Heat Treat Oven....it's alive (sorta)

jtdesigns

jtdesigns

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May 2, 2011
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Well, I got my oven built along with Auber Instruments PID. Seems like it is taking an extremely long time to get to programmed temperature. For example, I was doing to HT of some O-1 blades. It took over 45 minutes to get to 1430 but when I opened the door, the blade was a brilliant orange almost a white hot. I'm now confused. I thought proper temperature (non magnetic) was a dull cherry red. Could my thermocouple be misreporting. The outside case got pretty hot. What would be the best way to tell without buying a digital pyrometer?

Jeff
 
From your description, the most likely thing is that you've either got the input set for a different thermocouple to the one you are using, or you are displaying degC instead of degF. Check the input type is set correctly before you do anything else.
 
PID displays in faherenheit (Cool=10) and thermocouple is SN=0 which is a k type. Will have to verify that budget castings supply sent me correct thermocouple. Still scratching my head.
 
Shouldn't make the reading that far off, but the wires to the thermocouple should be right. The 2 wires that go to it are not the same each is supposed to match the material in the side of the thermocouple it is connected to. I do a check on mine once in a while with silver. Take a small piece of .999 silver. I have some bullion coins. Just a silver. Place it in a little box made with stainless foil and set your oven to something like 1740 and let it heat up. Place the box in it and wait 10 min or so and check it, if the silver is not melted go up 5f at a time untill it does, Silver melts at 1763 which is a good point to check for what we do. Save the silver you can keep adding it together and have a chunk.
 
The problem is that my oven has a hard time getting to 1450 according to the readout but the metal is brilliant orange at 1300 (certainly beyond 1450). Double checked with a magnet and it is definitely non magnetic ( but I didn't really need the magnet to tell me that based on the color). Something is wrong with my readout as I am convinced the oven is getting hotter than the display.
 
I haven't followed your build so I'm reaching here a bit.......is the wire from the controller to the thermocouple the correct material? Copper wire won't work, it'll overheat the interior.......(my bad experience). :(
 
What temp does your setup read at room temperature and/or in a toaster oven or some such? Is it off there too?
 
As Phil says, the next step is probably to pull out the thermocouple and run a test at more-or-less known temperatures. Ice water and boiling water are the easiest two in the average home.

The other thing to try, if you have a multimeter that reads millivolts, is to measure the voltage across the input terminals of the instrument, look it up on the tables and see if it agrees with the instrument display.

This should tell you whether it's the thermocouple or the instrument that isn't behaving as expected.

There's a set of reference tables accessible free, along with lots of other useful temperature measurement stuff, at

http://www.labfacility.co.uk/temperature-handbook.html

If you just want the table in degF, it's at

www.pyromation.com/downloads/data/emfk_f.pdf
 
Call me again. I have some small type K thermocouples that are simply dissimilar wires fused together. I will stick one in an envelope and mail it to you. Connect directly to PID and it will help you know if it is your thermocouple or your PID. They came from a metallurgist that had to do a bunch of testing. I have 3 of their PIDs (1 programmable and 2 simple)and have never had a problem with 1.

As I said before the yellow wire that connects the thermocouple to the pid has 2 different types of wire in it. The red insulated on should go to pos and the other neg on both the PID and thermocouple. If not it will not be accurate (but shouldn't be as far off as your talking).

Also make sure the thermocouple and wire connections are clean and tight. With the small amounts of voltage they deal with it is very important.
 
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Should i not be using the round ceramic sheaths that came with the thermocouple? Cant see why that would make a difference. One thing i did notice is that when i originally hooked up thermocouple i had the polarity reversed and i switched the wires around only at the ceramic connector and not at the terminals of the pid
 
Well, I guess it is just as important not only to have the polarity correct on the thermocouple but each of the wires hooked up properly. Also noticed that somehow one of the thermocouple wires was not firmly seated on the pid terminal block. Thanks to all your suggestions, it seems to be running correctly and having no problem getting to temp.

Jeff
 
Good to hear it's sorted now.

For anyone who encounters this in the future, here's what I think was going on.

I was taught that reversing the conductors of the extension/compensating cable results in an error of twice the difference between the temperature at the cold junction sensor (in the controller) and the thermocouple terminal block (presumably on the outside of your oven).

If you are using thermocouple extension cable for the wiring, it will be exactly the same material as the thermocouple legs themselves. Each junction of the dissimilar metals becomes a thermocouple (compensating cable will give a similar effect, but over a narrower temperature range).

I've seen it a few times myself, but usually the connection is far enough from the hot zone to be at about room temperature (we design things that way to allow the use of cheaper compensating cable with cheaper plastic insulation), so the errors are not that big.

Section 5.7 of the Auber manual shows a connection diagram for differential temperature using two thermocouples connected series, but in opposite polarity. I think your original connection was something like that, though probably with 3 "thermocouples", with each of the introduced junctions having the opposite polarity to the measuring junction.

I use a different thermocouple arrangement to most, so I don't know what sort of connector temperature is normal, but I have measured a surface temperature of 112 degC/233 degF on the outside of the soft firebrick of one of my homebuilt HT ovens.

I'd normally expect the thermocouple to provide an easier heatpath than the soft firebrick, making the connector block hotter than this, but there are lots of other variables and these will depend on some of the finer details of your design. Taking that value, and assuming the controller is at around 20 degC/70 degF, we are looking at a difference between the two of around 160 degF. Multiply it by two, because there are 2 introduced junctions, and you get an error of 320 degF.

It sounds like your error was probably bigger than this, with the most likely reason being a hotter connection block.

Regards

Tim
 
I think i might build a 110v version (smaller chamber) for my home workshop. Definitely going to insulate with more Kaowool.
 
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