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Joining EGT probe wires.
by
Baxter
on 25 Jan, 2007 15:39
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Okay, just looked through some pages but I can't find the answer Im after.
I appreciate that a small voltage is produced between a junction of 2 dissimilar metals (or something :lol: )..
I bought 2 VDO EGT kits, both with wire and thermocouple, one for a T3 whick is gunna need a right long wire, and one for the Caddy, which will need shorter wires.
The plan was to use the spare from the Caddy to lengthen the T3 one.
So what do I do to connect the 2 bits? Solder? Crimps?
I notice that the VDO thermocouple uses wires with rings crimped on to which you bolt the extension wires onto, but the other end is left bare and the gauge is fitted with spade terminals, so obvioulsy I am going to get at least 2 junctions there.
Does it really make all that much difference?
Your thoughts would be appreciated.
Simon.
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#1
by
QuickTD
on 25 Jan, 2007 16:06
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As long as the junctions are all the same type and roughly the same temperature relative to their mates in the opposite lead, the parasitic junctions will all cancel each other quite nicely. Soldering is a difficult process to control with regard to thermocouples, you may have the 2 wires in intimate contact, which would negate the parasitic junction, or you may end up creating 2 additional junctions between the wires and the solder if the parent wires don't touch. I would suggest mechanical connectors such as crimps or screw terminal blocks, much more easily controlled. You can use crimp butt connections but be sure the wires don't butt together inside the crimp or you will kill the parasitic junction in that lead and unbalance the system. Use proper "K" type thermocouple extension wire and mind the colour coding.
Also be careful of extending or shortening the wires on a direct acting (non-powered) pyrometer. Some of these can be sensitive to lead wire resistance, shortening the wire can greatly affect accuracy. Most amplified (12volt powered) pyrometers are minimally affected by lead wire resistance and shortening/lengthening the leads is OK.
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#2
by
Baxter
on 25 Jan, 2007 16:09
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And to think we speak the same language... :lol:
I think that made sense!
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#3
by
Baxter
on 25 Jan, 2007 16:12
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So, if the gauge takes a +12v feed then I am okay to chop away, if it is just a gauge with 2 probe terminals then be careful.
Okay.
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#4
by
2mAn
on 25 Jan, 2007 19:17
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WHY DO I ALWAYS SEE THIS AFTERWARDS!!! :oops:
i wanted to solder the wires together and have had a *** of a time with it. i still havent been able to get my egt gauge hooked up.
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#5
by
2mAn
on 26 Jan, 2007 12:18
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WHY DO I ALWAYS SEE THIS AFTERWARDS!!! :oops:
Because you don't use the search beforehand? 
Andrew
the search isnt very good, i did read as much as i could and no one mentioned how to actually connect the gauge to the thermocouple. i assumed soldering would be best, but boy was i wrong... :cry:
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#6
by
Hillshy
on 28 Jan, 2007 02:43
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i agree with libbybapa, nice explanation from quicktd.
the searches can indeed draw blanks somtimes. i just spend hours reading old posts as there is so much knowledge on the site, just makes great reading.
ps Mr brick great chat phone, thanks for the tips and advice. i found places that sold bolts but they couldnt offer and info on them, just the basics, i may just bite the bullet and get em from arp. still friggen expensive tho! :shock:
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#7
by
Baxter
on 28 Jan, 2007 04:13
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So, what would be wrong with twisting the wires then soldering? You would surely get a good mechanical connection between the wires, then solder to stop them pulling apart.
Otherwise,so long as the joints are parallel to each other, using the same connectors then I shouldn't get much of a problem?
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#8
by
QuickTD
on 28 Jan, 2007 08:00
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You could twist the wires together, but you need to form a gas tight connection or the metals will oxidize. Solder doesn't stick very well to thermocouple alloys (particularly the alumel) You'll need to clean the leads with sandpaper then twist and solder quickly to prevent the oxide from interfering with the connection or the solder bond.
The absoulute best way to join the leads would be TIG or resistance welding, if your welder will turn down low enough...
Otherwise,so long as the joints are parallel to each other, using the same connectors then I shouldn't get much of a problem?
Correct.
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#9
by
QuickTD
on 28 Jan, 2007 11:50
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The issue with soldering is that there is some chance that even if the wires are twisted together, that the solder might through capillarity seep between the two thermocouple leads forming a slight film and creating those extra two junctions.
Yup, that as well...
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#10
by
Vincent Waldon
on 28 Jan, 2007 12:14
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In the field (for example, wiring an industrial plant) thermocouple leads are arc-welded together... again to ensure only like metal touches like metal. No connectors are used, unless the connectors are made of exactly the same alloys that make up the two different wires on the actual thermocouple.
For more info that you'll ever ever want to know about the science of thermocouples Omega is probably one of the world's leaders:
http://www.omega.ca/shop/sectionSC.asp?book=temperature§ion=a