Well, I did a thing, It was a thing I knew better then to do, too. (Thermocouple connecting) Options

[VaccaRabite]

Continuing to live my life as an object lesson for others...

I recently had to pull my engine for some repairs. For me, part of pulling the engine includes removing the thermocouple from under the #3 spark plug and fishing it out of the engine tins. This runs to the CHT gauge in the cabin of the car.

And we all know you are not supposed to cut the wire. Everybody knows that.

So I figured I'd cut the wire, add a nice weatherproof automotive plug, and see what happens. All in the name of convenience and curiosity.

I drove 300 miles last weekend. Including going fast up a really long couple of hills (like miles long hills) in 5th gear.
So the results.... At cold start, the gauge reads the same as ambient temps.
Under normal driving, the CHT gauge read slightly cooler then before I added the plug, but within 10 degrees of what I expected.

However, when pushing the car, the error rate grew exponentially. Where I would expect the car to be running 340-350 head temps, the gage only showed about 300. And pushing 90 up hill in 5th for over a mile I would expect the gage to read 360+ the gage only ever got to 325. I did not push past what I know this engine required to get to 375 (which is my back off point).

The result of this experiment is that if I did not know and expect bogus returns on the gage, I could easily roast an engine and start dropping valve seats, while a quality CHT gage was reporting I was WELL under the 400 degree danger zone.

I have another cable I can swap in. Kind of a pain, but not a huge deal. I'll add it o the maintenance schedule.

But, yeah. In this case the group think is accurate. The very slight resistance that the plug will add to the cable will be enough to throw off your CHT monitor.

Do not cut the line between the thermocouple and your CHT gage.

Zach

[iankarr]

Is there any difference in the makeup of that extension wire? I mean, is the positive wire a different metal, etc. Or are the red and yellow sides the same and just color-coded to make sure it hooks up to the thermocouple correctly?

[Superhawk996]

Is there any difference in the makeup of that extension wire? I mean, is the positive wire a different metal, etc. Or are the red and yellow sides the same and just color-coded to make sure it hooks up to the thermocouple correctly?

Yes, the thermocouple extension wires are made of different materials between the positive (yellow) and the negative (red) wires.

Thermocouple extension wire is very similar to the sensing wire, but uses less of the expensive metals (Nickel, Chromium,etc.) content in its construction to keep cost lower. However, the overall characteristics of the extension wire are still very close to the sensing wire and are VERY different from simple copper wire. Because the extension wire is slightly different than the sensing wire - its use comes with some constraints such as a more limited range of operation temperature. Example: Extension wire needs to be kept below 200C (not a problem for 914 application) and you want the temperature across the thermocouple extension (end to end) wire to be as uniform as possible.

Introduction of a third metal (like copper extension wire or copper/tin connector terminals) into the circuit will add a 3rd temperature junction that is uncompensated and will affect the accuracy and calibration of the thermocouple.

@Iankarr

Thermocouple extension wire also has different temperature limits and tolerance than the sensing wire. There are more expensive grades of extension wire that are used when higher accuracy is needed (doesn’t apply to our application).

When I used to do data acquisition, it wasn't uncommon to have to run long thermocouple extensions from the test cell into a control room - sometimes 75' or so. It would get expensive stringing sensing wire the whole way - especially when you may have 40 or 50 thermouple channels under test. In the case of our 914's that distance from the engine to the cabin in really only 5 feet so using sensing wire the whole way wouldn't be unreasonable.