Detecting water...

Discussion in 'General Electronics Chat' started by Seidleroni, Dec 10, 2008.

  1. Seidleroni

    Thread Starter Member

    Dec 8, 2007
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    I'm looking to create a water detection circuit. I looked online and couldnt find a probe that could just easily (and cheaply) give me an open circuit/closed circuit or something I could work with easily with a microcontroller. Since water conducts, is there a simple circuit or something I can use to detect if water is between 2 probes?

    I'm thinking something like this: Have 2 pins on a micro: GND, and an input pin which is pulled UP to 5V via a resistor. Then the output of the 2 pins are connected to a probe, and if the water exists, the water will pull DOWN the high pin and I can get the value of the input pin inside the microcode. I"m just worried that this may cause issues somehow with the ground and voltages. Is there something better to use?
     
  2. flat5

    Active Member

    Nov 13, 2008
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    You will need amplification. An op amp will help. Water conducts if it has minerals in it.
     
  3. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Are you trying to measure a water level or water concentration?

    There are many solutions to detecting "bulk" water on the web. For water concentration, e.g., as in dissolved in jet fuel, there are fewer good hits. Most seem to use some version of a capacitance probe.

    One approach used weight. Basically, a wafer (like a cracker, I suppose) was suspended in the fluid to be tested, and the rate of weight loss was monitored.

    John
     
  4. Seidleroni

    Thread Starter Member

    Dec 8, 2007
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    I just need to detect if there is water there i.e. if there is a flood. What would be a good way to do this? I suspect any water in a flood will have minerals in it, no?
     
  5. jpanhalt

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    Jan 18, 2008
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    I've seen methods based on conductivity, reflected light, and floats.

    John
     
  6. Seidleroni

    Thread Starter Member

    Dec 8, 2007
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    What would be a good method to use with conductivity? Was the suggestion I used in my first post a good way to go?
     
  7. flat5

    Active Member

    Nov 13, 2008
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    Yes. You will measure resistance or current. One approach would be to amplify the current detected and send the result to a comparator. From there you can you can set a latch to alarm you or go directly to an alarm but when things dry the alarm will stop without a latch of some kind. If you provide more details of the situation the device will monitor and how you want to power it and be notified, many people may jump in and provide complete circuits.
     
  8. Seidleroni

    Thread Starter Member

    Dec 8, 2007
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    I'm looking to create a sensor that can detect if there is a flood. I want to be able to use some form of electronics to detect if there is water between 2 wires (or probes of some sort). What would be a good way to do this?
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    By detecting the change in capacitance between two metal plates.

    Air has a dielectric constant very slightly over 1.
    Water has a dielectric constant of approximately 78 at room temperature.

    Therefore, if the capacitor formed by the two plates was in a resonant LC or RC circuit, water coming between the plates would cause a huge decrease in the frequency of the circuit.
     
  10. KMoffett

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    Dec 19, 2007
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  11. flat5

    Active Member

    Nov 13, 2008
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    The Velleman kit. Now I feel very humbled :)
     
  12. Seidleroni

    Thread Starter Member

    Dec 8, 2007
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    I looked at the schematic, but the BC517 transistor lists a turn on current of 10 mA. But if the resistance of tap water is ~10Mohms, if I"m using a 9V battery, wouldnt the current be too small to turn on the transistor?
     
  13. hgmjr

    Moderator

    Jan 28, 2005
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    Take another look at the datasheet. The minimum current gain of the BC517 is pegged at 30000. Keep in mind that the BC517 is a Darlington configuration and so it is bound to be very sensitive to the current developed across the probe gap. It is probably also a good idea to keep the gap between the terminals in the probe very small.

    hgmjr
     
  14. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Take a strip of felt, an inch long or so. Soak it in really salty water for a while. Take it out and dry it in the oven. Attach a couple of clip leads to it Apply a few volts to it through a 100K resistor, and sample the voltage drop with an op amp. Dry salt: no conducty....wet salt...conducty.

    eric
     
  15. Seidleroni

    Thread Starter Member

    Dec 8, 2007
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    hgmjr,

    At what current would the darlington turn on? The datasheet lists 10 mA? but is that before the 30,000 gain? Also, would it be possible to use this circuit with a 5V power supply rather than a 9V? If so, what would need to be changed?
     
  16. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    One of the simplest devices is a cloths pin with contacts on each jaw and separated by paper or fish food flakes. Paper/fish food gets wet, contacts close. There are numerous variations on that theme, like putting a paper band around the lever side to hold the jaws open. Current is not a limiting factor.

    John
     
  17. Seidleroni

    Thread Starter Member

    Dec 8, 2007
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    I would prefer to use an electronics based method rather than using paper or fish food. Could someone explain to me my previous question: it seems that to turn on the resistor, I need voltage of at least 1.4V, but in the velleman kit, using a 9V supply and assuming the water's resistance is 3MOhms then i should only be getting 1.2 volts at the Base. What am I missing, these things are obviously working.
     
  18. eblc1388

    Senior Member

    Nov 28, 2008
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    Your assumption of water having 3MΩ is simply too high.

    I just dipped my DVM leads into my cup of water and I read ~200K. The leads are 1cm apart. I'm now drinking the left over water in my cup.

    Before I die of poisoning, can you do the same and tell us what value you have got for your kind of water?
     
  19. Seidleroni

    Thread Starter Member

    Dec 8, 2007
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    I took 2 measurements: one in hot water out from the kettle, and one in cold water from the tap. The hot water was about 500 KOhms, and the cold water was about 2 MOhms. It seems that the velleman kit wouldnt work in the cold water condition, right?
     
  20. Seidleroni

    Thread Starter Member

    Dec 8, 2007
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    If the resistance of the water is 3MOhms, then the Velleman circuit would NOT turn on, am I right?
     
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