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Originally Posted by Mypsi  So let me talk this through and make sure I understand. Thermocouples are calibrated for a specific thickness and length of wire; a thicker wire would lower resistance, giving a higher reading, and a thinner wire would raise resistance, giving lower reading; a longer wire would raise resistance, giving a lower reading, and a shorter wire would lower resistance, giving a higher reading. Is that correct?
So we have five k-type connectors (the little forks coming out of yellow plastic plugs) sticking out of the front of the oven. They plug into the rotary switch, and another length of wires (a couple inches long) with connectors on either end connects that to the digital readout. Most of what I have to fuss with is that small section of wire. The wire currently used is a single strand, kind of thick (1mm?), somewhat inflexible, brittle wire. I opened up the connectors to make the connections a little tighter, but it's not a very easy wire to work with. They broke a few times when I made the loops, and it's a little too thick to work well with such small screws. Since it's such a small length of wire in respect to the grand scheme, would it cause significant problems to replace that with a multi-strand braided wire, that would be more flexible, and wrap around the screws more easily? |
Not quite. The disimilar metal juction of the thermocouple outputs a very small (millivolt) voltage in proportion to temperature. The meter is simply reading that voltage and converting it to temp based on a table for that type of TC. The material make-up of the wire is specific to the type (K or R, ect) to not interfere with that millivolt signal. There is no real specific length to the wire. In a calibrated system a calibrated input is used (in place of the TC) and the meter is adjusted to offset any errors created by wire length. But we are talking about less than a degree here, probably alot less in the relatively short runs around an oven. And oven use it is not that "exact" temp critical an application, you will not notice a degree or two. You are correct about longer wire more resistance.
I understand what you are talking about now. TC wire is usually a solid conductor (single strand) and it sounds like it is getting very old and has some breaks in it. I would replace your single pair of junction wires, is a simple and easy thing to do. Use any K-type wire you want ( I would use the simple plastic covered 2 wire) and polarity is important, red is negative (-), have no idea why. You could also just skip the rotary knob and plug directly to the meter.