipment that has a built in capacitive level sensor (i.e. two prongs that act as a capacitor and the value of which changes as the fluid level changes). There is no circuitry included with the sensor, essentially all I have is a capacitor that changes its capacity. The range of capacity is about 30 μF when the tank is full to 0F when empty. I want to create a circuit that when the capacitance is 0F, i.e. the tank is empty, then a constant 5v is outputted which can go to sensor to say switch off the coolant.

    The circuit will have to be powered by 24vDC and be low power to be safe to work around.

    I have a basic knowledge of electronic components, but this is too complex for me.

    Any help is appreciated.

    Thanks,


    Dave
     
  2. Dodgydave

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    Jun 22, 2012
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    Last edited: Oct 13, 2012
  3. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Yeh i know expensive!, i would use a 555 oscillator and rectify the output into an op amp comparitor, like an F to V converter.

    http://www.google.co.uk/imgres?q=55...w=204&start=0&ndsp=22&ved=1t:429,r:0,s:0,i:69

    or better still a Capacitance meter, where your probe would be "CX" and use a 22K preset instead of the bank of resistors, then your output would vary in voltage as the capacitor varies, and feed the output to an op amp comparator to switch on a relay.
     
    Last edited: Oct 13, 2012
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    30uF? Is it really 30 microfarads? That's a lot of capacitance for a sensor.
     
  6. davidmills2002

    Thread Starter New Member

    Oct 12, 2012
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    I used a multimeter to measure the capacitance of the sensor when the tank was empty and full. 0F when empty, expected as there is no coolant acting as dielectric between the prongs and I'm pretty sure it was about 20-25 uF when full and so I thought a maximum range of 30 uF would be suitable. I can double check in the week.

    The key thing needed from this is to detect when the capacitance is 0F as that is of the most importance to alert when the tank is empty. A variable scale of the level of the tank would only be a bonus.
     
  7. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Are you sure your coolant is non-conducting? Did you check it with the ohmmeter?
     
  8. davidmills2002

    Thread Starter New Member

    Oct 12, 2012
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    I measured the resistance and it was never stable reading between 9 MΩ and 15 MΩ
     
  9. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    OK, I give up.:p

    Your sensor may not be a parallel-plate capacitor, but the capacitance equation is still relevant. Note that capacitance is propoprtional to the dielectric constant of the medium between the plates.
    I'm assuming the sensor is immersed in the coolant, except, of course, when the tank is empty. The dielectric constant of most liquids is much higher than that of air. That of ethylene glycol, for example, is 68. When coolant first appears in the sensor, the capacitance should jump as the medium between the electrodes changes from air to coolant. If you can measure those two capacitance values, it would be a great help in designing a circuit to detect when the tank is empty. The capacitance can never go to zero, but it can probably go to some tens or hundreds of picofarads.
     
  10. davidmills2002

    Thread Starter New Member

    Oct 12, 2012
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    Ok, I'll measure the capacitance when the level is just below the prongs of the sensor and measure it when it is just touching them. Hopefully be able to do that tomorrow.
     
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