>Many types of wire through ordinary bulkhead connectors are OK, but be sure
>you don't put thermocouple leads into those connectors. The reason is that
>the thermocouple junction consists of a bi-metallic joining of the two
>different types of wire that make up the lead wires. If you cut the leads
>and put pins on them and then into a bulkhead connector and out the other
>side with the remainder of the leads wires, you have created another
>bi-metallic junction at the connector location. This will prevent you from
>obtaining reliable temperature measurements at the place you really want to
>measure it. The thermocouple manufacturers do make bulkhead connectors that
>allow you to carry the leads through the bulkhead--just be sure you don't
>use an ordinary Cannon connectors or the like that are used for ordinary
>power and signal voltages and currents....
Actually only half true . . . the part about creating new bi-metal
junctions is true but consider that for every brass pin crimped to
a thermocouple wire in a plug, there is an equal and opposing junction
created in the mating plug. From an electrical performance standpoint,
there is very little error added to thermocouples by taking them
through a connector. However, it's NOT easy. Thermocouple wire alloys
are much harder than brass/copper alloys found in wire and connector
pins. It's difficult to get a sturdy, gas-tight joint on a thermocouple
conductor by crimping a brass pin to it. It can be done rather well with
machined-pins . . . solid barrels of brass closed onto the thermocouple
conductor with a quad-squash die tool. Another more time consuming way
works rather well. "TIN" the ends of your thermocouple conductors with
silver solder and then SOLDER the tinned conductors into solder-cup
type connectors. Only some TC alloys will solder at low temps with
tin/lead solder but a coating of silver solder over the outside of
the conductor will make it solderable with ordinary materials common
to electrical connectors. Yes, there is still more "contamination" of
the bi-metal measurement system but each lump in the line is offset
buy a similar lump close by in the mating connector and overall errors
introduced are insignificant.
A technique that has been recommended by some manufacturers of
TC instruments for hombuilts tell you to take TC wire through
Molex or other open-barrel, sheet-metal pins on low cost plastic
connectors . . . I don't recommend it. You can also purchase
machined pins of the same alloy as the TC wire you're working with
but believe me, you don't even want to know what they cost.
Bob . . .
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