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thermocouple wiring

 
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klehman(at)albedo.net
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PostPosted: Sat Feb 18, 2006 6:03 am    Post subject: thermocouple wiring Reply with quote

IF I understand this correctly, I would partly agree with both of you
Gary and Gilles.

I think it is fine to use almost any kind of connector to go from TC
wire to TC wire as long as all parts of the connector are about the same
temperature. So it is fine to use a connector that is entirely forward
of the firewall or entirely behind the firewall and there is no real
need for expensive TC rated connectors. Avoid using a connector as a
firewall pass through as the temperature will be different on each side.

TC extension wire is cheap enough ( I seem to recall $0.65 per foot for
the twisted pair) that I avoided any TC wire to copper wire transitions
but I do have TC wire to TC wire push together spade PIDG joints forward
of the firewall. This is consistant with the recommendations for wiring
the Grand Rapids EIS engine monitor. TC wire is almost certainly cheaper
and easier to install than any kind of extra junction compensation box.
As long as all parts of the transition from TC wire to a connector and
back to TC wire are at the same temperature, we will be fine.

Ken

Gary Casey wrote:

Quote:


I think you are over-reacting to the cold junction thing. True, for
instrumentation-grade measurements (temperature accuracy to a
fraction of a degree) it is a good thing to have no metal changes in
the circuit all the way back to the cold junction. However, that
would mean that all the connector terminals would also have to be
made from like materials. As long as both wires make the transition
to another metal at the same temperature the measurement error will
be small, like a few degrees. Is the extra accuracy worth the
effort? The commercial systems of which I am familiar use copper
wire for interconnects. If that's not true I still have time to
replace all the copper in my system...
Gary Casey



>Hi Bob and all,
>
>After examining carefully the TC probe sold with the UMA CHT, the
>situation
>appears even worse than I expected. The probe has only 2 ft TC
>lead wires,
>and then copper extension wires. So with my in-line engine,
>cylinder 2 the
>hottest, the "cold" junction will be INSIDE the engine compartment,
>where
>temperature can rise somewhat higher than the 75F the instrument is
>calibrated for!
>
>So, I bought TC connectors and extension wires from Omega to move
>the cold
>junction close to the instrument.
>Jean-Pierre Castiello, who designed the A320 brakes temp
>monitoring, is to
>craft for me an external compensation box based on the same AD594.
>Input is
>the J extension, output will directly drive the CHT air core movement.
>
>I will let you know the result...
>
>Thanks to all of you for the so precious informations I could
>gather from
>the list!
>
>Gilles
>
>
>


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