Build a Temperature Probe

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I was doing some digging to find a thermal probe,
and it turns out they are quite expensive – usually
hundreds of dollars. Being the resourceful person
that I am, I decided to build my own simple
thermometer as I wanted to have an objective way to
measure the performance of various heatsinks and
interface materials.

Here’s the cheap way to make a thermal probe:

  1. Get an ohm-meter with a DIGITAL readout.
    Radio Shack carries them, but they are usually
    quite expensive. Home Depot has them much
    cheaper – I picked mine up for $30 and it has
    a neat feature called “data hold” which freezes
    the readout.

  2. Go to Radio Shack and pickup a thermistor and
    some heat-shrink tubing. Alligator clips are
    a definite plus for making a secure connection
    to your probe and I highly recommend them if
    your ohm-meter only has needle-point connectors.

  3. First, get a pair of wires and solder them to the
    thermistor. Make sure you do not have any solder
    between the leads (a short). Test the connection
    with your ohm-meter after it cools off.

  4. Take the heatshrink and wrap it around the wires
    and the thermistor – you only want the small “head”
    of the thermistor outside the package. The heatshrink
    is not for looks, but it is actually there to protect
    the thermistor from being pulled apart – they are
    very fragile.

  5. Heat it up. You’re supposed to use a hair dryer, but
    I found a butane torch works muuuuch quicker. >:) If
    you burn a hole through the heatshrink, use a larger
    tube to cover the hole you made.

  6. After you are done, you should have a little thermistor
    head sticking out and two wire leads out the bottom.
    I recommend something like 30ga wire about 4-6″ long.

Now, here’s how to use it:

On the back of the thermistor package will be a chart
listing the temperature on one side and the resistance on
the other. A sample is included below –

10k Thermistor (part # 271-110A Radio Shack)

  • 30 8.313
  • 35 6.941
  • 40 5.826
  • 45 4.912

The left side is Celsius. For those of you in the States,
grab a calculator and use this formula to convert to Fahrenheit
(yeah, I spelled it right – I checked the dictionary! *g*):

Celsius * 1.8 + 32 = Fahrenheit

And there you have it. Place the probe where you want it, read the resistance and convert to temperature. I know this is a little rough, but you should get the idea. I built this thing to measure my GeForce 256 card, which I thought was getting waaaay to hot with the old heatsink.

Turns out I was right – 130F!!! A Lasagna Cooler fixed that – bringing it down to a nice 86F at room temp. Much better.


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