Temperature Over-run Using PID Control

[alamogunr]

For a variety of reasons I'm just now getting around to setting up the PID temperature controller that I got from JConn. It came preset for 750º and I left it there for the time being. My question has to do with the temperature range as the PID cycles the pot on and off. I can observe the pot switch pilot going off when the temp reaches 754º and on when it reaches 746º. Obviously, it doesn't go any lower than 746º because the element comes on. When the element goes off at 754º, the temp continues to climb until it reaches as much as 765º or more. I'm not overly concerned because when I used a thermometer, I'm sure that the temp varied more than that. I'm just wondering if the temp continues to rise because of residual heat in the element or does the element not cut off completely.

As I said, I'm not really concerned, just want to completely understand what is going on.

BTW, the boolit I was casting was the MP Di Carli wad cutter. After I got my cadence adjusted, it spit out beautiful boolits. I was a little concerned about the aluminum blocks but had no reason to be. Wish my Lee six cavities were as easy to use.

[dragon813gt]

It will tighten up to more you use it. But be aware the over/under shoots will change depending on how much lead is in the pot. If you auto tune it w/ a full pot that's a vastly different load than one that's a quarter full. I always keep some sprues out to cool the pot down if it really overshoots(typically when the pot is getting close to empty).

[alamogunr]

UPDATE! Yes, I used the auto tune. The instructions said I could do it as many times as I wanted without worry. The instruction didn't detail what "auto tune" did for me. I assumed that it smoothed out the process.

OB, my unit must be the MyPin because the auto tune procedure is exactly as you stated.

I had to take my wife shopping since she was afraid to drive, having just finished cataract surgery last Monday. When I got back and turned the pot on again, I paid more attention to what was going on. After everything settled down, I cast about 150 boolits. During this time, the overrun was no more than to 765º. After casting another 50 or so, I added the sprues back to the pot and added another ingot. This dropped the temp way down as expected. When it heated up again, the overrun was 780º. It seems that a longer heat cycle produces a greater overrun. Maybe the auto tune will modify that behavior.

Although it interrupts my rhythm, I like to add sprues and more lead when the pot reaches half full. I'll be doing more casting over the next month since I've wasted half the winter when I could have been stocking up both boolits and loaded rounds. I'll probably learn a great deal more about this device. So far I really like it.

[dragon813gt]

Only use auto tune one time. You start over every time you hit it. What it does is intentionally over shoot and then lets it cool down. This is why the load in the pot matters. If you only let the pot get to halfway mark use the auto tune when it's at that level. This may cause it to undershoot when full but I doubt it will be by much if it does. Or meet in the middle and tune it at three quarters full.

You can manually enter the P, I and D values if you want to go down that route. You can screw it up quickly if you do this so only make one tiny adjustment at a time.

[alamogunr]

Thanks! I will do it one more time at 3/4 full and use it that way for awhile.

I don't know enough to manually enter values for P, I and D. Not really that interested in getting that deep into it.

[Mike W1]

JConn uses the Mypin controller but I don't know which one. The only disadvantage of using AT everytime is that you have to waste time for the needed heat/cool cycles to complete before it sets the P, I and D values. The tip of the TC is what controls the voltage back to the controller for a reading so I don't believe pot level makes that much difference. One thing I've noticed with ebay TC's is they seem to be 4" in length whereas my Auber TC's are 6" in length. Auber also recommends that only 3" of the TC be immersed, why I have no idea. Mine are covered more than that in my Lee 10# pots and I get consistent control.

I'm relatively certain if you AT a unit, write down the PID values, and immediately do AT again you're gonna get a different set of values and nothing else has changed. Someone like bangerjim can correct me but I don't believe once you've AT'd a unit that it keeps on getting closer after it's done it's thing. Once AT'has completed the values are set and do not change further.

[dragon813gt]

Yeah, you are right. These are cheap PIDs and they don't learn as they go. But pot level does matter. It's basing it's values on how fast it gains and loses heat. A quarter full pot is going to heat up a lot quicker than one that's at three quarters. I've tired tuning my pot at various levels and it reacted differently every time. I honestly forget where it was when I last did it. But now it's slow to approach temp when full and doesn't overshoot until it's practically empty. At this point the lead is dripping out of the spout instead of pouring.

[retread]

I have MyPins on both of my pots. When first coming up to set temperature it overruns about 15-20 degrees then settles down to the set point and stays there plus or minus a few degrees for the contents of the pot. I do not add lead or sprues during a session and end when about 3/4" is left in the pot. High level, middle or low the PID stays within a few degree of the set point. From the sound of your and others experiences I must just be lucky.

[jsizemore]

When you dump a large load on it after it's "learned" what is necessary to maintain the temp, you confuse it and end up with the big swings in temp till it "learns" the new operating parameters. Consistent behavior yield consistent results. Drop you sprues in as you go.

[Chris C]

. Drop you sprues in as you go.

Just starting to cast my bullets. Have read about the process until my eyes are crossed.............but I've never heard or read that one.

[Rickshaw]

I started by adding the sprues back to the pot at every cast, what I found was the contoller kept compensating for the change in pot temp. The next time I cast, I collected 20 sprues at a time then added them to the pot. It gave me a little break and I added lead to the pot at 60 sprues. Pot temp was steady, production was efficient. This is definately a process, not an event. That's what seems to work for me, your mileage may vary.

[Beagle333]

I use a Jconn PID with my ProMelt and I open the sprue plate with gloves, so I add the fresh sprue back as I go. As long as I don't wait too long and add a pile of sprues or a new ingot, it stays within a couple of degrees of set temp. I have built a couple of PIDs using Mypins and my own have wider swings than the Jconn for some reason. It is very stable.

[Mike W1]

I use a Jconn PID with my ProMelt and I open the sprue plate with gloves, so I add the fresh sprue back as I go. As long as I don't wait too long and add a pile of sprues or a new ingot, it stays within a couple of degrees of set temp. I have built a couple of PIDs using Mypins and my own have wider swings than the Jconn for some reason. It is very stable.

Do you happen to know which model Mypin that Jconn uses? I built one using the TA4-SNR and didn't find it did all that wonderful a job of maintaining temperature. Suspect other models of the Mypin may operate a little differently and that might be the difference. Mine appeared to AT after 2 cycles and my Auber uses 3 cycles to AT. Haven't actually checked what my Rex's do in that respect but seem to be better than that Mypin was.

[Beagle333]

Not right offhand. I can't remember if it is written on it or not. I've only had the case off once, just to look at it when building my own.

I'll send you a pic of the unit and maybe you can ID it.

[Don Fischer]

Funny thing just occurred to me. When I first started casting my own handgun bullet's, very early 70's, we melted out lead in a heavy cast aluminum, pot we got in a second hand store and heated on the kitchen stove! We used MTY tooth paste tube's to harden the lead and we checked the hardness by throwing a bullet on a table to see how high it would bounce! Today I have a Lyman electric pot and I recently got a hardness tester! Tin toothpaste tubes are a thing of the past but there still are some tube's around for different ointment's made of tin, they are very small!

Has casting come a ways or what?

[popper]

The object of a PID is to get to a set temp quickly without overshot for a FIXED thermal load (in our case). Just wait till it settles down to the set temp and then start casting. I haven't AT mine since I first made it.

[alamogunr]

Sounds like experimentation is the order of the day. Keep track and use what works.

[Idz]

You'll never get tight temperature regulation with an on/off type controller in a lead pot because the hysteresis (lag time between sensor and heater) changes with lead level. If you really want tight control you have to use a proportional controller that reduces heater power as you approach the set temperature.

[Mike W1]

You'll never get tight temperature regulation with an on/off type controller in a lead pot because the hysteresis (lag time between sensor and heater) changes with lead level. If you really want tight control you have to use a proportional controller that reduces heater power as you approach the set temperature.

Always want to learn more but you're talking way over most of our heads. I only know what's worked for me with a Auber SYL-2352, Mypin TA4-SNR, and a couple REX C100's. I don't think any of those would be a proportional type controller but the on/off type. Correct?

I can see the power out to the pot going on and off via the indicator flashes. The Auber and the Rex a "pattern", the Mypin I had was constant flashes so maybe a different type unit than the other two. It didn't do as good a job for me as the other 2 do. bangerjim indicated there's different ways of doing it but I'm a little thickheaded on that.

My uneducated guess is the TC tip reads the temperature and the PID does the rest of the work. Being the tip is down in the melt I don't see that changing very rapidly even as the pot empties. The PV on mine seem to vary but little but perhaps that's due to the response time of the TC. I've gotten the impression there are some differences among TC's. Obviously I'm doing some guessing here with no real facts to back me up.

[dragon813gt]

You'll never get tight temperature regulation with an on/off type controller in a lead pot because the hysteresis (lag time between sensor and heater) changes with lead level. If you really want tight control you have to use a proportional controller that reduces heater power as you approach the set temperature.

Define tight. You're correct that if you don't throttle down the voltage as you approach the set temp that it will overshoot. But we don't require absolute control w/ a lead pot. +-5 degrees is more than acceptable. Mine will typically hold w/in a degree if left alone. I don't add the sprues back as I go. I wait until the pot is almost empty and then add them back. Once back up to temp it will hold there as I start the process all over.

Proportional controls are overkill for this application. But then again I built a modulating gas valve setup for my smelting pot. Almost all here won't spend that type of money.