GEA24 and Type K Thermocouple wire

Hi folks (especially G3Xpert, I hope)

I have read many times that you should never put a break/coupler/connector in your EGT/CHT Type K thermocoupler wire on it's way to the display device. Not being an electrical engineer (just a SW guy) I assume that this kind of thing would introduce more resistance into the wire thus affecting the displayed temps.

Is this still the case using the GEA24 in the G3X Touch? I would dearly like to put a service connection at the firewall...

A thermocouple is nothing more than a junction of 2 different materials. It will generate a small temperature-dependent current. The issue with splicing wires, connectors, etc., is that each time you go to a different material (e.g., type K to aluminum crimp, or copper wire to aluminum crimp) you create a new junction which acts like a thermocouple in series with the others. Now, in general, if you have equal but opposite junctions, at the same temperature, the net additional current will be zero - they cancel each other. So in general going from type K to an aluminum crimp to a type K wire is okay, as long as the crimp is short and somewhat insulated, so the temperature at each end is the same. Same thing for connectors: type K to a male pin to a female pin to a type K wire is generally okay, if both ends of the connector are at the same temperature. You want to avoid going to a different type of wire. What you also need to avoid is using a connector to feed thru the firewall. Then, the engine side may be hot while the passenger compartment side may be cool. This would introduce some error.

Resist the urge

As a software guy myself, my instinct is to put an "interface" anywhere there is a transition to or through something, like the firewall or bulkheads or whatever. The thought is "I could just unplug/unhook this right here and then work on it" or whatever. I have to resist that urge, because every interface like that in an electrical connection, in an airplane, that is moving, and bucking, and vibrating, is a point of failure or corrosion. Run wires without any splices or connections for as long as you can, and then only splice/cut where necessary, and then reconnect with the most solid connector you can get (solder shrinks etc. Something that seals and holds). If you must have a connector, use something well regarded (I think Deutsch has it's fan's here?)

For your specific case, on the EGT/CHT, my understanding is the same as yours: the wires have a very specific resistance to them, and splicing them changes that and will throw off the readings. However, some sensors say "Lengthening or shortening wires will not affect readings". You will rarely if ever have to replace EGT/CHT sensors, so I wouldn't bother just for practical purposes, and because you really can't splice them (but you can cut to length and then crimp on the pin).

Thermocouple connectors

There are thermocouple connectors specifically for this, see link for an example:

https://au.rs-online.com/web/p/sensor-accessories/4559534

The thermocouples wires that pass through the fire wall in my A-22 do so via thermocouple connector plugs and sockets, leading to a Dynon D180. This works well, with the added convenience of being able to switch over a thermocouple for testing by swapping plugs between sockets.

From the product literature:

Product Details
Miniature Panel Sockets

The miniature thermocouple fascia mounting sockets with clip are available in types K, J, T, E, N, R/S and B. All fascia mounting sockets are polarised by means of unequally sized pins.


Panel mounting
Nylon clip for quick, simple fixing to panel
Colour coded in accordance with ANSI specification
Maximum operating temperature 220 °C


What is a Thermocouple Connector?

These miniature thermocouple connectors are an accurate and convenient way of interconnecting your temperature sensing components. Using thermocouple connectors allows a chain to be formed from the measuring tip of your temperature sensor to the host computer or meter. It is important that all the components in the chain are made from the same thermocouple material to prevent any alteration or distortion of the original signal. To achieve this, thermocouple connectors have contacts made from the same material as the thermocouple they are being used to connect or a compensating material. The connector housing has the thermocouple type clearly printed on it and is also colour-coded for easy identification. The thermocouple wires are simply screwed down into place using two captive screw clamps.


Features and Benefits

• Contacts made from true thermocouple alloys or a compensating material for accurate signal transmission
• Colour-coded for quick and easy identification
• Moulded fascia for flush fitting on front of panel
• Accepts polarized flat pin contacts for an accurate connection
• Made from rugged glass-filled thermoplastic
• Able to withstand temperatures of up to 120°C
• Will accept thermocouple cable up to 4mm in diameter
• Simple captive screw clamp connection for quick assembly
• Polarity and thermocouple type clearly marked on the socket housing
• Supplied with clip for quick and secure mounting to the panel


Applications

These miniature thermocouple connectors are used in any application where thermocouples are installed, extended or replaced. Their robust construction makes these thermocouple connectors ideal for use in demanding temperature measurement applications.


Accessories

Aluminium faceplates are available for use with these panel mount thermocouple connectors
Stock number 397-1674 (2 way)
Stock number 397-1703 (4 way)
Stock number 236-4132 (6 way)
Stock number 397-1719 (12 way)
Stock number 762-1156 (16 way)
Stock number 762-1165 (20 way)
Stock number 762-1168 (24 way)

Extra connectors = more failure points (and TC wires work with very low voltages so extra connections should be avoided).

Thanks a lot for all of the replies, guys, much appreciated. I'm thinking I will just go my original plan and have a continuous run from the GEA24 to each thermocouple. When/if I need to remove the engine/engine mount I will just remove the thermocouples themselves from the engine and let them dangle.

Cheers

Think same

rv8bldr said:

Thanks a lot for all of the replies, guys, much appreciated. I'm thinking I will just go my original plan and have a continuous run from the GEA24 to each thermocouple. When/if I need to remove the engine/engine mount I will just remove the thermocouples themselves from the engine and let them dangle.

Cheers

Click to expand...

I had the same thought process. I found high temp thermo wire on line for a reasonable price and just made the runs with no connectors; the engine should never come out on a regular basis.

TC Wire Splicing

As others have mentioned, if a splice must be made, ensure the splice for each wire is in close physical proximity to one another, to avoid exposing the new junctions to different ambient temperature.

Thanks,

Justin