REFLECTOR: EGT/CHT probe harness lengths and terminal block

[Dave Philipsen]

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