Re: Thermocouple compatible bulkhead connector?

Rick Hole

Yes, but note that in a firewall connector you will expect a higher temperature on the engine side compared to the cabin side. There will be an error but it really is not significant. Even if you use the bulkhead connector as the point where you convert the thermocouple metal wires into copper (and you will do this somewhere, at least at the instrument) and error will ocurr. If high accuracy is required, a second thermocouple is placed at the junction to copper for "cold junction" compensation. But we do not require that level of precision. EGTs are relative, and CHTs, well, if I am operating say 5 degrees from red line I will already have taken action.


--- In Q-LIST@..., "John Loram" <johnl@...> wrote:

Hi Rick:
When you splice a connector into a thermocouple wire, you actually create
two junctions (the point I missed ): one junction is created where the
thermocouple wire is attached to the plug side of the connector, and a
second junction is create where you connect the thermocouple wire to the
socket side of the connector. These two junctions are of opposite polarity,
so, if the two junctions are at the same temperature (as they would be since
they are plugged into one another) they electrically balance out one another
and will have no effect on the temperature measurement.

building like crazy, -john-


From: Q-LIST@... [mailto:Q-LIST@...] On Behalf Of
Sent: Thursday, February 04, 2010 3:11 PM
To: Q-LIST@...
Subject: [Q-LIST] Re: Thermocouple compatible bulkhead connector?

I was curious about this as in theory the additional dissimilar metal
junction will cause errors in the readings. As an experiment I tried
connecting a type K thermocouple to a digital thermometer readout with a
length of copper wire spliced in. With the thermocouple immersed in a hot
oil bath I watched the temperature displayed as I used a hot air gun to heat
the junction of copper to thermocouple wire. I thought I could see 1-2
degrees change heating the junction from ambient to as hot as the gun would
deliver, but it was difficult to tell the change from random drift.

I have forgotten if I repeated the experiment with a type J thermocouple.
The metals involved are different and I would not assume the same results
without testing.

So my conclusion is, for type K as used in EGT probes and often used for
CHT, you can use the non-thermocouple connector or even use copper extension
wire without significant error. And as long as all the cylinder's wires are
the same temperature at the change of metal, all the errors are the same
magnitude and direction, so with EGTs where only the trend is important, you
are doubly ok.

Rick Hole

--- In Q-LIST@yahoogroups. <> com, "John
Loram" <johnl@> wrote:

Thanks for that Sam. Your comment forced me to rethink the problem and I
realized that I had not considered that I was running both sides of each
thermocouple leg through the same connector (duh!), and consequently, the
effect of the dissimilar metals of the connector would be cancelled out so
long as both the male and the female pins of the connector are the same

Cool! thanks again, -john-

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