There are PIDs' and PID's and Thermometers

RicinYakima

High Steppes of Eastern Washington
The temperature in the shop has dropped into the 40's so I can begin my annual bullet casting sessions. While working with the new RCBS pot, I was having trouble with fill out, so started dialing up the temperature. Today the pot was set for 750 and is showing 752. My fifteen year old calibrated PID "magic box" from my deceased master electrician friend shows 728 after an hour's heat soak.
Real Temp.JPG

The RCBS is set for 750 is reading 752. The grey PID is reading 728 with the probe on the bottom of the crucible and a third the way in from the side. After everything was cooled off, I removed the crucible and it appears to me that the temperature measureing device is on the off side of the fan/control (left in picture) and directly above the heating coil.

If you want to know the temperature of the alloy in the middle of the pot, maybe a old fashion thermometer is not such a bad idea?

Ric
 

358156 hp

At large, whereabouts unknown.
I built my own PID out of desperation a few years back and apparently lucked out with the only open place on the pot itself to mount my thermocouple. At least my temps between the PID & my mechanical thermometer usually match. The only time they didn't match required a thermocouple replacement. I tend to embrace new technology, but verify it with my old-fashioned ways until I finally get around to trusting it.
 

Spindrift

Well-Known Member
I have the same RCBS- pot. I also got problems with fillout. The mismatch between the PID and my mechanical thermometer is around 50˚....
 
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Mike W1

Active Member
I got hooked on PID's so have played around with the "toys" I've accumulated a bit. 4 PIDs, assorted TC's to go into the pots, screw-in TC used on the lube heater, the little "bulb" type TC that came with the VOM that will measure temperature, and a TC that measures on a flat surface. Remarkably when checking TC's (in the melt) they've all been at least within a couple of degrees of each other. And they respond to change way faster than a dial thermometer does. Rigged 2 TC's at different depths in the pot and though I didn't try to time it when adding metal very quickly both registered the same temp. Some fellow had mentioned that hot lead conducts heat very well. Being my TC's pretty well duplicate readings I just calibrated my dial unit with them though I only really ever use it when brewing an alloy in the big pot anymore.

In your case I'd GUESS the mounted TC on the pot might not give as accurate a reading as the one in the pot. That melted lead in the pot is gonna be the same temperature throughout the mix pretty darn quick.
 

RicinYakima

High Steppes of Eastern Washington
The specific heat of liquid lead is really quite low (about a 1/4 of iron). It doesn't take much heat to make the temperature go up in the crucible. Also did doesn't take much heat loss to cool (and solidify) it. Which is why lead alloys cast so easily.

And I think you are correct in that the TC on the pot is reading heat from the heating cool more accurately than the alloy in the pot. Now that I know, I can adjust for the crankiness of the RCBS pot.
 

Reloader762

Active Member
If you want to know the temperature of the alloy in the middle of the pot, maybe a old fashion thermometer is not such a bad idea?

For the majority of my bullet casting life I never used anything to measure my alloy temp, just got used to finding the range on the dial on my Lee pot that each mold liked to cast good bullets.

A few years ago I bought myself a lead thermometer made a little stand for it to hang down in the center of the pot and it makes controlling my pot temp range much easier especially as the mold block size and number of cavities changes. I pretty much know where to put the dial on the pot depending on which mold I'm using.
 
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Ian

Notorious member
Mine does exactly what Spindrift described, it runs the alloy 50⁰F cooler than what the PID display indicates. The temperature orobe is clamped directly to the heating coil at the bottom of the pot and isn't anywhere near the actual alloy or touching the crucible. I figure there is a bias programmed into the probe signal which facors element temperature as an jndirect function of pot tenperature. They got it wrong. Good thing is the PID holds steady and I can add my own "bias" to the set temperature to get what I want.

Can you imagine a new caster having to deal with this? I can see the internet arguments now...."You have to cast wheelweights at 800⁰, those guys who say 700 to 725 are all full of it!" Yeah....well....bad tools have started plenty of arguments before.
 

Walks

Well-Known Member
Have yet to use a PID. Guess I'm either too old school or having used the same 2 Pots for 35+yrs, I guess I'm "tuned" to the Pot temp. I do use an old style thermometer in the pot rigged with a wire frame to hold it just above the bottom of the pot. Have a ingot mold half covering the pot with just enough room to drop sprues back into the Pot.

I bought a Lyman digital therm. a couple years ago. It reads about 25degrees hotter then my long shanked therm. and 25degrees cooler the my short shanked therm.
So I use it to check when ever I add alloy. I usually cast about 720-735 for most aluminum molds, and about 20degrees cooler for iron. Unless it's little tiny .32 bullets, then crank it up and cast faster.

Cross My Fingers and Pray that these RCBS & LYMAN Pots outlast me.
 

RicinYakima

High Steppes of Eastern Washington
Walks, they most likely will. I have a 1937 Potter bottom pour 3 pounder, a 1950's SACEO 20 pound utility, a 1975 Lee 10 pounder, 1980 RCBS bottom pour, 1999 Lee 5 pounder, and now this RCBS dipper. Empty them when you are done and clean them regularly, and they should last forever.
 

Walks

Well-Known Member
Always empty when done, why I have so many ingot molds. Plus you never know what kind of alloy you're going to cast the next time. And the pot gets cleaned every time.