REFLECTOR: EGT/CHT probe harness lengths and terminal block

Joe Ewen Jewen at comporium.net
Sat Nov 24 09:04:27 CST 2007


The purist will also tell you that the terminal block should be of the same 
material as the TC wire (Allen Bradley, Phoenix Contact among others produce 
TC connection terminals.)  Using a nickel plated terminal would add 4 more 
TC junctions, which should have offsetting errors.

Another connection approach is to use a TC plug/receptical.  An example is 
from Omega.com, GMP-K-MF, cost is 5.50 per set.

http://www.omega.com/search/esearch.asp?start=11&perPage=10&col=&summary=yes&sort=rank&Date1=&Date2=&type=&search=THERMOCOUPLE%2BCONNECTOR

Joe

----- Original Message ----- 
From: "Dave Philipsen" <velocity at davebiz.com>
To: "Velocity Aircraft Owners and Builders list" <reflector at tvbf.org>
Sent: Saturday, November 24, 2007 1:54 AM
Subject: Re: REFLECTOR: EGT/CHT probe harness lengths and terminal block


>I _believe_ that you can install a terminal block inside the engine
> compartment _if_ you use the correct thermocouple extension wire from
> the terminal block on into the wiring harness.  Theoretically, whenever
> two dissimilar metals are connected a small voltage is generated that
> could cause an error.  However, connecting two wires of the _same_ type
> with a third, dissimilar, metal (such as a terminal block) would negate
> that effect as long as the temperature of the terminal block is the same
> from one side to the other.
>
> At some point the wire composition has to change from that of
> thermocouple wire to some other kind of wire either at the harness
> connection or a connector on the back of your EFIS.  The EFIS probably
> compensates for that by measuring the temperature of the case with a
> thermistor and modifying the equation appropriately.  Your connection
> from thermocouple extension wire to the dissimilar harness wire is
> probably ok as long as the temperature of that junction is close to the
> temperature of the EFIS unit itself.  You definitely wouldn't want that
> junction to be in the engine compartment since the EFIS doesn't know the
> temperature of you engine compartment and can't correctly compensate for 
> it.
>
> This being said, some purists will tell you that you _must_ use only
> thermocouple or thermocouple extension wire from the thermocouple all
> the way to the EFIS unit.  You can always wire it up with the terminal
> block and then test the probe(s) with a known temperature heat source.
> Boiling water is 212 degrees F at standard sea level pressure.
>
> Andrew Ellzey wrote:
>> Question for the collective. What is everyone doing when ordering CHT and 
>> EGT probes as for cable lengths? I am installing BM EFIS one system and a 
>> Lycoming IO-540. Can a terminal board be added on the inside of the 
>> aircraft firewall to make a disconnect point for future probe changes and 
>> to make the harnesses reach? I have the BM built A3 sensor harness and 
>> the wiring from BM is unshielded 22 gauge wiring for the CHT and EGT 
>> inputs. This seems to be a mistake on their part. Westach sells extension 
>> leads up to 12 feet that is supposed to not affect accuracy. Is the the 
>> correct way to go with a terminal block inside the aircraft firewall? Has 
>> anyone used standard 22 gauge wiring and not had problems with probe 
>> accuracy? BM sensor inputs are completely programmable.
>>
>> Andy Ellzey
>> XLRG
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>>
>>
>
> -- 
> Dave Philipsen
> Velocity STD-FG
> N83DP
>
>
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