Bending thermocouple wire

[drone_pilot]

Hello All,

I'm running the brown thermocouple wire from my SkyView EMS to the engine compartment for EGTs and CHTs. I've looked and can't find the exact answer that I'm looking for. Is it okay to leave the brown insulated thermocouple wires long and bundle the excess with either a tefzel tie wrap or Adel clamp forward of the firewall? I cannot see too much of an issue here except the wire appears to be solid rather than stranded. Please let me know your opinions.

[larosta]

As long as you do not place the bundle near a varying electromagnetic field (inductive or RF), or a heat source that would degrade the insulation, there should not be a problem with bundling it. We bundle the excess thermocouple wire all of the time in the instrumentation world.

larosta

[1001001]

Originally Posted by drone_pilot

Hello All,

I'm running the brown thermocouple wire from my SkyView EMS to the engine compartment for EGTs and CHTs. I've looked and can't find the exact answer that I'm looking for. Is it okay to leave the brown insulated thermocouple wires long and bundle the excess with either a tefzel tie wrap or Adel clamp forward of the firewall? I cannot see too much of an issue here except the wire appears to be solid rather than stranded. Please let me know your opinions.

You should never cut thermocouple wires unless you are prepared to recalibrate the sensor that reads them. Thermocouple wires are carefully sized to provide a consistent resistance so that the voltage produced by the junction is read by the sensor properly. The shorter the wires, the lower the voltage drop. Since thermocouples rely on a dissimilar metal junction producing a very low voltage signal proportional to temperature, cutting the long wires shorter will change the signal and hence the indicated temperature.

[mburch]

Hmm... you sure about that?

[Walt]

Originally Posted by 1001001

You should never cut thermocouple wires unless you are prepared to recalibrate the sensor that reads them. Thermocouple wires are carefully sized to provide a consistent resistance so that the voltage produced by the junction is read by the sensor properly. The shorter the wires, the lower the voltage drop. Since thermocouples rely on a dissimilar metal junction producing a very low voltage signal proportional to temperature, cutting the long wires shorter will change the signal and hence the indicated temperature.

I've never heard that before about EGT/CHT thermocouple wiring, do you have a reference I could look at?

In fact to the contrary my suggestion to the OP would be cut off the excess if it's practical to do so.

[Jesse]

I'm with Walt on this one. Cut off the excess and then secure.

[wjb]

Me, too. Thermocouples generate a voltage difference across a dissimilar metal junction that's a function of the temperature of the junction, but don't (and can't) generate any appreciable current, something that the IR (current*resistance) drop would affect. There's no issue cutting the wires to the correct length.

They are, yes, usually a single solid wire (not a fine gauge stranded set), so minimizing tight radius bending will contribute to their longevity.

Feel free to leave them long, or cut them as you see fit.

Checkout the Omega Engineering FAQ for more detail (http://www.omega.com/prodinfo/thermocouples.html). No association with Omega other than being a very happy customer over the years. (and, they probably make your thermocouple)

[1001001]

Originally Posted by Walt

I've never heard that before about EGT/CHT thermocouple wiring, do you have a reference I could look at?

In fact to the contrary my suggestion to the OP would be cut off the excess if it's practical to do so.

I don't have a reference handy (I'll try to find one) but in industrial control systems great care is taken to properly calibrate the resistance of thermocouple wires. If wires need to be shortened or lengthened the sensor must be recalibrated. In addition, only special "thermocouple wire" is used to extend thermocouples. In industrial controls, typically it is now simpler to run a short thermocouple wire to a local transmitter than to run thermocouple wire all the way back to the control panels as was done commonly in the "olden days." I work in an industry in which we have equipment that has been running since the 1940s alongside brand new stuff. It's interesting to see the changes.

Anyway I assume that these EGT sensors are Type K thermocouples, based on the typical temperatures encountered. Is that correct?

[Mich48041]

I worked with thermocouples in an industrial plant. The thermocouple wire was randomly cut to whatever length needed to reach from the thermocouple probe to the instrument input. Like other have said, there is so little current flow that the resistance of the wire is not a factor. A few feet more or less in an aircraft will not make any difference. I agree with the majority.

[1001001]

Originally Posted by Mich48041

I worked with thermocouples in an industrial plant. The thermocouple wire was randomly cut to whatever length needed to reach from the thermocouple probe to the instrument input. Like other have said, there is so little current flow that the resistance of the wire is not a factor. A few feet more or less in an aircraft will not make any difference. I agree with the majority.

Wow, maybe I have been told bad information over the years. Based on the places I have worked and the people I've worked with I have always been told that thermocouples are very sensitive to wire length and type.

I guess I may need to learn some new things about thermocouples based on the near universal disagreement on the subject I'm seeing here. I guess it is off to the books for me!