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Thermocouple Welding and Soldering

Thermocouple Welding and Soldering

Thermocouple Welding and Soldering

I am looking for an information source about welding and soldering Type T thermocouple (T/C) extension leads to another extension lead and also to the thermocouple wire.  

Specifically information regarding the technique, sources of error, and the pro/cons of welding vs soldering.

Why is there a special procedure/technique for welding Type T and what is the special process?

Does anyone have experience with either technique?

We have 68 Type T T/C channels located outside in New England.  We currently have Omega MTC-12 connectors mounted on masts with the male connector on an umbilical to the T/Cs.  The female bulkhead connector is mounted on a 4x4 enclosure.  The female bulkhead mount (admittedly) does not have the specified environmental backshell.  As expected we are experiencing erroneous readings and open T/Cs.  The connections should not have to be undone for 20-30 years. I would like to propose removing the connectors and welding or soldering the T/C cable wires at the connection at the 4x4.  Also, I would like to weld or solder the extension cables to the industrial T/C leads at the T/C peckerhead.  

PS: Does anyone have a better name for the "peckerhead"?

RE: Thermocouple Welding and Soldering


It is acceptable to make a thermocouple junction by soldering the two metals together as the solder will not affect the reading. In practice, however, thermocouples junctions are made by welding the two metals together (usually by capacitive discharge) as this ensures that the performance is not limited by the melting point of solder.

Notes on using thermocouples
Dr. Robert J. Moffat, Stanford University

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.

Beaded Thermocouple Temperature Measurement of Semiconductor Packages

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)

RE: Thermocouple Welding and Soldering

The formal term is "connection head".

soldering works, properly done. having been done the same road make sure that the junctions you make are near ambient.

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