F

Flying Scotsman

Well Known Member
Just a quick question...I'm ready to connect up the Dynon EMS harness to the EGT and CHT probe wires, but the set of probes/wires that came with the Skyview are uber-long. I mean like several feet long. They're beautifully made, and I hesitate to goober them up by trying to cut them and re-install/crimp on connecters (I'd rather use what's there already). Anybody have any experience/recommendations for dealing with these longish thingies?

Thanks much...

Steve
 
Do not cut CHT/EGT probes...

I think it might be the Dynon manual that prohibits it, could be somewhere else though, that I read the thermocouple probes are calibrated for their wire length and any trimming thereof upsets the calibration.
 
Probes

DO NOT cut the actual probe wires. These already have connectors on them. The harness wires (covered in brown insulation) are meant to be cut as they have no connectors. In my installation I brought all the probe wires to a convenient location on the lower aft arm of the engine mount on each side. Gently bent the probe wires to make all the connectors arrive at roughly the same point, and tie wrapped them there. Then cut the harness wires to length and installed connectors. Coupled it all together and tie wrapped the harness to the engine mount as needed. Please remember to use the self sticking silicon tape under any tie wrap around the engine mount.
 
I'm routing my CHT/EGT wires now... was thinking that two 8-position terminal strips (L&R sides of the firewall) would be a nice way to handle these connections. Any reason to avoid terminal strips for this? I'm building an experimental piper with a really short engine/firewall distance; probe wires reach the firewall no problem.

REALLY perfect would be a bulkead/pass-through type terminal strip, so the harness wires would connect on the cold side of the firewall. Anybody seen such a thing in 8-position with possibility for at least one side to be spade connectors? My sensors are pre-terminated with 1/4" female spades, harness is unterminated right now (dynon).

thanks everyone!
 
I'm with the guys that say don't mess with the thermal couple wires

I'm with the guys that say don't mess with the thermal couple wires. I just tied them into the wire bundle behind the instrument panel. They work fine.

Bob Axsom
 
No Terminal Strips!!!

If you make a junction in thermocouple wire using terminal strips, unless you get magic terminal strips that are made of the correct thermocouple alloy, you will introduce an error. The error comes from the small difference in temperature at the two connections to the terminal strip.

Now, if the terminal strip is not covered by anything, chances are good that both terminal junctions for each wire will be at the same temperature, and you are probably OK. After all, we don't care about accuracy down to better than 5 degrees or something.

BUT....

If you were to make a firewall pass-through terminal strip, so that one junction of each thermocouple wire was forward of the firewall, and the other junction was aft of the firewall, then you will introduce HUGE errors.

Each place where the alloy changes forms a temperature-sensitive junction. So if you go from Chromel to copper then back to Chromel at a junction strip, then the temperature difference between the two connections will make a spurious voltage, an error. Thats why REAL thermocouple connectors have plugs and sockets that are made from the correct alloy and are specific to the type of thermocouple.




alpinestar said:
I'm routing my CHT/EGT wires now... was thinking that two 8-position terminal strips (L&R sides of the firewall) would be a nice way to handle these connections. Any reason to avoid terminal strips for this? I'm building an experimental piper with a really short engine/firewall distance; probe wires reach the firewall no problem.

REALLY perfect would be a bulkead/pass-through type terminal strip, so the harness wires would connect on the cold side of the firewall. Anybody seen such a thing in 8-position with possibility for at least one side to be spade connectors? My sensors are pre-terminated with 1/4" female spades, harness is unterminated right now (dynon).

thanks everyone!
Click to expand...
 
didn't realize the sensitivity of the connections... basically rules out the pass-through terminal strip idea. I still like the thought of a conventional terminal strip on the firewall from a neatness point of view... it would be uncovered and (hopefully) an even temperature. I think I'll go this route & leave enough slack to revert to a non-terminal strip approach if needed. Without the terminal strip, I have a lot of slack to coil up & secure somehow... just gets to be a rats nest & I'm picky about wiring neatness! any other ideas welcome.
thanks for the input.
 
I was afraid to cut the probe wires, so I bent and routed the probe wires, then trimmed the EMS leads, so that the connections weren't on top of one another, then slid them into a length of firesleave. Sealed one end to keep oil and dirt out.





Routing the wired on the starboard side, prior to trimming the EMS leads:


 
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I would not cut them, and have the joints accessible as in 120 hours I have had to replace 3 of my Dynon EGT probs, just waiting for the forth to go! You can tell when there gone when one of your EGTs starts wandering a bit, then reads low. I thought I had an injector problem on the first one. Now it's just a regular thing!

Dave
 
There is nothing wrong with cutting the brown leads that come from computer harness. Do not cut the egt probe lines! Terminate them with the crimp connectors that are provided with the probes. If they are not provided, call Stein and get them from them.
 
All you wanted to know...

and were afraid to ask. Omega has been in the measurement business for many years. Here is their page on thermocouples. Read on and have fun. By the way, the "magic block" is an isothermal block which they mention in the article. Some of the mystery comes from the older gauges which have a cold compensation calibration inside.

http://www.omega.com/temperature/z/pdf/z021-032.pdf

Good luck,
Paul
 
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