Heat treat oven

[John Hasler]

If the wires aren't welded or clamped air-tight the thermocouple may be inaccurate and unreliable. It's hard to get a reliable air-tight junction with 900 degree temperature swings. You can weld the junction by twisting the wires at the tip together and then briefly touching the other ends to a 12V car battery.

There is always at least one other junction in the loop. It doesn't matter as long as all the junctions at the cold end are at your reference temperature (room temperature, in this case). The voltage developed is a function of the difference between the hot junction and the cold one.

The old way to compensate for variations in room temperature was to add another thermocouple of the same type to the loop and stick it in a mixture of ice and water. All the connections were made at a big block of copper so that they would all be at one temperature. The modern way is to attach the reference thermocouple to a calibrated thermometer IC such as an AD590 and feed it's output into the electronics.

For type K yellow is positive.

[SteveM]

If the wires aren't welded or clamped air-tight the thermocouple may be inaccurate and unreliable. It's hard to get a reliable air-tight junction with 900 degree temperature swings.

I guess the reason it's worked for me is I've been using my tire pyrometer for cooking, which isn't as critical

I have to take dinner out of the heat LONG before it gets to 900 degrees.

Steve

[stephenc]

Today's winner ... Bob d

I switched the jumper from nc to no and I have heat .. a lot of heat really fast heat .
I only left the heat on for about a minute , I am assuming this thing got left out in the rain the night before the auction so better safe then sorry .

But back to the thermocouple.... it works , sorta erratically .
With a bic lighter for a heat source it won't register any heat , then it jumps up a few hundred degrees and back to nothing .
At this point I can't see what it would hurt to fuse the end like suggested so I'll give that a try later after a run a few warm up - cool down cycles to remove any moisture lurking in the brick

[Magicniner]

I have a 1950s electric furnace to which I fitted a new thermocouple and £15 PID controller shortly after moving it into the workshop, it works flawlessly and it's a modification which I heartily recommend!

- Nick

[SteveM]

I have a 1950s electric furnace to which I fitted a new thermocouple and £15 PID controller shortly after moving it into the workshop, it works flawlessly and it's a modification which I heartily recommend!

Can you point to a link to what controller you used?

Steve

[Magicniner]

Steve,
It's a Mypin T series PID



Bigger than most of models at the cheapest end of the market but it's been working well for about 7 years now, I found the tape on the bench and included it in the shot for scale,
Regards,
Nick

[johnfreese]

Until you have verified the temperature controller works do not trust it. I had a bad experience with a borrowed furnace. I was annealing a piece of cast iron. I set the controller for the desired temp. I found out the controller did not work. Melted my cast iron part into the floor of the furnace. Destroyed the lining and elements getting the iron out. No parts available. Spent $700 to replace the furnace. Should have installed a separate thermocouple to read temperature.

[Magicniner]

Until you have verified the temperature controller works do not trust it.

I "Added" the PID and kept the original thermocouple & meter
I use an infra red pyrometer and check all 3 agree reasonably closely every so often and always before a critical job like solution hardening stainless parts and hardening and tempering D2 Tool Steel parts.

I'm always a bit stunned that some people will spend a day making a part, or batch of parts, setting up their machines and measuring the work to ensure it all turns out just so and then throw it in a furnace assuming the reading is right, it's like assuming your DRO is set because it was set for the same tool last week

- Nick