This just in from the MAKE zine blog ( http://makezine.com/blog/ ) , a cool DIY site and magazine. It's a link to a picture instructable on how to make the probe wire thingie for a thermocouple.... Since the DIY spirit seems to run high round here, I figured this might be helpful for some people!


here's the link to the (as yet incomplete) instruction set:

http://www.instructables.com/id/ENVQPD6 ... /?ALLSTEPS

excerpt from the site:

"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."

"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."





This does not bode well...

esp item #2



Sean

I can get TC wire cheap at my local monster surplus electronic warehouse. If I wanted to solder the TC wires, are there any special requirements (heat limitations on the solder joint, solder type bead size etc)?

If I wanted to solder the TC wires, are there any special requirements (heat limitations on the solder joint, solder type bead size etc)?

I think it's best to use a capacitive discharge welder for making TC's.
Maybe Greg Scace will chime in on this one, I'm sure he would know.

My meter is good for 0.3C accuracy so something a little tighter than 1c would be good. I would probably need a TC calibrator as well to test the TC's.

If your temperature requirements are below approx. 350F, standard 60/40 resin core solder works well for type T wire pairs. Special limits type T wire is not too expensive and does not seem to suffer any harm as a result of the heat of soldering. The positive lead is pure copper, which is easily wetted by the 60/40 solder.

Twisting the ends of 24 ga T-wires tightly together over a length of around 5mm, followed by soldering, produces an accurate probe with a reasonably small junction mass.

If your temperature requirements are higher, or if the probe will contact food or drink, this is not the right plan.

Type K wire does not solder easily (for me) at all, so you might want to weld a bead instead. If you can get a pure carbon electrode and a high amperage 12 to 24VDC power supply, you can strike an arc between the carbon and the wire pair that will make a nice bead (car battery works, but keep the arc away from the battery - H2 is explosive).

Technically, an inert gas (argon) environment should be used when making this weld, but I have made many probes this way (without the argon) that tested just fine.

These days I am using a capacitive discharge spot welder to weld beads. For 24 ga wires, somewhere around 20 VDC seems to be about right. A pair of needle nose pliers (or alligator clip) gripping the wire pair makes one electrode; the other electrode is a 1/8 inch pure carbon rod threaded into an aluminum carrier. A quick swipe of the wire pair across the carbon rod creates a very bright, hot arc that melts just the tips of the wires. Surface tension draws the molten metal back into a little spherical bead. Pure magic ;-}

Jim

Sounds like it would just be easier to purchase them. I doubt I could adjust my MIG welder down enough to bead the two wires without evaporating a foot of copper even with a gas shield. I may get a few feet and try some silver solder for fun, maybe even strike an arc on the MIG.

Good to know. I've never tried leaving one "just twisted," but that sure would save a lot of time and trouble.

Jim G.