REFLECTOR: EGT/CHT probe harness lengths and terminal block

[Brian Michalk]

All of the new electronic monitors will sense voltage generated across 
the thermocouple from the Seebeck effect.  If you are using a self 
powered analog gauge, then it is a current sensing device.  Since 
current is proportional to resistance, it is important to have your 
leads trimmed to the same resistance for an analog system.

Now, the electronic ones don't have the need to be trimmed.  However, 
the delta difference between the cold junction and the TC amplifier chip 
will show up as a bias on the instrument readout.  What this means is 
that your cold junction, the point where the alloyed TC wires end should 
  be very close physically to your box of electronics.  If you use an 
extension, it needs to use the same TC wiring alloys.  If you don't go 
through that, then at least make sure your cold junction has all of the 
TC wires terminating at the same temperature.  This would be best on the 
cold side of the firewall, as it's going to be closer to the same 
temperature as your electronics box.

If you want to measure peak EGT's, it doesn't matter what you do with 
the TC junctions because you don't care about absolute temperatures, you 
care about trend information.  If on the other hand you want to know 
CHT's, then you need accurate probes to keep from cracking cylinders.

I have two sets of probes for CHT's.  Six to a computer, and six to an 
analog gauge.  I had to trim the length of the analog ones to be very 
close in resistance.  Then I took some measurements and sent the gauge 
back to Westach where they recalibrated it to the TC resistances I 
provided to them.

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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