DIY thermocouple - Page 2

#11: Post by **espressme** » November 25th, 2006, 6:02 pm

cannonfodder wrote: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.

Hi,
Old master welder at the College says:
"With a TIG, turn on the shielding gas, then turn on the High Frequency, and not the current. Works great for thermocouples."

Best regards to All!
richard

November 25th, 2006, 7:35 pm

Now if I only had a TIG I would be in good shape. Unfortunately I only have a MIG. I will have to try Jim's twisted wire when my meters arrive.

#13: Post by **JimG** » November 26th, 2006, 1:37 pm

How many do you need? What is the application?

Jim

November 26th, 2006, 2:13 pm

another_jim wrote: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, what type wire are you using in your roasters? I tried a type T with the plastic housing and of course the housing melted. For < 1000 F in type T what wire would you recommend?

#15: Post by **Marv** » November 27th, 2006, 12:22 am

Jim,

This may be a dumb question, but are you able to use just the wires twisted as a bean probe in moving beans or just as an environment probe?

I am building a little roaster and need to solve the thermocouple issue. I am planning on have one at the exhaust and hopefully on in the moving bean mass.

Thank you

Marvin

November 27th, 2006, 10:44 pm

OkcEspresso wrote:Jim, what type wire are you using in your roasters? I tried a type T with the plastic housing and of course the housing melted. For < 1000 F in type T what wire would you recommend?

Type T, as far as I know, is not much use above around 150C. I use K type with Capton insulation for all my temperature measuring needs, since I don't want to get more accurate than 1C absolute or 0.1C on comparative readings, and this set up works up to 600F, and unlike fiberglass, is very abrasion resistant, and works on every meter. If you want more accurate than that that, you don't only need better TCs, but also 14 or 16 bot A/D converters on nist traceable gear -- way too expensive for my "not that obsessive about temperature" tastes. For higher temps and better abrasion resistance, you need woven mica insulation, another expensive item.

I get the wire at Omega, which sells k type, 30 gauge, capton insulated in 50 foot lots.

November 27th, 2006, 10:49 pm

Marv wrote:Jim,

This may be a dumb question, but are you able to use just the wires twisted as a bean probe in moving beans or just as an environment probe?

I am building a little roaster and need to solve the thermocouple issue. I am planning on have one at the exhaust and hopefully on in the moving bean mass.

Thank you

Marvin

I've got them above the beans, but they last in the moving ones too (where I've had them before).

Use a needle nose to twist the wires together, then fold them over. Don't overdo it, or they'll break, and have a good, non-nicking set of wire strippers, or they'll really break. Check them on boiling and ice cube water to make sure they are OK I've done about ten so far, and they've all been ± 1C, so, given my manual dexterity, I figure the method is pretty idiot proof.

November 28th, 2006, 12:23 am

I found This while looking for thermocouple information. simple but to the point.

Also found these

Notes on using thermocouples
Dr. Robert J. Moffat, Stanford University
http://www.electronics-cooling.com/Reso ... n97_01.htm

Copper-Constantan: Copper-Constantan (Type T, color coded blue and red) generates about 40 µV/°C (22 µV/°F). Neither wire is magnetic. Junctions can be made by welding or soldering with commonly available solders and fluxes.

Copper-Constantan thermocouples are very susceptible to conduction error, due to the high thermal conductivity of the copper, and should not be used unless long runs of wire (100 to 200 wire diameters) can be laid along an isotherm.

Most thermocouples needed for electronics cooling applications can be made in-house from bulk thermocouple wires bought as spools of insulated pairs. If a thermocouple welder, or any "fine-wire" welder is available, welding is generally quicker and easier than soldering. Any solder which wets both wires can be used to make the junction. Keep the weld bead or solder ball within 10 to 15% of the wire diameter. All other factors being the same, a thermocouple with a soldered junction is just as accurate as one with a welded junction.

and...

JEDEC SOLID STATE TECHNOLOGY ASSOCIATION
Beaded Thermocouple Temperature Measurement of Semiconductor Packages
http://www.jedec.org/download/search/jep140.pdf

ASTM STP 852, Manual on the Use of Thermocouples
ASTM Manual 12, On the Use of Thermocouples

5.1.3 Thermocouple fabrication
Thermocouples may be fabricated using a fine oxy-hydrogen welding torch, a capacitive discharge micro spot-welding technique, or any industry acceptable method which fuses the two wires into a homogenous bead. Not recommended are metal joining techniques such as eutectic solder or wire twisting which could result in a thermocouple junction far removed from where the wires appear to cross. Poor joining techniques could result in monitoring errors and/or performance instabilities.

(Note that the targeted use of these 24AWG to 40 AWG TC fabricated wire thermocouples is for semiconductor package monitoring use)

The fine oxy-hydrogen welding torch got my attention. Could I use my oxy-acetylene torch with a very fine welding tip and carbon rod? Any thoughts?

#19: Post by **Marv** » November 28th, 2006, 1:15 am

another_jim wrote:I've got them above the beans, but they last in the moving ones too (where I've had them before).

Use a needle nose to twist the wires together, then fold them over. Don't overdo it, or they'll break, and have a good, non-nicking set of wire strippers, or they'll really break. Check them on boiling and ice cube water to make sure they are OK I've done about ten so far, and they've all been ± 1C, so, given my manual dexterity, I figure the method is pretty idiot proof.

Hi Jim,

Thanks for the reply.

How long are your thermocouples? To be specific 1) how long are the exposed wires - ie how much insulation is stripped off

and 2) when you fold this over is it in the midway point to have double thickness for strength

and 3) are the wires twisted before or after you fold them over ?

Thank you

Marvin

#20: Post by **gscace** » November 28th, 2006, 11:14 am

Jepy wrote:
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.

Some thermocouple wire solders quite easily - Type T in particular. Soldering is just fine. Welding is required for K.

-Greg