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

[Shivers]

K Type Thermocouple positive leg is composed of 90% nickel, 10%chromium and a negative leg is composed of 95% nickel, 2% aluminum, 2% manganese and 1% silicon

[Spoke]

K Type Thermocouple positive leg is composed of 90% nickel, 10%chromium and a negative leg is composed of 95% nickel, 2% aluminum, 2% manganese and 1% silicon

Question would be how far away from the joining of the 2 dissimilar metals can the 2 leads change to one type of conductor maybe not made of the same materials as either dissimilar metals?

When the thermocouple is connected to the gauge, both metals change to the type of metal found on PCBs. So somewhere away from the dissimilar joint it is ok to use different metals. Where is that point?

[Superhawk996]

K Type Thermocouple positive leg is composed of 90% nickel, 10%chromium and a negative leg is composed of 95% nickel, 2% aluminum, 2% manganese and 1% silicon

Question would be how far away from the joining of the 2 dissimilar metals can the 2 leads change to one type of conductor maybe not made of the same materials as either dissimilar metals?

When the thermocouple is connected to the gauge, both metals change to the type of metal found on PCBs. So somewhere away from the dissimilar joint it is ok to use different metals. Where is that point?

Anywhere two dissimilar metals join will become a new temperature sensing junction with it's own EMF produced at that junction.

In the case of attachment at the gauge, this is the cold junction and it is compensated.

If a dissimilar metal is introduced somewhere between the hot junction and the cold junction it too could be compensated (if temperature at that junction is known) but often that junction will be highly non-linear and would make it tough to compensate for.

I think the thing that throws people on thermocouples is that the sensing is not due to resistance change but due to the Seebeck effect that generates its own voltage potential between the two dissimilar metals as temperature changes. Resistance only begins to come into play when extension cables become very long (100' or so). I'm not saying resistance doesn't matter . . . just that resistance change isn't what drives the operation of a thermocouple.