DIY thermocouple

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chino

#1: Post by chino »

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
LMWDP #094

lennoncs

#2: Post by lennoncs »

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

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another_jim
Team HB

#3: Post by another_jim »

The best diy solution is to go to Omega and buy 50 feet of K or whatever type of thermocouple wire (I like the 30 gauge capton K for all around usefulness), and about 20 plugs. This will come to about $75 and do you for 20 TCs. Use a fine gauge wire stripper and just twist the sensing ends together -- this will work fine if you are looking for 1C accuracy. For some bizarre reason, in US TC wire, the red conductor is always the negative, rather than the +ve.
Jim Schulman

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cannonfodder
Team HB

#4: Post by cannonfodder »

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)?
Dave Stephens

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Jepy

#5: Post by Jepy »

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.
Image

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cannonfodder
Team HB

#6: Post by cannonfodder »

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.
Dave Stephens

JimG

#7: Post by JimG »

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

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cannonfodder
Team HB

#8: Post by cannonfodder »

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.
Dave Stephens

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another_jim
Team HB

#9: Post by another_jim » replying to cannonfodder »

There's no need to weld or solder TCs, they work and last with the ends twisted together. I have four on my roaster and they've run for years that way.
Jim Schulman

JimG

#10: Post by JimG » replying to another_jim »

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

Jim G.