Intro: Making a Thermocouple
A thermocouple is a device used to measure temperature by utilizing the Seebeck effect. This instructable will show a very simple method of making a thermocouple so you can precisely measure the internal temperature of your thanksgiving turkey.
I'm sorry that the one picture I have came out terrible. The picture with the flash off was even worse (low battery syndrome). I was debating weather or not to post - but considering I couldn't find instructions anywhere on the web on how to do this, I decided to go forth and conquer. At least the video in step 2 is somewhat amusing.
If I can - I'll build my own bridge and make my own meat thermometer probe. Why? you ask. Why not? What's cooler than measuring the temperature of your turkey with your trusty multimeter. And in my case, my trusty cheapo multimeter and calculator :P
Step 1: Materials
1. Thermocouple wire : we got ours from http://www.omega.com/prodinfo/ThermocoupleWire.html
You can probably do it with your own wires - but the must be dissimilar materials and your results may vary. If the materials are not suitable, you will know as soon as you try to calibrate.
2. A capacitor bank or other method to spot weld a tiny wire.
Step 2: Actually Making the Thermocouple
1. Strip back the outer insulation
2. Strip back each individual wire and expose about .24-.5 inches of wire.
3. Bend the wire to make a contact point**
4. Weld (see next step for a suggested setup).
Now you can use your thermocouple (a whole other set of instructions). We used Labview 7 a DAQ board and a conditioner board (which provided a 1/4 Wheatstone bridge).
**If you have more than one contact point, you're measurement will be the average temperature over all of the contact points rather than the temperature at the contact.
Step 3: Welding
We had a fancy little device that provided a ground (pliers) and a hot "plate." We would grasp wire in contact with the pliers and touch the hot plate (closing the circuit). A high amperage discharge would occur and the wire would spot weld.
I suspect it was a capacitor bank. All you'd need is a sufficiently large bank of charged caps. Take a pair of insulated pliers and attach the ground wire to an exposed bit of metal. Then grasp your thermocouple wire in the pliers and touch the positive end of the charged cap bank.
I would personally start small and add caps as necessary ;)
Step 4: Calibration
Okay, so lets say you have the tools to use your Thermocouple (I don't, but the measurements lab did). As with any other measuring device, you must form a calibration curve to show your measurements are accurate/precise and credible.
Given the tools/supplies available. We made an ice water bath and a boiling water bath. Ideally, we would have added salt to the water to help it get colder. We used a normal thermometer to measure an analog temperature. Then, we started recording the thermocouple data in Labview and went from the room to the ice water bath. We saved this data and repeated for the hot water bath.
As thermocouples have a linear relationship to temperature, linear interpolation can be used to determine any temperature within the thermocouples range. You can also determine the response time of the thermocouple based on the data collected.