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Thermal Resistance at a single point

  1. Aug 13, 2009 #1
    Hi All

    I am having a design problem. I want to melt a fishing line which is in contact at a single point with some material. The problem is that I have a 5 V of supply and Power should be between 1-3W. Some other constraints are that the material to which the fishing line is connected should be short and the temperature on material should be around 200 degree C.

    So is there any suggestion that which kind of material will be suitable for such purpose?

    And one more thing, the material to melt the fishing line should be of high thermal resistance or low?

    If this topic should be in the materials section then sorry in advance , I will put it there then.

    Thanks and Regards
     
    Last edited: Aug 13, 2009
  2. jcsd
  3. Aug 13, 2009 #2

    vk6kro

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    The classic answer would be to get a few inches of Nichrome resistance wire and heat it with an electric current and melt the fishing line. You would need to current regulate the power to the wire. A series auto light might be enough.

    The problem might be getting such a small quantity of resistance wire. Buying a whole roll of it would be very expensive.

    Or (if you didn't live in Australia or the USA) you could try a laser to cut the fishing line :
    like this one:
    http://www.dealextreme.com/details.dx/sku.11315
    This can apparently light a match, so it would probably melt fishing line.
    You probably won't even be able to buy one, so I won't list the dangers of lasers. If you can get permission to use one for scientific purposes, you would already know these dangers.
     
    Last edited by a moderator: Apr 24, 2017
  4. Aug 14, 2009 #3

    vk6kro

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    Just curious, though. Why don't you use a pair of scissors or a scalpel blade and just cut the fishing line?
     
  5. Aug 14, 2009 #4
    Hi vk6kro

    Thanks for your reply. Well the reason I cannot use a scissor is that the fishing line would be in space (on a satellite) :)
    I actually want to use it to hold the solar panels for a small satellite (CubeSat) and for the antennas.
    So you suggest to use a Nichrome wire for this purpose?
     
  6. Aug 14, 2009 #5

    vk6kro

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    You could use Nichrome wire, I guess. That would be a possible ignition source though.

    Another way would be to have a bimetallic strip that bent when heated and this moved a blade which cut the fishing line. My wife has a "thread cutter" which is a hook with the blade on the inside of the hook. This could cut fishing line if it tried to pull the line through a hole in something.

    You could have an "actuator" which is an Iron rod being pulled into a coil. This could be made quite small and light if it only got used once. The rod moves and pulls a cutter into the coil and cuts the fishing line.

    You could have a very small motor with a sandpaper cylinder on its shaft. The motor starts turning and the sandpaper rubs against the fishing line and wears through it.

    You could have a syringe with air in it. When there was no air pressure above the plunger, like in the vacuum of space, the plunger would move upwards pushing a rod with a cutter on it. This would cut the fishing line.

    Are you really going to do this (ie do you work for JPL?) or is it a design exercise?
     
  7. Aug 16, 2009 #6
    Hi

    Thanks for the bundles of ideas. But the thing is that system has to be kept simple somehow because it is only one time operation. Therefore i proposed that the fishing line is hooked up to a small resistive wire under stress and the the fishing line is melted *somehow* the solar panels under tension (because of being torsion spring loaded and being held by the fishing line) will open. Since it is for a fairly smaller satellite, so a simple solution is required.
    The thermal conductivity of a common alloy of Nichrome is 11.3. Do you know if I would require a material of higher thermal conductivity or lower?

    I'm working on a project in a University in Germany. Well the solution is likely to be used for a future satellite :)
     
  8. Aug 16, 2009 #7
    Although using an actuator is the best solution but its expensive for space applications.
     
  9. Aug 18, 2009 #8

    vk6kro

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    One thing I would do first is to put the fishing line in some liquid Notrogen and while it is there, bend it to see how brittle it becomes. I don't know if it does or not, but it would be worth checking.

    Nichrome wire is not a convenient material to use. It is necessary to silver solder it and it has a fairly low resistance per inch of wire unless you get very thin wire.

    I wondered how you would get on with using, say, a 1/4 watt 15 ohm resistor with 5 volts on it.
    This would dissipate 1.66 watts and probably destroy the resistor, but first it would get very hot and probably melt any fishing line wrapped around it.
    It would be a very easy thing to try if you found the idea attractive. Make sure there is no tension on the resistor leads as resistors can be pulled apart this way.
     
  10. Aug 18, 2009 #9
    I don't know if dipping in nitrogen would be of anyhelp in this case. I am interested in using a resistive wire instead of a resistor. So the re-defined constraints now are that a voltage of 4V and a maximum current of 1.5A can flow through it. So with that in hand the resistance should be 2.66 and power would be 6 W.

    So the problem now is that the length of the wire should be anything in between 1-2 cm and the thickness should be as such that it should be able to hold the the fishing line which is itself under tension.

    I don't know for sure that if this length of the wire would be able to produce such a heat to melt the wire. Any ideas?

    and one more thing, I was also thinking may be something like a solder iron tip which can also be used as a heating element for melting the nylon wire. I don't exactly know what they are called, but they look something like a thermocouple.
     
  11. Aug 18, 2009 #10

    vk6kro

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    Dipping in liquid nitrogen would be to test the properties of fishing line under extreme cold, which is what you get in space. If it gets brittle, it is no use to you.

    I have seen mini 6 volt soldering irons. They would probably work on 4 volts

    Much cheaper though is a 2 cent 15 ohm resistor.
    I would form a loop of fishing line through a small washer and put the resistor in the loop. If the fishing line were under tension, it should melt in less than 10 seconds.

    Nichrome wire is available in very thin gauges, so maybe you could find some of that.
    Try this site:
    http://www.heatersplus.com/nichrome.htm
    They have some interesting charts. Their 31 AWG wire has about 8 ohms per foot and they sell it by the foot, so you wouldn't need a whole roll of it. You could wind about 4 inches of it into a coil and run the fishing line through it.
     
  12. Aug 19, 2009 #11
    Actually fishing line is commonly used in cubesats for their antenna deployments. So low temperature is not a problem for it.
    The link was of great help, but i think its in US. And it takes a long time to ship to europe and the cost of shipment would be far more than the item's.

    I was wondering that if a 0.5mm diameter wire of nichrome could take the tension of the fishing line at a single point of contact?
     
  13. Aug 20, 2009 #12

    vk6kro

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    Nichrome wire is very thin and several metres of it could be put in an envelope and reach you in a few days. Shouldn't be expensive, but if it is, use Google to find a supplier that suits you better.

    Better to find a store in your own town that will sell you a small quantity. I think they would give it to you if you tell them it is going in a satellite in space.

    Nichrome wire is also very strong so it could hold fishing line, but it may tend to cut through it.
    You could anchor the fishing line to a part of the case and then have the Nichrome wrapped around the fishing line but not pulling on it. Apply current and heat up the fishing line to melt it.
     
  14. Aug 20, 2009 #13
    Yea, I have found some local shops here in germany and I will order them soon.
    And that is my concern now that if I only hook it up to nichrome wire then it might get cut. So i'm planing to pass it through 2 hooks and wrap the nichrome wire around it. I can use a washer like thing, but I am unable to visualize what you previously said about that.

    May be you can elaborate that a bit more.
     
  15. Aug 20, 2009 #14

    vk6kro

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    I have attached a drawing of the scheme I suggested earlier.

    A fishing line is under tension and a loop of it is passed through a washer or even a smooth ring as used for fishing. The resistor is held in the loop by the tension of the fishing line.
    Actually, I should have made the resistor a bit longer so it couldn't get pulled through the hole in the washer. Anyway you get the idea.

    When the resistor gets hot enough to melt the fishing line, the loop breaks and the ends of the fishing line can move apart.

    If you are tying fishing line to a ring with knots, be careful that the end going to the antenna does not have a ring on it after it melts or this could get caught on something.
     

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  16. Aug 20, 2009 #15
    Thanks for the descriptive explanation :)
    will try out some different techniques for sure. by the way, what is your specialty field?

    Still my problem remains. That how can I calculate theoretically the resistance require to reach around a 150 degree C temperature with a 1.5 A current through it and a power supply of 4 V. The resistance coming out in this way is around 2.67 Ohms but to achieve this resistance for constraints of a short (20mm) long wire and around 0.5 mm diameter wire seems not possible theoretically. Whereas the fact is that I have seen the fishing line melting with more or less the same specifications.
     
  17. Aug 20, 2009 #16

    vk6kro

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    That is why I suggested the resistor technique. Resistors can pack a lot of resistance into a small space and you can make an easy connection to their wires.
    If you have to use Nichrome in a small space, you have to use only a small piece and limit the current with electronics. You do not want half a metre of nichrome wire getting very hot inside a satellite.

    Hang a 1 KG weight from some fishing line and try different methods of releasing the weight.
    Should be a fun thing to do.
     
  18. Aug 24, 2009 #17
    Nylon becomes brick hard, and it is already super tough at room temperatures...
     
  19. Aug 24, 2009 #18

    vk6kro

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    Good. Sounds like you have tried this.

    Did the nylon stay flexible at those temperatures? This has to work in space where it will get very cold.
     
  20. Aug 26, 2009 #19
    We use nylon at 3K or so, the Nylon has a large coefficient of expansion. It is pulled already tight, and does so even more once it is cold. It is important for us that the structure it holds does not move. From what I am being told it gets very stiff and I believe it. It will definitely not be as flexible as before and probably even stay in shape unless you have a little force pulling on it, but it shouldn't break. The fact that it survives the tension of our cool down indicates that it will not be brittle. Overall I think it will behave very similar to room temperature if it is thin.
     
  21. Aug 26, 2009 #20
    wow..pretty interesting to know this.
     
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