First, therocouples are nice thing to have, but unless you are doing some serious baking or commercial baking they are not really neccessary (good thing my wife does not read this
). I put 3 in because I "just had to know exactly how my oven works", but have not used them of late as I have learned to "read the oven" and have a good idea of what temp it is. So unless you are in the mode of above, you won't be using them much after you have cooked a dozen times or so and if one goes bad, no big.
Ok you want to use them. I put my 3 on the same side or slice of the oven (top, mid, floor) and installed a removable door in the roof, so all I have to do is remove some shingles, then the door, and pull out a TC and install a new one. I would recommend TC's in stainless sheath, they are sealed, so will last the longest and easily slide in/out of drilled hole in brick.
TC's do fail over time. They tend to "drift" or read lower values as they progress to failure, but I doubt you will notice since they tend to drift in single degrees at a time. Even on a TC's worst day it will most likely be much more accurate than a "cheap oven thermometer" so that would not be a good test. Also must remember that the bead, or disimilar metal junction at the tip is what is reacting to temp, so the TC only reads what the temp that bead is exposed to. You need to know where exactly that tip is. There will be different temps at different parts of the oven and that is excluding the convection currents of the open interior.
Here are a couple of failure modes for TCs and two are specific to K-type (because that is the best choice for our ovens, IMHO)
1. Ground loop failure, TC circuit becomes shorted to ground somewhere or shorted to the other wire. TC will read low or close to zero and will change temp.
2. Corrosion due to oxygen of Alumel/Chromel bead, the higher the temp the faster the rate and can be seen by bead becoming a greenish color. Will read gradually lower temp and then fail "open circuit". The main reason I prefer stainless sealed sheaths.
3. Grain growth in Alumel at Chromel junction, at our temps this would probably take tens of years, more applicable to temps above 1000 deg C. Fail "open circuit".
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Originally Posted by Mypsi  I've been wondering about this, too. Our oven is close to 20 years old, and was around long before I started using it. There are 5 thermocouples, but only 4 give readings. They go into a switch box, then to a thermometer, and all the wires are a little shaky, so it's a balancing act of fiddling with wires to get good readings. I suspect that they are reading low, because I picked up a cheap dial oven thermometer to put inside, and it actually reads around 50 F higher than the thermocouples (that's not right, is it?).
So I have a few questions:
What happens when a thermocouple goes bad? Does it just stop reading, or does it get less accurate?
mr.jim, can you explain a little more how they would be replaced? I've got these wires coming out the front of the brick, that I imagine were placed during construction, and are snaking through inches or feet of brick. Some notes from a previous baker seem to imply that at least one might be in the back of the oven. Are you saying that I should be able to pull this out, replace it, and feed it back in?
If I replace the outside wiring, what kind of wire should I use? Right now it is a thick single strand that seems kind of brittle, and I think I could get a better connection with a thinner, braided wire. |
I would suspect loose wire connections or even some oxide build-up on connections. You can try and open connectors, clean and retighten. The same for the rotary switch if you can access it. As for replacement, you need to know what type of thermocouples they are. You can replace the wire running it as close to the original as possible or replace the TC's and the wire at the same time. As stated above you must get the wire made for the type of thermocouple you have, generically matched. The wire itself is not match to that exact thermocouple and does not have to be the same length as long you are not adding tens of feet then any temp difference readings will be less than a degree or so.