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Thermocouple question...

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steve(at)tomasara.com
Guest

Posted: Sun Aug 31, 2008 12:42 pm Post subject: Thermocouple question...

Bob and group,



First I would like confirmation that I'm correctly interpreting my 

inherited EGT/CHT installation.   What I found on my pusher project was 

that the EGT and CHT thermocouple wire ended on the firewall at a 

terminal strip (and possibly even on the way to the terminal strip in 

the CHT case).  The rest of the run to the instrument (an EC-1) is in 

copper and included another terminal strip junction and a lap-solder 

junction at the instrument.  The wires coming out of the instrument 

appears to be tinned copper.  Instrument manual clearly says the 

extension wiring must be type "K" thermocouple wire but the wire exiting 

the instrument (that used to be connectorized) appears to be tinned 

copper.  My interpretation is that  they were using on the connections 

to the instrument wires as the cold junction and assumed the temperature 

at these junctions were the same as that in the instrument (which seems 

like a good enough assumption).  From this I would believe that the 

readings used by the previous owner would be in error (presumably low) 

by the difference in temperature between the back of the panel and under 

the cowl.  Is this right?



My intended fix is to use thermocouple extension wire continuously from 

the thermocouple sensor wire up to behind the panel.  And since I am 

converting from one cylinder coverage to all four plus a spare, I'll be 

adding a five position, double pole rotary switch to each set (EGT and 

CHT).  To both get the extra couples to cancel and to be able to fit 

acceptable sized rotary switches, my plan is to run all the thermocouple 

wires to a set of terminal blocks (or an equivalent) at convenient place 

behind the panel.  There the signals switch to smaller gauge copper 

(four six-wire cables) up to the rotary switches (wire length less than 

3', signal length less than 6').  Back at the terminal strip the 

post-switch EGT pair and CHT pair go back to type "K" extension wire and 

run back up to the instrument  for connection (via connectors in place 

of lap-joints) for correctly located "cold" junctions.  Did I get this 

right?  Is there a significant negative to the "less than six feet" of 

signal path on copper behind the panel?



Thanks in advance,

Steve Stearns

Boulder/Longmont, Colorado

CSA,EAA,IAC,AOPA,PE,ARRL,BARC (but ignorant none-the-less)

Restoring (since 1/07): N45FC O235 Longeze Cothern/Friling CF1 (~1000 Hrs)

Flying (since 9/86): N43732 A65 Taylorcraft BC12D

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nuckolls.bob(at)cox.net
Guest

Posted: Sun Aug 31, 2008 4:37 pm Post subject: Thermocouple question...

At 02:40 PM 8/31/2008 -0600, you wrote:

 	  Quote: 	
		  



Bob and group,



First I would like confirmation that I'm correctly interpreting my 

inherited EGT/CHT installation.   What I found on my pusher project was 

that the EGT and CHT thermocouple wire ended on the firewall at a terminal 

strip (and possibly even on the way to the terminal strip in the CHT 

case).  The rest of the run to the instrument (an EC-1) is in copper and 

included another terminal strip junction and a lap-solder junction at the 

instrument.  The wires coming out of the instrument appears to be tinned 

copper.  Instrument manual clearly says the extension wiring must be type 

"K" thermocouple wire but the wire exiting the instrument (that used to be 

connectorized) appears to be tinned copper.  My interpretation is 

that  they were using on the connections to the instrument wires as the 

cold junction and assumed the temperature at these junctions were the same 

as that in the instrument (which seems like a good enough 

assumption).  From this I would believe that the readings used by the 

previous owner would be in error (presumably low) by the difference in 

temperature between the back of the panel and under the cowl.  Is this right?


	


   Yes . . .

 	  Quote: 	
		  My intended fix is to use thermocouple extension wire continuously from 

the thermocouple sensor wire up to behind the panel.  And since I am 

converting from one cylinder coverage to all four plus a spare, I'll be 

adding a five position, double pole rotary switch to each set (EGT and 

CHT).  To both get the extra couples to cancel and to be able to fit 

acceptable sized rotary switches, my plan is to run all the thermocouple 

wires to a set of terminal blocks (or an equivalent) at convenient place 

behind the panel.  There the signals switch to smaller gauge copper (four 

six-wire cables) up to the rotary switches (wire length less than 3', 

signal length less than 6').  Back at the terminal strip the post-switch 

EGT pair and CHT pair go back to type "K" extension wire and run back up 

to the instrument  for connection (via connectors in place of lap-joints) 

for correctly located "cold" junctions.  Did I get this right?  Is there a 

significant negative to the "less than six feet" of signal path on copper 

behind the panel?


	


   How many total measurement points are you installing? "four,

   6-wire cables" brings an image of 24 conductors for 12

   thermocouples total. One of my favorite "tiny" switches

   for thermocouples is a Grayhill GH5602 . . .



http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=GH5602-ND



   which only handles 6 thermocouples. If you need a full 12

   positions, then consider the Grayhill GH7103



http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=GH7103-ND



   which is also quite compact.



   Consider using T/C wire all the way from the splices

   at the engine couples, through the switch and to the instrument.

   You can use "connectors" in the form of d-subs (use

   machined pins and 4-quad crimp tools to install) and/or

   2-screw barrier strips to join K-wire to K-wire but avoid

   k-wire to copper joints until you are at the rear of the

   instrument case. Suggest TT-K-24S wire at:



http://www.omega.com/pptst/XC_K_TC_WIRE.html



   You can get this wire in short amounts for just over

   a dollar a foot. See quote form at the bottom of

   the page cited above.



   I think I'd bring copper wires at the instrument out to

   a female wire-to-wire connector (Omega SMPW-K-MF) shown on



http://www.omega.com/pptst/SMPW_SMP_HMP_HMPW.html



   You could bond the connector off the instrument's pigtail

   to the back of the instrument case with E6000. Depending

   on room inside the instrument, you might be able to

   even modify the instrument to accept a midge panel jack

   as shown in:



http://www.omega.com/pptst/MPJ.html



   The 'secret' to accurate thermocouples is do-unto-Chromel-

   as-you-do-unto-Alumel all the way from the temp-sense

   junction to the instrument . . . even inside the instrument

   if possible but at the back of the case would probably

   suffice.



   Bob . . .

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steve(at)tomasara.com
Guest

Posted: Mon Sep 01, 2008 5:39 am Post subject: Thermocouple question...

Bob,



Thanks for the help.  I'm having a lot of fun on my re-wiring project 

but it is really time-consuming...  I really appreciate having your 

book, you and the group available to sanity check my details...

 	  Quote: 	
		   **How many total measurement points are you installing? "four,

          6-wire cables" brings an image of 24 conductors for 12

          thermocouples total.**

Each of the 6-wire cables is basically used as a copper intermediary for 
	


one of the switch poles (four total for the combinations of EGT/CHT and 

Yellow/Red).  Each cable has 5 goes-to-the-switch signals (from one side 

of 5 thermocouples) and a sixth wire that goes from that cables 

switch-pole common to a section of TC wire required to move the cold 

junction back up to the instrument.  This gives 10 thermocouples total 

(which I'll think about extending to six since the grayhill switch will 

let me).

 	  Quote: 	
		   **One of my favorite "tiny" switches

          for thermocouples is a Grayhill GH5602 . . .

       

       http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=GH5602-ND

       

          which only handles 6 thermocouples. **

Those grayhill switches look exactly like what I was getting ready to go 
	


look for. 



 	  Quote: 	
		   **Consider using T/C wire all the way from the splices

          at the engine couples, through the switch and to the instrument...  but avoid

          k-wire to copper joints until you are at the rear of the

          instrument case. **

How do I bond TC wire directly to the switch without having a copper 
	


intermediary?  That seems like a better approach but I don't have any 

good ideas on how to do it (I figured silver-soldering to the switch 

wasn't likely to work...).  Since all my ideas required a copper 

intermediary, I figured I might as well take advantage of it and remote 

the TC to copper junctions (other than the required cold-junctions 

behind the instrument) a couple feet away from the crowded spot on the 

panel where the switches and instrument reside.  <new idea follows...> 

If I do end up stuck with a copper intermediary, it seems like the run 

from the switch to the instrument doesn't have to go back to the 

pre-switch TC-to-copper junctions and therefore could be just a couple 

of inches long (the switches will be located at the instrument).  Since 

each would still require a switch-to-copper junction at the switch, a 

copper to TC junction, and a TC junction back to copper (at the 

instrument for a cold junction) all within a few inches it seems a 

kludgy way to do it but it does save running a pair of TC wires a couple 

of feet in a wire-crowded area. 



(Annoying theoretical thought follows...) Assuming the "new idea" still 

balances all the thermocouples correctly (which I *think* it does), and 

since the section of TC wire could be arbitrarily short (implying 

arbitrarily iso-thermal) it's seems reasonable to argue that IF you knew 

that the copper wire from the switch and the copper wire to the 

instrument were the same alloys (which you don't) that the couples at 

each end of the arbitrarily short TC wire would cancel and, therefore, 

you could eliminate them without affect.  The implication being that the 

resulting performance would be equivalent to that with the cold couples 

at the back of the instrument when, in fact, they aren't there.  Clearly 

I'm missing something here and don't have it all figured out yet...

 	  Quote: 	
		   **Suggest TT-K-24S wire at:

       

       http://www.omega.com/pptst/XC_K_TC_WIRE.html

       

 **

I've got some of this wire in 20 Gauge for making thermocouples for 
	


not-too-hot areas.  For the 10' runs up to the panel, I've got some 

EXPP-K-20S stranded "extension" wire (but need more...).  I picked the 

20 Gauge just to follow the "no airframe wire smaller than 22 Gage" 

rule-of-thumb and the stranded variety for improved motion-related 

durability.  (Sounds like I might have over-thought the problem.)  If 

there is a good way to bond TC wire to the switch then I'll ditch the 

copper intermediary and switch to the 24 Gauge extension wire for the 

couple of feet required to go up to the switches.



I'm assuming it's not worth "home-brewing" from scratch either EGT 

probes or plug-washer CHT probes so I haven't purchased any high-temp TC 

wire.  But can you replace the wire in plug-washer CHT probes and, if 

so, do you have a part number for an appropriate wire to buy for that 

purpose?



Seemingly simple principles sure lead to a lot of required details to 

get it right.  Sort of a Mandelbrot or "emergent behavior" sort of 

thing. <grin>



Thanks in advance,



Steve Stearns

Boulder/Longmont, Colorado

CSA,EAA,IAC,AOPA,PE,ARRL,BARC (but ignorant none-the-less)

Restoring (since 1/07): N45FC O235 Longeze Cothern/Friling CF1 (~1000 Hrs)

Flying (since 9/86): N43732 A65 Taylorcraft BC12D

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Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

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