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Extending thermocouple leads

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Radioflyer

Joined: 04 Feb 2009
Posts: 43
Location: Boston

Posted: Thu Aug 02, 2012 8:27 pm Post subject: Extending thermocouple leads

I think I understand how to do things right with thermocouples, but want to make sure. I have a set of (GRT/EIS) J- type, CHT bayonet thermocouples. The 2 foot leads are terminated from the factory in what I believe to be standard spade terminals. I need to lengthen these leads about 12 feet to reach my MGL Avionics quad CHT display instrument. So I plan to get some J-type wire, crimp on some mating spade terminals and similarly terminate the other end very close to the instrument. (One set of spades will be in the hot engine compartment and the spades on the other end of the extension will be in the cabin.)



My understanding is that the parasitic couples at the spade terminals will be cancelled out, so I should get accurate temp readings at the instrument. Correct?



Is it worth the trouble to search for stranded J-wire or is solid good enough?



--Jose

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nuckolls.bob(at)aeroelect
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Posted: Fri Aug 03, 2012 7:18 am Post subject: Extending thermocouple leads

At 11:27 PM 8/2/2012, you wrote:

 --> AeroElectric-List message posted by: "Radioflyer" <skyeyecorp(at)airpost.net>



 I think I understand how to do things right with thermocouples, but want to make sure. I have a set of (GRT/EIS) J- type, CHT bayonet thermocouples. The 2 foot leads are terminated from the factory in what I believe to be standard spade terminals. I need to lengthen these leads about 12 feet to reach my MGL Avionics quad CHT display instrument. So I plan to get some J-type wire, crimp on some mating spade terminals and similarly terminate the other end very close to the instrument. (One set of spades will be in the hot engine compartment and the spades on the other end of the extension will be in the cabin.)



 My understanding is that the parasitic couples at the spade terminals will be cancelled out, so I should get accurate temp readings at the instrument. Correct?

  

   Sort of . . .



   What do the 'spade' terminals attach to?



   The "wrong+wrong=right" thing applies only

   when pairs of wrongness are applied to EACH

   lead . . . not opposite leads. When thermocouple

   gages of yore were brought to an instrument on

   the panel, they too were terminated in ordinary

   terminals onto studs at the back of the instrument

   in the cockpit environment. These too introduced

   small errors that were acceptable in the grand

   scheme of things. A few degrees of error in hundreds

   of degrees of reading.



   Putting ring terminals onto a thermocouple at

   the 'cold junction' or 'reference junction' end

   of the wire introduces little error . . . the

   hotter the environment of the polluting junctions,

   the greater the absolute error . . . but the

   percentage of error vis-a-vis local temperature

   is about the same.



   Ring terminals used to splice a t/c wire run

   with a nut-n-screw would be an example of many

   wrongs adding up to not-too-bad. But the single

   spade on one end of each side will induce some

   small errors.



   Having offered all that, the maximum error you

   might encounter is insignificant in this situation.

   You're not going to make decisions based on a few

   degrees of reading. It's departures from 'normal' and

   trends that deserve your attention.



   So don't fuss over the details too much . . .



 Is it worth the trouble to search for stranded J-wire or is solid good enough?



   From a measurement perspective, solid and stranded

   can be interchanged and spliced in the same system.

   You can get small quantities of a variety of t/c

   wires from Omega at



  http://tinyurl.com/cwnomt3



   Their part number builder says they'll sell you

   25' of TT-J-24S for $30.00



   eBay may offer additional opportunities at



  http://tinyurl.com/cr35ff4 



   I've got a bunch of thermocouple wire stashed in

   boxes yet unpacked from the move . . . else I'd

   offer to send you a hunk of something suited to

   your task. But in the 105 weather I'm not inclined

   to rummage around out in an un-conditioned part

   of the shop!



 

 

   Bob . . .    [quote][b]

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Radioflyer

Joined: 04 Feb 2009
Posts: 43
Location: Boston

Posted: Fri Aug 03, 2012 10:51 am Post subject: Re: Extending thermocouple leads

Bob, thanks for your reply. You asked what the spade terminals were attached to. They are crimped on to the J wires of the thermocouple sensor. The wires are red and white and one of them was magnetic.



You are right that a few degrees of error would not much matter for CHT readings. I don't have a sense for how much error my "J-spade-J-spade-instrument" setup would introduce, but if within say ten degrees then it would be ok. I just want to be sure I can reliably tell that my cylinders are not exceeding max allowable temperature.



--Jose

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nuckolls.bob(at)aeroelect
Guest

Posted: Sat Aug 04, 2012 5:13 am Post subject: Extending thermocouple leads

At 01:51 PM 8/3/2012, you wrote:

 	  Quote: 	
		  



Bob, thanks for your reply. You asked what the spade terminals were 

attached to. They are crimped on to the J wires of the thermocouple 

sensor. The wires are red and white and one of them was magnetic.


	


    Do the terminals terminate to a 'sensor' under

    the cowl or to an instrument behind the panel?



 	  Quote: 	
		  You are right that a few degrees of error would not much matter for 

CHT readings. I don't have a sense for how much error my 

"J-spade-J-spade-instrument" setup would introduce, but if within 

say ten degrees then it would be ok. I just want to be sure I can 

reliably tell that my cylinders are not exceeding max allowable temperature.


	


    Sure. One way to check your instrument's

    calibration with as-installed thermocouples is

    to get one of those in-the-cup immersion heaters

    for travel-coffee/tea and use it to raise a cup

    of distilled or R/O water to boiling. You can

    do this right next to the engine where you've removed

    a cht sensor bulb and immerse it in the water.

    Wait several minutes while the reading stabilizes.



    Compare with boiling point of water at your

    present atmospheric pressure conditions:



http://tinyurl.com/bmptwag



    and you can see just how close the instrument

    is to reading true at about 200F.

   Bob . . .

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Radioflyer

Joined: 04 Feb 2009
Posts: 43
Location: Boston

Posted: Sat Aug 04, 2012 6:24 pm Post subject: Re: Extending thermocouple leads

The extension (thermocouple) wire pair would eventually connect to an instrument on the panel in the cabin. So, Engine side: thermocouple--spades--J wire pair extension--thru firewall. Then Cabin side: extension cont'd--spades--copper--instrument on panel. The extension is 12 feet long because this is in a pusher aircraft. Spades to copper to instrument because I can't connect the J type thermocouples wires directly to the DB15 on the instrument.



What I'm thinking is that the parasitic couple between spades and thermocouple wire pair in the hot engine side, and the other parasitic couple of thermocouple wire pair and spades in the cooler cabin will not much affect the absolute temp readings. 



--Jose

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retasker(at)optonline.net
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Posted: Sun Aug 05, 2012 9:36 am Post subject: Extending thermocouple leads

You are absolutely correct.  In fact, it may not affect the readings at all.



The way a thermocouple works is that any given metal will generate a voltage from one end to the other if the temperature is different from one end to the other.  If one chooses a different metal for 

a second wire it will generate a different voltage across its length. If one hooks a meter between the two wires at one end it can be calibrated to read the difference in temperature between the two 

ends because the sum of the two outputs will vary with temperature. If there is no difference in temperature between the two ends, there is no voltage generated so no temperature reading displayed.  

You can make all kinds of connections and add other materials between the two ends and you will see no temperature reading as long there is no temperature difference.  Taking that one step further, if 

there is no temperature difference between the each end of the things you add, then, even if there is a temperature difference between the far end (cylinder head) and the meter end, it will still read 

correctly.



So, if a spade connection is the same temperature from end to end (which is pretty likely since they are copper - a very good heat conductor) there will be no net voltage generated across the 

connection. Which means that the reading you get at the meter (or whatever you are using to read the temperature) will still be the same as if you had no spade connection.



Where you would get into trouble is if you add a long splice in the thermocouple wire that is not thermocouple wire.  For example if you add several feet of copper wire to your too short thermocouple 

wire.  The likelihood of that being the same temperature at each end decreases in proportion to the length of the extension.



Bottom line:  You will likely see no difference in reading with or without the spade and pin connections.



Dick Tasker



Radioflyer wrote:

 	  Quote: 	
		   



 The extension (thermocouple) wire pair would eventually connect to an instrument on the panel in the cabin. So, Engine side: thermocouple--spades--J wire pair extension--thru firewall. Then Cabin side: extension cont'd--spades--copper--instrument on panel. The extension is 12 feet long because this is in a pusher aircraft. Spades to copper to instrument because I can't connect the J type thermocouples wires directly to the DB15 on the instrument.



 What I'm thinking is that the parasitic couple between spades and thermocouple wire pair in the hot engine side, and the other parasitic couple of thermocouple wire pair and spades in the cooler cabin will not much affect the absolute temp readings.



 --Jose





 Read this topic online here:



 http://forums.matronics.com/viewtopic.php?p=380062#380062




	


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