Scored a kiln today

The best setup would be two TC's. One in the muffle and one in the chamber. Have a thermocouple selector switch to select one or the other. Start with the one in the chamber, and when the kiln has stabilized after 30 minutes or so, switch to the one in the muffle for an exact reading during HT.

A thermocouple selector switch is a pretty handy thing. It can be connected to many TCs and then to a single readout. That allows you to see the temperature of the shell on the HT oven, the chamber front, back middle, muffle, etc. Other TCs can be in the quench tank, grinder motor or platen, etc.

Shop around and you can find the switched or a pre-made thermocouple switch box.
 
That's not a bad idea. Also makes using the chamber for axe heads easier.
 
Run the PID off the TC in the muffle. Run a calibration cycle once it is in the muffle. I don't particularly see the use of the one in the full chamber other than to let you know about heat loss, etc. Your temp inside the muffle, will never be at the temp outside because there will be a hole in the top of the chamber. So if you ran your PID off of the original TC, your muffle temp would never be there, even ignoring the constant argon purge. Even if you would eventually reach equilibrium temp (you wont, it is thermodynamically impossible) it would take a lot longer and be far less energy inefficient. THe PID should work just fine calibrated off the intermuffle temp.
 
kuraki said:
That's not a bad idea. Also makes using the chamber for axe heads easier.
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This is my plan for my electrical sandpot Ht oven I work on .....
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My reason for starting with a TC in the chamber was to get a better heating curve. If the TC was inside a steel muffle, it would take a long time for the PID to read any temperature increase/decrease. Coil firing swings could be large as the response time to high/low readings will be very slow. If you have only one in the muffle you would have to run the kiln for a good hour or so to get it equalized and steady at the target temp.

With a TC in the chamber and another in the muffle, once the entire chamber and muffle are stable at the target, switching to the muffle TC will provide an accurate reading of the blade temp to the PID.
 
Stacy E. Apelt - Bladesmith said:
My reason for starting with a TC in the chamber was to get a better heating curve. If the TC was inside a steel muffle, it would take a long time for the PID to read any temperature increase/decrease. Coil firing swings could be large as the response time to high/low readings will be very slow. If you have only one in the muffle you would have to run the kiln for a good hour or so to get it equalized and steady at the target temp.

With a TC in the chamber and another in the muffle, once the entire chamber and muffle are stable at the target, switching to the muffle TC will provide an accurate reading of the blade temp to the PID.
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Are there PIDs that allow switching between two TCs, or do you need to install a DPST to do the switching manually? I agree with the response time bit, but after a calibration cycle at a specific temp, the PID should work near perfectly and run flat out until it gets close the the desired muffle temp. Unless I am missing something, the cal cycle should eliminate the overshoot.
 
I agree that it would eventually learn to balance the firing cycles to keep the target. My concern is that the learning cycle will be for a shielded TC. It would take a long time for the PID to zero in on the delay time in response. This could lead to a long heat-up time of an hour or more before it stabilized at the target.
My two TC idea was mainly theoretical, and would have to be tested to see how much it shortens the time of stabilization.
 
I will test with my manual probe first to see just what it does as it sits.
 
kuraki said:
I wasn't looking but saw it on the local craigslist and the price was right. 7200 watt and 22.25 x 17 inside with ramp control, 6 program slots and 2300f. Paid $500. Seller claimed she fired it only 3 times and it looks it.

So since it's vertical, my plan is to cut a hole in the top, drop a stainless tube in, and connect argon to the tube. I'll use wool to cover the tube. Tube to baffle from radiant heat spikes, and reduce gas volume required.

View attachment 944249 View attachment 944250 View attachment 944251
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Today I was repair some vaccum pump from one friend and I thought on this thread ....So , why you don t make that tube closed chamber and use vaccum or argon gas inside ?
 
I'm going to use argon inside. I'd rather use a constant flow than worry about a pressure vessel or vacuum seal.
 
I think the best, but more expensive, option is have two TC's and two PID's because of different calibration characteristic. One set (TC in chamber) for heating up and second set (TC in tube) for hold on temp. PID with TC in tube should be calibrated to exact temperature and this TC could be unshielded for faster response.
 
I really don't think that the external PID will help. All it will do is prevent a large overshoot. Which your PID calibration will do even in the tube.

Basically, the calibration cycle will run the thing full out until the tube gets to temp. I am guessing that you will see an overshoot of a hundred or so F inside the tube (you probably ought to calibrate with gas flow). After the cal cycle, it should just hit the temp without significant overshoot. If it doesn't figure it out, just manually program your PID values. Plenty of info online will help. May take a couple of test cycles, but you will get it.

If you really want to go all out, you could do the thermo (I would ignore the gas flow if I was gonna do this, cause it would make the math a lot harder and probably not be a big enough deal to make a difference) and then program accordingly.

What you don't want is to bring the main chamber to your target temp. You NEED the main chamber to overshoot initially by presumably a few hundred degrees, then to ramp down a bit (it will still need to be a deal hotter than the chamber) as the tube warms up. Because you don't know what the Ideal overshoot and hold time for the main chamber is, the external TC reading isn't much use to you.

The whole point of a PID is to (nearly) eliminate the sinusoidal over/undershoot pattern that you would get with a switching controller. The layer of insulation (your tube) would increase this over/undershoot, but the PID should be able to hack it.
 
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I think it is easier with two PID's.

1. Set 1st PID (chamber) to target temperature + (as You wrote) about 150-250°.
2. Run kiln on 1st PID.
3. Turn on 2nd PID (tube), only for read temperature in tube.
4. When reached target temperature switch to 2nd PID to run kiln.
5. Calibrate 2nd PID, should be easy. (do it 1st time only)
6. Run kiln at the temperature for desired period.
7. Drink coffee, read newspaper, pull blades out for HT.

Am I wrong?
 
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