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Q & A: a nichrome thermometer

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Most recent answer: 10/22/2007
Q:
Hi Van, If I know the resistance and the voltage accrossing a nichrome ribbon, how can I calculate the temperature of the ribbon? Thank you.
- Siyu (age 16)
Mawson Lakes College, Australia
A:
There are two possible questions you could be asking -- how to use nichrome as a temperature sensor, or how to use nichrome as a heater. I'll try to take a stab at both.

The resistivity of metals like nichrome increases with temperature, and the resistance of a particular piece of wire will depend on the resistivity and things like how long the wire is and how thick it is. It may well depend a bit on how the electrical contacts are attached also.

You can use nichrome as a thermometer by measuring the current with a known voltage across it, that is, by measuring its resistance. You should calibrate it with a known thermometer so that you can translate the resistance into temperature. The temperature may not be the same everywhere on the wire, so your readings will be more reliable when the wire is all at the same temperature.

Second possible question: You are flowing a current through the nichrome ribbon and want to know how hot it's going to get. This one has more external factors complicating it. The power dissipated as heat in the wire is V*I, where V is the voltage, and I is the current. If you know the resistance R, Ohm's Law says V=I*R, and so the power is P=I**2*R, or V**2/R. As mentioned above, the resistance changes with temperature, so watch out. The power is the rate at which energy is converted from electrical potential energy to heat -- it is not a temperature. If the thermal energy stays in the wire (that is, you've got a good insulator around it), even with a small power, the wire will continue to get hotter and hotter until it melts or breaks. If the wire is in a jar of icewater which is constantly stirred, then the temperature of the wire will be lower than if the wire were insulated, even with the same voltage across it.

It might be possible to use both of these features of nichrome -- that is, you can use it as a heating element which senses its own temperature. Just measure the resistance and calibrate it with a known thermometer.

Tom

(published on 10/22/2007)

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