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Thermocouple wire connections

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glcasey(at)adelphia.net
Guest

Posted: Thu Feb 23, 2006 6:05 am Post subject: Thermocouple wire connections

Now I'm second-guessing myself.  I wired my thermocouples by  

connecting them to copper wire (twisted pairs) in the engine  

compartment.  The firewall connections were prewired by my panel  

supplier to bulkhead connections with gold-plated terminals.  I'm  

sure they used copper wires on the back side of the firewall.  I'm  

trying to convince myself that the cold junction compensation is  

still good, but I'm worried that the transition to copper, being  

inside the engine compartment, effectively places the cold junction  

there and therefore I'll get an error in reading equal to the  

temperature difference between the engine compartment and the  

instrument on the panel.  I'm tempted to pull all the wires and  

replace them with thermocouple wires that will penetrate the firewall  

and go directly to the instrument.  That's a LOT of work , especially  

this late in the project.  Tell me I don't have to do that.



Gary Casey

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brian

Joined: 02 Jan 2006
Posts: 643
Location: Sacramento, California, USA

Posted: Thu Feb 23, 2006 6:49 am Post subject: Thermocouple wire connections

Gary Casey wrote:



 	  Quote: 	
		   instrument on the panel.  I'm tempted to pull all the wires and  

 replace them with thermocouple wires that will penetrate the firewall  

 and go directly to the instrument.  That's a LOT of work , especially  

 this late in the project.  Tell me I don't have to do that.


	


You don't have to do that. The errors will be small, especially if all

your junctions that have similar metal pairs along the wire are at the

same temperature.



-- 

Brian Lloyd                         361 Catterline Way

brian-yak at lloyd dot com          Folsom, CA 95630

+1.916.367.2131 (voice)             +1.270.912.0788 (fax)



I fly because it releases my mind from the tyranny of petty things . . .

- Antoine de Saint-Exupery

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_________________
Brian Lloyd
brian-yak at lloyd dot com
+1.916.367.2131 (voice) +1.270.912.0788 (fax)

I fly because it releases my mind from the tyranny of petty things . . .
- Antoine de Saint-Exupery

nuckollsr(at)cox.net
Guest

Posted: Thu Feb 23, 2006 7:17 am Post subject: Thermocouple wire connections

At 06:03 AM 2/23/2006 -0800, you wrote:



 	  Quote: 	
		  



Now I'm second-guessing myself.  I wired my thermocouples by

connecting them to copper wire (twisted pairs) in the engine

compartment.  The firewall connections were prewired by my panel

supplier to bulkhead connections with gold-plated terminals.  I'm

sure they used copper wires on the back side of the firewall.  I'm

trying to convince myself that the cold junction compensation is

still good, but I'm worried that the transition to copper, being

inside the engine compartment, effectively places the cold junction

there and therefore I'll get an error in reading equal to the

temperature difference between the engine compartment and the

instrument on the panel.  I'm tempted to pull all the wires and

replace them with thermocouple wires that will penetrate the firewall

and go directly to the instrument.  That's a LOT of work , especially

this late in the project.  Tell me I don't have to do that.


	


    What instruments? Do they feature dynamic cold junction compensation

    or static compensation. In either case, you're correct that splicing

    to copper wire at any location OTHER than proximity to where DYNAMIC

    cold junction compensation takes place, calibration of the instrument

    is a flag waving in the breeze. If it's static cold junction compensation

    (a la UMA) then you've got two flags waving in the breeze.



    I'm mystified as to why someone would take the time, effort and

    expense to PACKAGE such a device (the hard part) and then

    drop all their cookies on the floor by not putting $5 dynamic

    cold junction compensation in and educating their customers

    on the best we know how to do with thermocouples.



    Thermocouple temperature measurement technology goes back 100+

    years when measuring the microvolt shifts in output was damned

    difficult. Now we can do it for peanuts and it seems to have

    been blown off by some purveyors of technology.



    Bob . . .

      < What is so wonderful about scientific truth...is that >

      < the authority which determines whether there can be   >

      < debate or not does not reside in some fraternity of   >

      < scientists; nor is it divine. The authority rests     >

      < with experiment.                                      >

      <                            --Lawrence M. Krauss       >

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