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I want to embed a wire inside a plastic and heat it to specific temperatures (80$^\circ$C 100$^\circ$C 130$^\circ$C 180$^\circ$C). I want to find a material that is resistive enough for resistive heating, but also I want the wire's resistivity to vary fairly strongly with temperature - this way I can know the actual temperature that I'm getting by looking at the resistance during the heating process.

My problem is that I cannot seem to find a material with the properties I want. I tried nichrome wire and carbon fibers, and while they are resistive enough to give me heat, their resistance does not change enough (for my desired temperature ranges) to be measurable with my setup.

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  • $\begingroup$ What's the question, exactly? Are you asking how to use a wire to heat plastic while measuring the temperature of the plastic? Or do you want to measure the temperature of the wire? Are you asking where to find "properties of materials?" Are you asking what kind of wire to use? $\endgroup$ – Air Apr 24 '15 at 22:54
  • $\begingroup$ Sorry for the confusion. Yes I am essentially asking for what wire to use. However, I would not be opposed to hearing some new ideas on how to achieve my goal. And no, running a thermocouple next to my heating wire is not an option for my application. Thanks. $\endgroup$ – Nic Apr 25 '15 at 8:32
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Just about anything you can find will exhibit a temperature dependence on its resistivity. The first hit looking up nichrome says it's temperature dependence is 0.004/°C.

A better model for resistivity is the Steinhardt-Hart(sp?) equation, but you probably don't need something that elaborate. Since you only want to control the temperature to 4 discrete values, you can figure out ahead of time what the total resistance of a particular diameter and length of wire will be at each of those 4 temperatures.

To measure the resistance, you need to measure the voltage across the wire and the current thru it, then divide. With the right analog electronics, these values can be presented to A/D inputs of a microcontroller, which can compute the resistance, determine the temperature, then vary the current via a controller accordingly.

I'd probably implement the resistance to temperature function as a piecewise-linear table lookup. That's quick to compute and can model any non-linearities without lots of cycles.

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  • $\begingroup$ That's the idea, however I have tried it and the resistance change is so small it gets lost in the noise. Remember I am only going to between 80 - 180 degrees Celsius which is why the change in resistance is so small. $\endgroup$ – Nic Apr 25 '15 at 8:36
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    $\begingroup$ @Nic: Most metals don't have a large thermal response in resistivity, but there are materials that do. I haven't looked up what thermistors are made from, but they exhibit a large response. The problem will be finding "wire" of anything that isn't a metal. However, a factor of 0.004/degC can be measured with careful analog design. Think about it. 0.004 is 1 part in 250, or one count out of 8 bits, and that's just for a 1 degC change. There is plenty of signal there. $\endgroup$ – Olin Lathrop Apr 25 '15 at 12:08

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