Message-ID: <39AD2DC3.17F599AB@regandesigns.com>
Date: Wed, 30 Aug 2000 10:52:35 -0500
From: Brent Regan <brent@regandesigns.com>
To: Lancair List <lancair.list@olsusa.com>
Subject: Connectors in OAT wiring
Reply-To: lancair.list@olsusa.com
Mime-Version: 1.0

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If one end of a conductive (and some semiconductive) piece of material is
hotter than the other, a voltage gradient will be present between the ends.
The magnitude of the voltage as a function of temperature varies from
material to material and is called the Seebeck coefficient, after the guy
who discovered that if you join two pieces of different material and heat
the junction end you can measure the DIFFERENCE between the two voltages at
the cold end. The thermocouple is born.

Contrary to popular belief, the voltage is not generated at the hot junction
of the thermocouple but along the wire itself. One wire has a higher voltage
across it than the other, due to a different Seebeck coefficient. Think of
it as a 1 volt and 3 volt battery connected positive to positive. If you
measure the voltage across the negative terminals, you will see the
difference, or 2 volts.

So here is my question. If both wires of the thermocouple pass through a
connector where the materials used are the same for both circuits and the
entire connector is at one temperature, what does it matter what material is
used in the connector?

I understand that if the connector passes though the firewall then the
difference in temperature across the connector pins will produce a voltage
that may cause an offset, but if the connector was at one temperature, no
offset voltage would be generated. On the other hand, the materials used for
thermocouple wire are alloys of iron and aluminum, both subject to
oxidation. Any connector using these materials risks developing an
insulating oxide on the contact surfaces.

I would propose that gold plated inline contacts would be the best choice
for thermocouple connectors under the cowl. Being a connectorphobic, I
silver soldered all 14 of the thermocouple leads on my engine. Since I
installed inconel EGT probes (I fried the stainless ones) I haven't had a
single problem.

Regards
Brent Regan

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