Corrections for connectors of differing metals are difficult to do unless you know the temperatures on both sides of the junction in real time. This is practically impossible, since you would have instrument both sides of the connector with more thermocouples. Any two dissimilar metal junctions respond to temperature with a unique function. If you have, say a copper connector under the firewall, one end of the copper adds a temperature dependent voltage and the other end reduces the voltage same amount, but only if both ends are at the same temperature. So that is the reason for the problem crossing the firewall. The connection calibration at the instrument or engine monitoring box is built in to the software so is predictable and transparent to the user.
I am attaching some info on how I did my firewall connectors for EGT and CHT. I have 6 cylinders, so I built up two plugs, each carrying 3 EGT’s and 3 CHT’s, one set on the left and one on the right (see last photo below). The first pic below is on the front of the firewall, right side. This is before I connected the wires into the individual plugs, but you can see the 6 holes for them in the aluminum “container” that I built to contain all 6 individual male plugs. When you disconnect the plug from the firewall, there are 12 prongs! The second pic is on the back side of the firewall. There is no aluminum “container” on this side since they are held down by the two phenolic blocks on either side of the hole. The 6 separate two-prong plugs have a convenient hole in the housing that allowed me to put piece of welding rod through them 3 at a time and gang them together. The ends of the welding rod are clamped to firewall by the phenolic blocks. The black screw heads on the firewall side (first photo) go through the firewall into threaded holes in the phenolic on the back side to secure the plug in place and seal the hole. I am loading the spec sheet for the connectors and some Nuckolls article on thermocouples into the files section, if you want more info.
Jay Scheevel, Tri-Q2 N8WQ, 92 hours
From: main@Q-List.groups.io <main@Q-List.groups.io> On Behalf Of Joe Hood
Sent: Thursday, May 28, 2020 6:20 AM
Subject: Re: [Q-List] Firewall electric thru connections
I used to enjoy wiring up Canon plugs on A-4s. Had no idea on the dissimilar metals and "cold junctions." Can't the instrument be re-calibrated?
Found a pretty good primer on thermocouple extensions here (I learn something new every day):