Modifying the RCBS Pro-Melt 2 - PID Thermocouple

Dimner

Named Man
Continuing where this thread drifted into some talk about modifying the PM2...


I'm at a point where I no longer want to mess with the asinine thermocouple that is attached to the heating element. :headbang:The modification here will be to detach the current thermocouple from the PM2's PID and install a new thermocouple that will be mounted externally and take temperature readings directly from the melted alloy.

Part of me really wants to just gut the whole internals and make an external assembly with a better quality PID controller and one that I can actually get full instructions for. However, I'm stubborn and frustrated about overpaying this much for a bottom pour pot with an inferior and poorly thought out PID design. Therefore, shelling out any additional cash for anything more than the external thermocouple is a no go.

So today, I stripped the external parts of the pot so I could get to the electronics inside. I'm 99% sure I have located the thermocouple lead, but I thought I would ask the group here to verify if I am correct. Silver braided wire that is coming from the crucible side of the pot and then leading to the PID with red and black wires. Black has a blue crimped forked connector. Red has a red crimped forked connector. That sound right? Now just swapperoo?

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Ian

Notorious member
Yep, my pro dipper pot thermocouple reads way off, I forget how much but it's simething like 50 or 100⁰ cooler than any of my thermometers say. Sensor ON the bottom element=engineering stupidity. I think RCBS took a hint from Lee Precision and Ruger and are making cheap, DIY kits disguised as functioning items but have to be tweaked by the user to work right. Except......I have two Lee 4-20s, one over 20 years old and the other bought last fall, both worked properly out of the box although I make my pet tweaks to them. For the cost of a Lee pot and an Amazon PID kit with included 6" immersion K-thermocouple, SSR, heat sink, and power supply and an additional prefab project box and some scrap wire I can build a better furnace cheaper than RCBS's cheapest and better than their best.
 

Dimner

Named Man
Got the new thermocouple wired this evening. Man those electronics are packed tight. Had to use the magnet electrical taped to the index finger trick to get some of the nuts in place.

Here she is with the duct tape and bubble gum mount I came up with. Kept the lid attached because I'd forget where I would put it otherwise.

Tomorrow I'll put it through her paces and see how it goes vs an external electrical temp monitor.

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Dimner

Named Man
Yesterday and last night I spent some time with the modded pot. Happy to say the PID is working great now. Using an external digital thermometer with thermocouple, I did some side by side readings to see if what the pot's PID is saying is really what is happening in the melt.

I started with putting ten 1/2lb lee ingots in the pot (empty pot). Before turning it on the PID read 62 and the digital thermometer read 65.7. I set the PID to 700 and it only took 14 minutes for the 5lbs of lead to melt. PID read 716, external 712.3, Then I watched the pot for another 10 minutes and recorded the min/max using the digital thermometer to see how the PID would swing in temperature.

PID (readings observed every 60 seconds)
Min = 691
Max = 724

Digital Thermometer (records the melt every second or more)
Min = 699.8
Max = 727.3

All in all that's not so bad for starting out from cold ingots.

Next, I added ten 1Lb ingots to the pot. In an attempt to see how things went as if I was adding to the pot after getting the level down to about 20% left of melt. Pot still at 700. During this period before everything melted, the pot went down to the lowest 575, exactly 15 minutes later everything was melted. PID read 700. External Thermometer read 703.8. This time I waited 10 minutes before starting the min/max test. I ran the min/max test for 35 minutes. Noting readings every 5 minutes. This time, I also noted the average the digital thermometer provided at the end of the 35 minutes.

PID
Min = 700
Max = 700

Digital Thermometer
Min = 699.8
Max = 707.2
Avg = 703.4

Lastly much later that day, I ran the same kind of test from a cooled and solid pot. Filled nearly to capacity. Close to room temp. Before I turned it on the PID read: 112 External: 116.2. It required 41 minutes to reach the 700 degree setting. Then i let it go for another 30 minutes to record min/max/average.

PID
Never budged from 700*

External Thermometer
Min = 699.1
Max = 707.5
Avg = 703.2

The last 20 minutes, the melt never hit the min or max again. It hovered around 704.3*.

I'm really impressed with the actual PID temp controller. I expected it to be less then stellar, but it seems to keep the melt temperature very stable. That's even though the actual PID doesn't display the actual swings. However, it's the melt that counts.

Why do all of this observation and record keeping? Well, two reasons.
1) I couldn't find an actual full manual for this particular PID unit. I was worried I would need to auto tune the thing. If I could avoid auto tune, that would be the best scenario. Turns out, no auto tune needed

2) Piece of mind. I can forget this whole thing, and knowing my brain I will, so a day off and on doing this test serves as a long term piece of mind that in most cases my melt will be +/- 4 degrees
 

Jeff H

NW Ohio
Sorry I missed this one Tuesday.

You did a nice job, and shame on RCBS for putting together something that looks like some of the crap I've come across in the field.

One thing I'd MAYBE change is that if the fork terminals on the TC wires at the controller did not come on the wires from the factory, remove them and connect the wires directly under the compression plates of the terminals. The way a TC works is by connecting two very specific dissimilar metals (depending on the range) and introducing any other non-specific metals in the circuit could induce a small - probably infinitesimally small error, but if a guy's gonna be anal, he may as well be as anal as practicable.

They make terminals specific to the TCs but they are not necessary if they are meant to be left in place - otherwise you'd have used the proper jack/socket (again, specific to the TC type) and the fewer connections the better, especially in process heating, where heating/cooling cause things to loosen over time.

Again, this may not be realistically necessary, but industry experience dealing with PO'd customers doesn't wear off easily after numerous earfuls over the phone in the middle of the night. Not everyone pays attention to the prints and I've had installers do some dumb stuff, like run stranded copper wire in lieu of TC lead wired and shielded cable, and IN conduit with a bunch of 480/3/60. Magnets make the (our) world go 'round and magnetic induction is a wonderful, wonderful thing that makes it possible to not have to go outside to poop or drink warm beer. Magnetic induction can be our nemesis when inadvertently used to induce garbage in our signal wires. It was a LOT of copper wire, spread over the layout of a several-hundred-foot long paint curing oven.

Anyway, that's a nice job and you (and others) are ABSOLUTELY correct that you do NOT put your process variable (actual-temp-sensor) on the heat source - you put it on the product being heated. That was incredibly dumb of them. It could have been sort of hacked/cobbled a bit if they'd put an offset in the controller, but even that is incredibly hokey. They likely did not develop this themselves, but deferred to the 'wisdom" of shrewd sales and businessmen halfway across the world. I've dealt first-hand with that bunch and they will cut any corner they can find and in their world, it's perfectly ethical to do so.

Wouldn't have hurt RCBS to have consulted with someone who knows a little about this, and I'd bet there are many in our little world who would have done it for FREE. Come to think of it, maybe that's exactly what happened!:oops:

LEE has done OK with their pots and you know you're getting the most affordable tool - that's never been a secret. I will say that if they ventured into this upgrade with the current crew on deck, they'd muck it up as badly. These things are literally no more complicated that a common, $15 toaster, until you introduce closed-loop control with a PID unit. ANY electric lead pot can be more easily converted CORRECTLY than incorrectly and getting the controls away from the heat is a GOOD thing.
 

Dimner

Named Man
Thanks for the kind words Jeff. I happened to do exactly what you explained with my TC wires. Installed without forks and under the plates.

Good point on the offset. These units do have the ability to incorporate a temperature offset. Attached are the instructions. I decided not even to mess with the instructions as I noticed the a static offset was not meaningful. The offset value needed seemed change from session to session and based on the amount of alloy in the pot. So, yeah, no thank you offset!

Maybe I will try and still find instructions for this PID and see if I can get it to turn on using the last setting and then figure out something for keeping the fan on. In a perfectly designed unit, it should have an on/off switch. Turn on and it defaults to your last set temperature. Turn off main plug and the pot switches off and the fan continues until it senses an appropriate temperature. I could always wire something from those tiny micro arduinos to the fan and rig something. But, man, I wish it just would be designed properly.
 

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Jeff H

NW Ohio
..........

Maybe I will try and still find instructions for this PID and see if I can get it to turn on using the last setting and then figure out something for keeping the fan on. ..........

If the setpoint (desired temp) is not retained when you turn the unit off, something is wrong. That is a basic, no sh... Sherlock feature. What is "wrong" could merely be RCBS's choice in controllers, but even the cheapest ones I've seen maintain their previous setpoint after being shut down. That just seems weird.

If the fan you are talking about is intended to cool the electronics, you could hardwire that to run all the time. That way you never forget to trun it on. I'd put an inline fuse in it, at least. The ideal situation would be to remove the controller and SSR from the heat, but that 's more work. If you have the fan, may as well use it.

I would like to sit down and design one today, but time and the fact that AutoCAD and Solidworks have made it very difficult to obtain/maintain a license even for official educational purposes sort of messes that up. I never really designed mine. I used discarded/damaged, warranty-return controllers at home so I could walk a customer through using one over the phone. When I didn't need them any more, I dedicated one to the lead pot. All I was after was a temp reading because free junk from the trash was cheaper than an analog thermometer. I figured what the heck and added an outlet controlled with a mercury-displacing relay another customer shipped back to me (and denied it) because he'd have gotten in deep doo if anyone knew there was even that much mercury in his plant. Otherwise, I'd have used a SSR.

Now that you have gotten your feet wet - and I admire the determination to dig in and sort this mess out - maybe think about shopping components and start to gather them and eventually build a self-contained/remote PID controller with an outlet to plug ANY pot into for when your RCBS pot eventually .... goes to pot. Pun intended. Not that the pot will go bad, but if the controls irritate you enough in the future, you could bypass all of them with such a unit and keep on casting.

If I were to build one today, I'd do a dual-PID unit, with one of them being a "ramp-soak" controller and a separate outlet for those who want to control a toaster oven to heat-treat or powder-coat. I'd use surface-mount glass-fuse holders on the front and jacks for the TCs and maybe a switched power source for fans, like those in some toaster ovens.

I've upgraded mine a little over time, and the most convenient upgrade was to add TC jacks with receptacles on the controller's enclosure. I can switch between my lead pots (2), which each have a dedicated TC, and my pizza oven, which I use for PC'ing. I had been controlling that one via PID for heating Kydex years ago, before I got tired of people who couldn't do it try to dictate how/what I did with it FOR them. You're way ahead of the game. For a long time I just laid the controller on the bench while using it and stuck it in a ziplock bag when not using it. I didn't even have mine in a box for years.
 

Dimner

Named Man
I did seriously consider gutting the internals of the RCBS pot and doing an external PID, however, the stubborn bull headed consumer in me wants this pot to work as I intended with as little money as possible (real or imagined). So I went this route. Also, I wanted the changes to be easily reversible incase I decided to sell the pot and just go Lee + the Eternal PID I already have for my ladle pot!

As for the temp setting memory, I think RCBS was able to pick a PID controller in which that could be turned off as an option. I agree any PID controller worth it's salt has that feature as a default.

Still cannot for the life of me find a manual for a tQidec C10B. I can find a quasi manual for the CHB102, which seems similar, but it's not a full blown manual. I think if I were to have the manual, I'm pretty sure I could set it to run the fan after I flip a switch until the TC senses a specific temp. Most of these PIDs have some bit of DC out. Or if it's just circuit level DC output for logic, I can setup a logic switch for when the TC senses a temp and turn it off that way. Then tell the whole thing to shut down after that. First, documentation please!!:sigh:
 

Jeff H

NW Ohio
I did seriously consider gutting the internals of the RCBS pot and doing an external PID, however, the stubborn bull headed consumer in me wants this pot to work as I intended with as little money as possible (real or imagined). So I went this route. Also, I wanted the changes to be easily reversible incase I decided to sell the pot and just go Lee + the Eternal PID I already have for my ladle pot!

As for the temp setting memory, I think RCBS was able to pick a PID controller in which that could be turned off as an option. I agree any PID controller worth it's salt has that feature as a default.

Still cannot for the life of me find a manual for a tQidec C10B. I can find a quasi manual for the CHB102, which seems similar, but it's not a full blown manual. I think if I were to have the manual, I'm pretty sure I could set it to run the fan after I flip a switch until the TC senses a specific temp. Most of these PIDs have some bit of DC out. Or if it's just circuit level DC output for logic, I can setup a logic switch for when the TC senses a temp and turn it off that way. Then tell the whole thing to shut down after that. First, documentation please!!:sigh:

That's a bummer about the manual. You might be much happier with the controller once you find it. It's possible that there isn't a consumer/user manual for it if it's meant for integration in some non-user-servicable appliance. Maybe?

I get it about the stubborn, "bullheaded consumer." The delusion, these days with which we torture ourselves could be our eventual demise. It's like phones - "apps" - they do what they do, yoiu get what you get and "the masses" seems appeased by the limitations because they don't have to think past it. Used to be things got better because people demanded it. Now things get dumber because people accept it.
 

dannyd

Active Member
I talked to RCBS when that pot came out they were trying to copy where Magma put theirs on the master caster. Told the guy that won't work and in reality RCBS as never designed a casting furnace before, so you may want to ask for help.