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Ideas- Thermal Conductor, Electrical Insulator, Waterproof — Parallax Forums

Ideas- Thermal Conductor, Electrical Insulator, Waterproof

realitybiasrealitybias Posts: 16
edited 2007-05-27 16:29 in General Discussion
I need ideas. We are using a nichrome-based wire heater element to heat up water, but stray current in the water from the exposed nichrome wire presents tough problems. I need an idea for a substance that could be wrapped around it, that will withstand the high temperatures that glowing nichrome wire can sometimes reach, while insulating the nichrome wire from stray electric currents. Any ideas?

Comments

  • stamptrolstamptrol Posts: 1,731
    edited 2007-05-24 19:16
    Get yourself either an element out of an electric water heater ( check local plumbers); if thats too big, go to an automotive store and get an engine block heater. They're very compact.

    The way you are set up now, I really hope you're using very low voltage!

    Cheers,

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    Tom Sisk

    http://www.siskconsult.com
    ·
  • realitybiasrealitybias Posts: 16
    edited 2007-05-24 19:24
    Dont worry, we are using low voltage (about 10 volts, DC wall adapter). We do need one that is very small though, and we can't have stray currents, because we use electrodes to measure water conductivity as well.
  • stamptrolstamptrol Posts: 1,731
    edited 2007-05-24 19:34
    The other source of small units is a chemistry lab supply house.

    They have many heaters in glass tubes. Also, check out the "Heater" section of Omega Engineering.

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    Tom Sisk

    http://www.siskconsult.com
    ·
  • RDL2004RDL2004 Posts: 2,554
    edited 2007-05-24 20:09
    How much water are you heating? Go to your local pet store and look at aquarium heaters. They come in a large range of wattages and many are fully submersible with decent thermostatic controls.

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    - Rick
  • realitybiasrealitybias Posts: 16
    edited 2007-05-24 20:13
    Not very much water at all... I'd guess about 16 ounces of water. We would need to be able to order these en masse, though, and they would need to have some sort of aspect that could be controllable through the boe bot BASIC stamp (ie, turn them on and off when the boe-bot recognizes the temperature gets too high or low).

    They also cannot interfere electrically with other sensors that are submerged. Our nichrome wire heater, with exposed wire, heavily interfered with our other sensors.

    You've all got good ideas... if we could find a cheap way to do this, either by coating the heater element in a thermo-conductive electrical insulator, or buying an implimentable comercial version, it would be great.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2007-05-24 20:52
    If the nichrome wire can be pulsed, you could just read your sensors between pulses when there's no current flowing. This would be easy if the pulsing and sensor reading were both controlled by the same BASIC Stamp.

    -Phil
  • realitybiasrealitybias Posts: 16
    edited 2007-05-24 21:03
    Well, the heater element and nichrome wire is controlled using a relay switch. I'm not sure if that will cause problems with pulsing (I'd be pulsing a relay on and off. I suppose it would be possible, but it might lag a bit). That is an excelent idea, though. I'd prefer to be able to keep it turned on, but this may be a feasible solution.
  • metron9metron9 Posts: 1,100
    edited 2007-05-24 21:30
    How much current at 10V are you using.

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    Re-arrange what's inside the box then...
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  • realitybiasrealitybias Posts: 16
    edited 2007-05-24 21:36
    If I remember correctly, we use 10V wall adapters, 800-1000 mA.
  • metron9metron9 Posts: 1,100
    edited 2007-05-24 21:56
    How about a 1 ohm resistor 100watt http://dkc3.digikey.com/PDF/T072/P1622.pdf

    Solder on the wires
    Dip the whole thing in epoxy

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Think Inside the box first and if that doesn't work..
    Re-arrange what's inside the box then...
    Think outside the BOX!
  • agfaagfa Posts: 295
    edited 2007-05-25 00:55
    Sounds as though Phil Pilrim hit it on the head.· I'm not sure that it has to be continually pulsed but take measurements when the heating·element is off.· a stamp could easily do that.

    You say you are measuring conductivity.· Is the nichrome wire alterning the conductivity?·
  • BeanBean Posts: 8,129
    edited 2007-05-25 01:17
    You might want to check these out http://www.minco.com/products/heaters.aspx

    Bean.

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    ·
  • realitybiasrealitybias Posts: 16
    edited 2007-05-25 02:00
    Its worse than that. The nichrome wire is not altering the conductivity, it is quite literally burning out things on the breadboard. The conductivity sensor is simply two metal leads that go into opposite sides of the fishtank. Because they are wired into the boe bot directly as a resistor, we can use an RCtime circuit to see how strong of a resistor the water is, dependent on how much salt is in it. The current from the nichrome wire goes through the water randomly, and burns out elements inside of the boe bot.
  • LawsonLawson Posts: 870
    edited 2007-05-25 03:54
    heh, sounds like a second relay is needed to disconnect the conductivity sensor whenever the bare wire is heating the water. A resistor in series with each conductivity sensor lead would also protect the BoeBot board.

    just a thought,
    Marty
  • Tracy AllenTracy Allen Posts: 6,666
    edited 2007-05-25 05:55
    10 volts at 1000 mA is 10 watts and 10 ohms of resistance. There are ceramic resistors on a hollow ceramic core, insulated with ceramic. [noparse][[/noparse]urlhttp://www.allelectronics.com/matrix/Power_Resistors.html Maybe you could improvise something like that by wrapping your nichrome wire around a a form like a glass tube, and then insulate it with a dip of JB-Weld or by wrapping it and the connections well with silicone tape. Silicone tape (available from Rat Shack) is self-sealing and can withstand pretty high temperatures, but don't let the wire get "red hot". Use longer wires in parallel if necessary to get the 10 ohms without danger of red hotness.

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    Tracy Allen
    www.emesystems.com
  • metron9metron9 Posts: 1,100
    edited 2007-05-25 14:53
    OOps I added a 0. In my above post.

    I don't see why you need to bother with nichrome wire why not just use a chip resistor dipped in epoxy then you don't have to worry about overheating the element. If it's getting red hot now insulating the wire will only make it hotter and you will smoke any insulation to get the amount of heat you are getting now. You need to disburse the heat over a larger area. If you are heating water the maximum is 212 degrees F anyway, why do you need an element that puts out 1000F. With 16oz of water you need 1 BTU per degree of heat so from room temp to 212 is 242 BTU's

    That's about 120 watts or energy over 1 hour. If you are only using 10 watts for 1 lb of water with no thermal loss you can get about 14 degrees increase over 1 hour from room temp. You need to reduce that by the thermal loss of the container holding the water.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Think Inside the box first and if that doesn't work..
    Re-arrange what's inside the box then...
    Think outside the BOX!
  • agfaagfa Posts: 295
    edited 2007-05-25 20:01
    I've done water·testing before, when we measured conductivity, the water temp was never taken into consideration.· Maybe I'm just ignorant about all the facts.· What is the importance of heating the water?· Just wanted to know, maybe a learning oportunity for me.
  • realitybiasrealitybias Posts: 16
    edited 2007-05-27 05:23
    No, it has nothing to do with the conductivity. The goal was to use the boe bot to create a self regulating fish tank system that heats and cools, keeps salt water concentration stable, reads temperature and salt water conductivity, and circulates the water constantly.

    Because the nichrome wire sent several stray currents through the system, it made the self regulation of the system impractical, and in some cases, damaging. I remember one groups boe bot started smoking, and they blew out a 555 circuit.

    I found online a length of silicone-wrapped heater wire that can go up to 150 degrees C, which should be about right for our purposes.
  • Robert KubichekRobert Kubichek Posts: 343
    edited 2007-05-27 16:29
    Couldn't you use that same wire inside (and potted) a length of high-temp-pyrex tubing?
    The tubing can be bent in custom shape(s) to fit your needs..
    It would be much better safer than silicon-wrapped heater wire...
    There would be NO chance of stray currents inside of a glass envelope..
    Also, the more wire you use, the lower the max temp of the wire to get the same
    thermal ouput...
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