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Thermocouple Wire Connections

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Valin

Joined: 13 Apr 2010
Posts: 31
Location: Colorado

Posted: Mon May 18, 2015 11:39 am Post subject: Thermocouple Wire Connections

Hello,



We're on the verge of connecting our Garmin GEA 24 Engine Airframe Processor/Box to our engine's thermocouples. Each engine thermocouple has a pigtail of two wires with ring terminals on them.  My question is, do we have to use the ring terminals to make the connection to our thermocouple wires running from the GEA 24?  



I know the thermocouple temperature measurement is all about the electrical resistance being observed so I assume one needs as low a resistance connection as possible for best accuracy.  Maybe two ring terminals held together with a screw/nut is the best -- but, wondering if knife type connections or something else would be as good...?



Thanks,



Valin

Lancair Legacy Project

Colorado



Sent by iPhone

recapen(at)earthlink.net
Guest

Posted: Mon May 18, 2015 12:06 pm Post subject: Thermocouple Wire Connections

There's plenty in the archives on this one....



As long as both sides of the connector are on the same side of the firewall and physically tight, you should be OK.  The trick is that the actual wire has almost no elasticity to it so relying on a crimp connection is the weak point.  I have experience with this in the first few hours - after that...I used the method suggested by the avionics manufacturer.....crimp plus solder quick disconnects with insulation 

to keep the poles apart.  Happy now after a couple hundred hours on it.....



--

stein(at)steinair.com
Guest

Posted: Mon May 18, 2015 12:15 pm Post subject: Thermocouple Wire Connections

Those connections aren't necessarily as important as having them all

consistently the same (whether it be ring terminals, butt-splices, knife

splices, pins, or whatever).  Sure there will be increased resistance at

that junction, but if they are all uniform you won't see an unequal result.



Cheers,

Stein 





--

Valin

Joined: 13 Apr 2010
Posts: 31
Location: Colorado

Posted: Mon May 18, 2015 1:48 pm Post subject: Thermocouple Wire Connections

Thanks guys. 



Stein, I see where it's okay as long as each temp probe has the same bias for EGTs. Seems like I'd want the CHTs to be as accurate as possible, though...  Is there a best connection type for accuracy?



Thanks,



Valin



Sent by iPhone



[quote] On May 18, 2015, at 2:14 PM, Stein Bruch <stein(at)steinair.com> wrote:

 

 

 

 Those connections aren't necessarily as important as having them all

 consistently the same (whether it be ring terminals, butt-splices, knife

 splices, pins, or whatever).  Sure there will be increased resistance at

 that junction, but if they are all uniform you won't see an unequal result.

 

 Cheers,

 Stein 

 

 

 

 

 --

millner(at)me.com
Guest

Posted: Mon May 18, 2015 2:09 pm Post subject: Thermocouple Wire Connections

On 5/18/2015 12:38 PM, Valin & Allyson Thorn wrote:

 	  Quote: 	
		   I know the thermocouple temperature measurement is all about the electrical resistance being observed

Not really... the input is high impedance, so a little resistance in the 
	


connectors is not a big deal. The big deal is not extending the wire 

with non-thermocouple wire, as that would introduce two cold 

junctions... one at the terminals on the thermocouple, and the other at 

the other end of the (presumably) copper extension wire. If one end is 

cooler than the other, you'll modify the observed TC reading.



Any connector should work; the voltages are small, though, in the 

millivolt range. But as long as the two ends of your connection pair are 

at the same temperature, you're not introducing an error.



Paul

kjashton(at)vnet.net
Guest

Posted: Mon May 18, 2015 2:25 pm Post subject: Thermocouple Wire Connections

Omega.com sells a line of connectors for thermocouple wires but i agree with Stein.  If the connections are made in the same area of temperature, the errors are negligible.   I have owned a Cozy that used ordinary copper wire between the thermocouples and the gauges up front.  Seemed to work pretty well.   I use thermocouple wire (type J or K) all the way on the two airplanes i built but i am not sure it really makes much difference over copper wires.

After fiddling with ring-terminal connections, I would just solder the connections and leave a little extra wire so you can just cut the connection if you need to pull the engine.

-kent 

[quote] On May 18, 2015, at 5:46 PM, Valin & Allyson Thorn <thorn(at)starflight.aero> wrote:

 Thanks guys. 

 Stein, I see where it's okay as long as each temp probe has the same bias for EGTs. Seems like I'd want the CHTs to be as accurate as possible, though...  Is there a best connection type for accuracy?

 Thanks,

 Valin

 Sent by iPhone

> On May 18, 2015, at 2:14 PM, Stein Bruch <stein(at)steinair.com> wrote:

> 

> 

> 

> Those connections aren't necessarily as important as having them all

> consistently the same (whether it be ring terminals, butt-splices, knife

> splices, pins, or whatever).  Sure there will be increased resistance at

> that junction, but if they are all uniform you won't see an unequal result.

> 

> Cheers,

> Stein 

> 

> 

> 

> 

> --

nuckolls.bob(at)aeroelect
Guest

Posted: Mon May 18, 2015 3:31 pm Post subject: Thermocouple Wire Connections

At 03:14 PM 5/18/2015, you wrote:

 --> AeroElectric-List message posted by: "Stein Bruch" <stein(at)steinair.com>



 Those connections aren't necessarily as important as having them all

 consistently the same (whether it be ring terminals, butt-splices, knife

 splices, pins, or whatever).  Sure there will be increased resistance at

 that junction, but if they are all uniform you won't see an unequal result.



 

   Exactly . . .



   Somebody once opined that what was good for the

   goose was also good for the gander. In this instance,

   what's good for the chromel is good for the alumel.

  

   In other words, how ever you splice the wires, the

   path of electrons across metalic joints in one wire should

   be equal and opposite those on the other wire. While

   this still creates tiny errors, they're largely 

   cancelling and insignificant to system operations.



   For more details, see:



  www.aeroelectric.com/articles/excerpt.pdf 



 

 

   Bob . . .    [quote][b]

millner(at)me.com
Guest

Posted: Tue May 19, 2015 6:19 am Post subject: Thermocouple Wire Connections

Well, it subtracts the difference across the copper wire from your 

readings... so as long as accuracy of your CHT numbers isn't important, 

plus or minus 50 degrees, I guess that's OK...



Paul



On 5/18/2015 3:21 PM, Kent or Jackie Ashton wrote:

 	  Quote: 	
		   i am not sure it really makes much difference over copper wires.

Valin

Joined: 13 Apr 2010
Posts: 31
Location: Colorado

Posted: Tue May 19, 2015 9:56 am Post subject: Re: Thermocouple Wire Connections

Thanks everyone and especially Bob and the article you wrote on thermocouples.  I learned I didn't have a complete and accurate understanding of thermocouples and their principles.  



We are connecting to the thermocouples with thermocouple wires.  We already have the tiny ring terminals, screws, and nuts so might just go ahead and use them to connect to each thermocouple's pig tail wires that already have ring terminals on them.  



Thanks again!



Valin

millner(at)me.com
Guest

Posted: Thu May 21, 2015 2:00 pm Post subject: Thermocouple Wire Connections

>> Explain, plz, difference of what? How do you arrive at plus or minus 

50 degrees? Just interested in how you got to that much variation.



Each junction between different types of wire registers a voltage 

difference... that's how thermocouples work.



Our aircraft thermocouples are typically chromel-alumel (exception is 

CHTs for Insight GEM which are iron-constantine. Almost everyone else 

uses only chromel-alumel)



If you use copper wire to extend a lead, then you make two junctions 

near the engine, chromel-copper and alumel-copper. And you make two 

junctions near the instrument, same pairings.



If the end near the engine is 50 degrees warmer than the end near the 

instrument, not uncommon, then you introduce a net voltage change... 

'cause the chromel - copper - chromel won't cancel out due to the 

temperature difference. This is *unlike* the earlier discussion of 

dissimilar metal connectors; there both ends of the connector are pretty 

much the same temperature.



The voltage change and error is proportional to the temperature 

difference between the uncompensated junctions (chromel-copper-chromel 

and alumel-copper-alumel). It's not directly proportionate, 'cause the 

thermocouple effect is non-linear; but for this kind of differences, 

it's close enough to linear to estimate fairly accurately.



Paul

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