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Thermocouple Lead Extension Splicing

Freemasm

Freemasm

Well Known Member
The question is not about extension lead materials, TC theory, cold junctions, etc. You basically have to introduce a junction in engine TC leads. Whatever you wind up doing will introduce some error. Easy to come up with an argument for/against any approach. What have people done to make splices that have a live-able installation?

- EI cold splice hardware. I want to believe this will be acceptable, 20 for $20 includes shipping and the odd sized allen wrench.

https://iflyei.com/product/olc-2/

- The "standard" Alcor TCs come with crimped-on ring connectors on the leads (see pic). Crimp or solder matching crimp rings to the extensions?

Other approach? What has been done here to prove reliable, close enough, measurements?

Your help. comments would be appreciated
 

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Thermocouple Splicing

The law of intermediate metals states that a third metal may be inserted into a thermocouple system without affecting the emf generated, if, and only if, the junctions with the third metal are kept at the same temperature.

https://instrumentationtools.com/la...ermediate metals,kept at the same temperature.


What this means to your case is that you can use copper terminals on the ends of the thermocouple extension leads to make the connection. We have even used terminal strips to do this. It is VERY IMPORTANT that you keep the assembly that had the new junctions that you have created with the intermediate metals (copper) at the same temperature. Simple keep them close together and wrap them with a bit of insulation material so that everything in the "splice" is as close as possible to the same temperature. This is a pretty common technique in instrumentation installations.
 
Well said larosta.

Half my TC’s were long enough to reach the RDAC without splices and half needed extensions. I used high quality crimp/heat sealed splices. After two years all the TC’s seem to track each other to a few degrees.
Mike
 
Thanks. Posting with backing info is always appreciated. From other opinions here and other places:

- Joints should only be soldered, no intermediate terminal
- Joints should only be silver soldered
- Terminals are OK but only if soldered; no crimping
- etc.

Since it appears I have an informed audience; another question please. Since the EM force is already established at the dissimilar metal joint, why is it necessary to have the lead extension materials the same as the lead? Why wouldn't leads made of copper work if the lengths are relatively the same? Can you get me smarter here as I'm not conceiving the need. Thx.
 
Thermocouple Cold Junction Compensation

Short answer - Your instrument needs to know the temperature / EMF of the thermocouples that are created at the point where the thermocouple wires are connected to, in your example, the copper extension wires. In your aircraft instrument the reference is established at the point where the thermocouple wires are connected to the instrument. In the case of using copper wire for the extension you would need to move this reference sensor to the point where the connection is made. It can and is done, in the case of an aircraft installation it gets difficult and would require modifications to your instruments.

Here is a document that describes what the reference junction does in the circuit.

Cold junction compensation for thermocouples (Omega)
https://www.omega.com/en-us/resources/thermocouple-junction-principles
 
Much thx to those who replied. Not conceptualizing it the way I should but the explanations have helped a lot. One realization (true or not), the EI connectors may be designed such that they are as much thermal isolators as electrical insulators. Would seemingly help keep the paired junctions closer in temp. If I’m guessing wrong, tell me. Thx again.
 
Stranded vs solid

Garmin recommends stranded extension leads, which makes sense. This is much harder to find than solid. Any life issues experienced with solid extensions? Thx.
 
Solid thermocouple extension wire is pretty durable and should be OK in this application. Just be diligent about supporting it by securing it to structure where possible and adding service loops to eliminate strain or flexing the wire as much as possible where it passes from one moving structure to another.
 
JPI gives you another wire that goes to the instrument on your panel and it ends just like your picture. You join them both with a supplied nut, washer, & bolt. tighten them up and you're done.

MGL gives you a male bayonet on one wire and a female on the other wire going to the panel. Push them together and you're done.

So apparently it's not so critical.
 
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