What's this?

Mike W1

Active Member
#1
TC Holder.JPG

Well it's a cobble job of scrap to enable multiple TC's to read the melt at different depths at the same time. Only used it once so far but indications are there's not much difference from bottom to top of the melt. Next session I'll leave the 2nd TC in place to see how long it takes the heat to migrate upwards from a cold start. Might also enlighten me on temperature drop even when adding a "hot" ingot. It looked like a 340°F ingot added to 6-7 lbs of hot melt dropped it about 30° C. Suspected it might be quite a bit and it's why I use a feeder pot which doesn't drop the temp.
 

fiver

Well-Known Member
#4
I done a study and wrote up a report a couple of years back on the boolits site, but it was with 2 thermometers in a 20 lb LEE pot.
what I found was smaller [1/2 lb or less] ingots pre-warmed was the best way to go about adding lead continuously, otherwise it took almost 3 minutes to bring the pot back up to temp.
[basically a full heating cycle]

this looks to be a lot more accurate.
 

popper

Active Member
#5
not much difference from bottom to top of the melt. The thermal conductivity of liquid lead is such that there is effectively NO difference in temp, top to bottom. I only add sprue and drippings after 1/2 pot (gives the shoulder a break). Then I flux again, just beezwax. Don't use tin but stiring gets the junk on top and wax coagulates it for skimming.
 

Mike W1

Active Member
#6
Well I didn't learn anything earth shattering. Confirmed in my own mind that I don't think a shelter or hut on top of the hot plate is of much value, at least if you cast inside as I do. I do find the little adjustable rig helps ensure the mold sits flat on the hot plate for maximum heat transfer. Copy of my notes follows.
Mold Leveler.JPG

Flat TC.JPG
Setup is 2@ TC's in lead pot, one at 1/2" from bottom and side of pot and one immersed 1" into the melt. Object is to observe temperatures of the melt and to time from a cold start how long before things stabilize. Lee 10# Production controlled by Rex C100 PID unit. Also used another Rex C100 to read the 2nd TC. Also used a VOM hooked to a flat surface TC to monitor the mold temperature after the mold sat on a hot plate controlled by a PID set at 400°. Object of this was to see if any value was added by comparing twixt use of a covering over the mold or notl. The "hut" is insulated with 1/8" cement board on all 4 sides and top. Inside dimensions are roughly 4.5x4.5x4.5 inches.

11-2-18 Shop temperature 90° F
Just used the bottom TC on this run. About 7 lbs in the pot, set at 365° C (689° F) Placed "hut" and mold on hot plate and turned everything on. Pot reached set temperature about 20 minutes. Had an interruption so didn't measure mold temperature till 26 minutes. After shutdown placed and left in melt the 2nd TC about 1" into the melt.

Mold temperature after 26 minutes using the "hut" about 340° F

11-3-18 Shop temperature 90° F
Turned on with same settings as before. This time without the insulated hut. While pot was coming up to temperature the top TC was 40-50° C lower than the one on bottom of the melt. That gap narrowed a bit as bottom got closer to 365°. Within seconds of alloy melt the temps merged.

Set temperature of 365° reached in 19:35 minutes.
Top TC hit 365° in 19:45 minutes.
Once alloy was melted (325° C) both TC's read the same within 10 seconds of each other
Hot plate set at 400° on PID read about 390° across the disk.
Mold was 330° F at both 23 & 26 minutes elapsed time.

My thought had been a cover over the mold was not worth the bother and think this confirmed that. Possibly would help in a breeze but I cast inside.so it's not needed for me. The gizmo in the one photo enables various molds and handles to sit flat on the hot plate and I think that's prudent. And as a recent post pointed out once you're up to temperature it is pretty consistent throughout the melt unless you add sprue's, etc. Using a feeder pot set 1° hotter I''ve never noticed temperature drop of more than a degree or two.

quote "popper
not much difference from bottom to top of the melt. The thermal conductivity of liquid lead is such that there is effectively NO difference in temp, top to bottom."

Checking pot recovery times. 365° C Pot temp
1. Added 4 ingots 335° F Average - 8-1/2 minutes recovery time
2. Added 1 ingot 340° F dropped to 335° C - 5 minutes recovery time
3. Added 1 ingot 325° F dropped to 321° C - 3 minutes 9 sec. recovery time
4. Added from Feed pot about 5 ingots at 365° C - dropped 3 ° C
 

Rally Hess

Well-Known Member
#7
When I first looked at this I thought it was a counter weight for the hot plate. Some of the hot plates slide around because of the hard plastic feet on them.
Wife bought me one such hot plate at a garage sale. The legs had recessed seats for the screws so I just put a dollop of silicone caulk over the screw heads, then put a piece of wax paper on each and pressed down to form flat feet. Worked well enough, and longer than the element lasted.