G

gmpaul

Active Member
Today I drilled a hole in my firewall for my wiring. Added a split o-ring and started poking the wire through. All I need to do now is add some extension wire. This is when I had a Brain Cramp!

Wouldn't it be a clean installation to use a military bulkhead connector? You know the 12-pin with J and K pins for temps.

I Googled up connectors and the prices weren't bad until you add type J & K pins. Then Oh My Gosh...the prices are out of site!

Here's my question: What did everybody else use for thermocouple connectors?

Thanks in advance,

GP
 
Well I may be missing it.....but

gmpaul said:
Wouldn't it be a clean installation to use a military bulkhead connector? You know the 12-pin with J and K pins for temps. I Googled up connectors and the prices weren't bad until you add type J & K pins. Then Oh My Gosh...the prices are out of site!
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Paul what J & K pin? Do you mean J and K junction thermocouple for CHT and EGT (respectively)? What does that have to do with a bulkhead connector? I think I got it. My answer (opinion) is a firm don't.

Why are you making more work, effort and cost. Are you going to remove your engine or tear your plane down every other week? What do you gain by doing this? Will your gauge work properly with a connector in the extension wires for the EGT/CHT? The answer may be NO.

When you run the wires, consider they will be there for the rest of your life or the life of the plane. There is no reason to make a service connection for the EGT or CHT. Think LIGHT WEIGHT, LOW COST and SIMPLE. Buying expensive mil-spec plugs does not really meet these criteria. Again I ask why? It look cool?

gmpaul said:
Here's my question: What did everybody else use for thermocouple connectors? Thanks in advance,GP
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I get in trouble trying to answer questions I don't have all the info on, so I may be missing something; if so sorry.

What thermocouple connector does everyone else use? Usually the one that came with the gauge. CHT or EGT probes, probe extension wire and the gauge are a matched set.

Thermocouples (probes) and their extension wires or junctions fall under a SPECIAL kind of wiring. It usually is a MATCHED set with the gauge / instrument. In some cases (most), it requires careful attention to the instructions the gauge and probe came with. If you need to extend the probe wire sometimes plan wire can be used, other times you need special K or J junction extension wire. The manufactures instructions can tell you this.

Hope that helps, if not may be some one else will be able to help, or may be you can say what kind, model and brand of gauge you are talking about, CHT or EGT and what you are trying to do. From what I gather I WOULD NOT introduce extra "service plugs" or bulkhead plugs in the wires.

Lets say its OK to cut wire to and from a thermal couple (EGT/CHT probe) and you could put a big Mil-Spec plug break in the firewall for example. What would that do for your REALLY?

Good luck.
 
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After talking with EIS, EI, and other engine monitor companies, it seems there is little if any specific requirements. J or K thermocouples can be connected to just about any type of connector. However, the requirement is to use thermocouple wire (usually solid conductor), and *never* solder them.

With that said, some thermocouples use a "grounded" type connector. If you have those kinds, they are *extremely* sensative to what type of wire, the lenght of it, etc. However, with that said, I know of no engine monitor system that uses a grounded connector.

In my install (and most of the Lancairs flying), the thermocouples have RMS connectors that are used to take the wiring thru the firewall, these are metal, fireproof CPC type connectors in high density. Also, in my installation, I'm going to use the "Omega" type 2 pin connectors for each CHT and EGT probe. These can be had in either regular temp or ceramic for high temp environments. Cessna uses the regular temp ones on all their new airplanes, however the price difference was only about .20 to go to the ceramic, so I went that way.

Here's what they look like from my 182T
DSCN0190_edited_1.sized.jpg


I don't have the web address handy, but if you google omega and thermocouple, I'm sure you'll find them. They do sell on the web and direct too.

These 2 pin connectors as just used to make removal or replacement easy and usually go about 4-6" away from the probe, or where ever the metal spring ends on some probes.

Hope this helps.
 
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I just drilled holes and fed them through

I installed the EI 4 probe EGT and CHT gauges and used the wiring they supplied. In this system there are three wire segments for each element of the thermocouple pair the initial thermocouple metal half segment, an extension segment and a segment at the instrument. The three segments are interconnected with identical insulated male blade and female rolled edge terminals. The interface is reversed on each pair so the user is unable to cross wire the thermocouple probe half to the wrong instrument input. The company cautions you not to reterminate the wires unless it is necessary and to stow the excess behind the instrument panel. Obviously they do not want you to install a bulkhead connector interface in the wire run. The conductors are solid (not stranded) silver colored metal but I do not know the content (probable one whose resistance is not sensitive to temperature and is not a good thermal conductor) - I suspect the all of the wire in the segments except the dissimilar metal in the thermocouple segment is the same - I know the terminals are identical. The Alcor single probe EGT was similarly segmented but the interfaces were small standard looking "ring" terminals overlapped and fastened together with small (#4 probably) screws and nuts. The point is to make the circuit interfaces rock solid with as low and invariable resistance as possible so as not to drop or vary the the small voltage produced by the thermocouple across the instrument. I drilled holes in the firewall put a rubber gromet on the wires, inserted it into the firewall hole and sealed with Dow 736 hi-temp RTV.

Bob Axsom
 
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Thanks again Guys,
I'm sorry that I did not give enough infomation. My egt and cht probes came in a kit frome GRT. They were about 10 to 12 inches too short. I should have specified a length when ordering but I did not know at the time. I called Grand Rapids and they told me to just go ahead and use a standard terminal block with thermocouple extention wires. The difference in the reading wouldn't be much. This answer still bothered me so I called (my guru) Mell, and he confirmed their answer.

Alan, thanks for the picture, I'll probably go that way.
 
GP.. the important thing is to use the same materials in each lead for each pair of wires.
Each dissimilar metal to metal joint will create it's own little thermocouple.... if you have the same joints in each lead, these unwanted thermocouples will cancel each other out...

gil in Tucson
 
Thermocoupler wire.

i too am at the stage of wiring my cht's and egt's for my Odyssee install from MGL Avionics. The egt's have about 3 feet of wire on them whereas the cht's only have about 6 inches. I was planning on placing the RDAC X on the instrument subpanel as to keep it out of the elements and heat as much as possible, so even the egt leads will not be long enuf. After talking to MGL they said icould use pretty much any twisted copper wire of 18 or 20 gauge as extension. However after reading this thread I am certainly confused on if this is actually the proper way to procede. And of course it is Friday evening!!

Does anyone know how much error could actually be produced usingf regular copper wire? A very small difference would not be so bad, but say over 25 degree error would be.

Thank you.

jarvis
 
Omega

Here is a link to some tech info on thermocouples by Omega. Check out the sections on extension wire vs thermocouple wire and "cold junction compensation" for a discussion of what happens when dissimilar metals are used.

They'll also send you a CD with all of their catalogs and applications tutorials etc por nada.

http://www.omega.com/temperature/Z/zsection.asp

Have fun!
 
MGL RDAC-X

jrvssgl said:
i too am at the stage of wiring my cht's and egt's for my Odyssee install from MGL Avionics. The egt's have about 3 feet of wire on them whereas the cht's only have about 6 inches. I was planning on placing the RDAC X on the instrument subpanel as to keep it out of the elements and heat as much as possible, so even the egt leads will not be long enuf. After talking to MGL they said icould use pretty much any twisted copper wire of 18 or 20 gauge as extension. However after reading this thread I am certainly confused on if this is actually the proper way to procede. And of course it is Friday evening!!

Does anyone know how much error could actually be produced usingf regular copper wire? A very small difference would not be so bad, but say over 25 degree error would be.

Thank you.

jarvis
Click to expand...

FYI- The RDACs from MGL are built to withstand the harsh conditions of living in the engine compartment. The biggest advantage to this is it eliminates a bunch of wiring through the firewall to you panel. I don't know about you, but I've got enough wires behind my panel....
 
i agree it would be a nicer install on the front of the firewall, as far as wiring concerns go, but to me the rdac does not look all that robust. It is not a sealed unit either. Are you going to build it into some kind of an enclosure or just leave it hangin out? Even in the engine compartment the leads will still have to be extended.
 
rdac

Hi Jarvis,

I am just about to install my RDAC XD. MGL states right at the begining of their installation manual "this part is intended to be installed on an aircraft firewall (engine compartment side)". As stated in another reply this saves running alot of wires thru firewall and it is very easy to get to and visually inspect after plane is finished. I have been using MGL products for six years now and they have always been correct in their installation recomendations.

Steve
 
In answer to Jarvis' question, the junction between a thermocouple wire (alumel or chromel to copper) is called a cold junction. This kind of junction will result in an error in indicated temperature approximately equal to the ambient temperature where this cold junction is located. In reality this can be a sizeable inaccuracy if the junction is located forward of the firewall. The other kicker is that it won't be a constant error because it will change with ambient temperature changes. I guess it all depends on how accurate you want your readings to be...

On another note, if you need to solder thermocouple wire, silver solder works just fine. Anybody who's worked on OH-6/MD-500 helicopters will be familiar with this repair technique...
 
It will be OK...

Canadian_JOY said:
In answer to Jarvis' question, the junction between a thermocouple wire (alumel or chromel to copper) is called a cold junction. This kind of junction will result in an error in indicated temperature approximately equal to the ambient temperature where this cold junction is located. In reality this can be a sizeable inaccuracy if the junction is located forward of the firewall. The other kicker is that it won't be a constant error because it will change with ambient temperature changes. I guess it all depends on how accurate you want your readings to be...

.......
Click to expand...

...if there is an "equal joint" in both leads.

It doesn't matter what the "so-called error" is that changes with temperature - if the same type of joint is in each lead, and at the same temperature (because the two joints are next to each other), the "error" voltage is identical in each lead, but with opposite signs, and cancel each other out. Temperature changes (of the wire/connector combo) have no effect.

Back to the original #1 post, a military connector with crimped pins would be OK...
 
The large electronic houses have alumel and chromel pins to use in Mil Spec circular connectors. ITT Cannon or Amphenol. Check Mouser or Newark. I have found the prices at Mouser a little lower. You want a crimp type contact. On large aircraft there are many connectors between the engine and the indicator. It is ok to use a connector just get the right pins and sockets and you are all set.
 
Resurrecting... I'm about to start installing my CHT / EGT thermocouples and Type K TC wiring. The EIS box side wire has been pinned and installed in the d sub per Garmin diagram. However, I have to connect the wire from the box to the actual thermocouples (Alcor.) These devices have two staggered ring terminals, and the TC wire is bare ended. How do I make the connection? The Nuckols method suggests making hairpins and silver soldering the connection, or using special TC connectors. I'm thinking that I want a service connection since the previous EIS system was inop, and have been leaning towards the connectors (about $6/pair,) but I was thinking about *maybe* using d sub pins without the shell to make the connections. Would this work, or would the dissimilar metals throw off the temperature readings? I'd heat shrink the pins for protection.
 
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I have the Dynon engine sensor kit. I had read about issues with the crimped on connectors sometimes being a problem so I used the Omega connectors. They are a little bulky but seem to be a solid connection. The connectors have an eyelet in the center so I ran a tie wrap through them to lock the connectors together.

Disregard the wiring mess as I have not finished the final routing and clamping yet.


b557d9f9-e774-4404-8f9d-d074d929cdd0_zps1c6e3d92.jpg
 
CHEAP + SIMPLE

I like Walt's connector.
For a mechanical connector, it looks tops, because the wires overlap = 1 connection. Much better than two pin/socket crimps and the pin/socket connection. Total = 3 connections.
I went CHEAP and got some un-insulated high quality ring terminals. I cut off the rings and overlapped the thermocouple & extension wires, then crimped them with a full cycle crimper. after that, I slid shrink wrap over the joint and put the heat gun to it.
Very inexpensive, very light, and very small. Not too ugly.
Works fine.
Easily repeated, and if you must separate the connection, just cut it out. You only lose about 1 inch of wire to remake it. You should have enough service loop to make a few future repairs.
 
Interesting topic.

The issue of joining thermocouples with extension wire and its effect on accuracy has a lot to do with the device doing the actual measurement of the voltage generated by the thermocouple and what sort of reference junction compensation is done.

Most ICs used for TC measurement (all?) provide reference junction compensation by measuring the temperature of the reference junction. Hence for those devices that are located in a "warm" area in front of the firewall the temperature of the measurement device and the TC connections is accounted for (assuming the connections are close to the IC and therefore the "same" temperature. The temp ranges compensated for are typically -20 - +85 degrees C providing 2-3 deg C accuracy over the measurement range of the TC.

Now if your electronics are in the cockpit and you make an extension junction in front of the firewall with anything other than TC wire of the same type as the TC then accuracy will vary as the temp varies in the cockpit versus the engine compartment.

Hence the best way to avoid dealing with all this is an engine compartment mounted sensor box that communicates to the display via some sort of bus (serial, ethernet, CAN), etc.

So the system design award goes to MGL for this case IMO.

Military pins of same metal and TC extension wire solve the problem for other system approaches.

The signal conditioning required to pull a millivolt level signal out of a noisy environment is impressive. We can thank the auto industry for driving the cost of the chips down to a few bucks. With open source software on the way we can look forward to cheaper system costs in the future as well....
 
I bought a roll of thermocouple wire from omega, which is way cheaper than buying the extension leads, and I silver soldered all the joints and put heat shrink on the joints. Silver solder is an acceptable procedure for this material. Perhaps that info is on the omega site but I think I read it in the Aeroelectric Connection.

4 yrs and no issues.
 
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