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Thermocouple wire connections

 
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glcasey(at)adelphia.net
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PostPosted: Thu Feb 23, 2006 6:05 am    Post subject: Thermocouple wire connections Reply with 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.

Gary Casey


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brian



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

PostPosted: Thu Feb 23, 2006 6:49 am    Post subject: Thermocouple wire connections Reply with quote

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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nuckollsr(at)cox.net
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PostPosted: Thu Feb 23, 2006 7:17 am    Post subject: Thermocouple wire connections Reply with quote

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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