Modified Magna Master caster

[PMK]

I have put a PID thermostate on a Master caster, replacing the element and controller that came on it for better heat controll. Has anyone elce replaced the thermocouple? I welded a the mound right on the bottom behind the pore on the bottom of the pot and am ammazed at the way the temp veries from the top of the metal. This temp will be at 550 and the thermomiter in the metal at the middle or top of the pot will be at 725! Im amazed at the difference in the temp difference from what we are pouring and what is in the top of the pot, or where we normaly check the temp.

[montana_charlie]

This temp will be at 550 and the thermomiter in the metal at the middle or top of the pot will be at 725! Im amazed at the difference in the temp difference from what we are pouring and what is in the top of the pot, or where we normaly check the temp.
I have frequently wondered about that, but couldn't think of an easy way to check for a difference.

Maybe that explains why guys casting big bullets seem to favor the dipper over the bottom spout.
It might also explain why so many bottom pour casters run their pots at maximum temperature.

Now I find myself wondering just where in a pot the average themometer takes it reading from...down at the tip, or somewhere in the middle of the stem.
Or, maybe it's an average of both areas...
But it's my suspicion that the hottest metal in contact with the stem is going to make the reading on the dial.

Anybody ever seen the insides of a stem-type thermometer?
CM

[PMK]

I have the thermometer in the lead, and it touches the bottom, although the temp on the pid that just reads the bottom states its 550F, the Thermometer states its 750, but as the (pot, 2/3 full at this point) runs down and there is only 1" of lead in pot thermometer now reads 580F.

[montana_charlie]

So...that seems to say that, with the pot set for 750°, the lead at the bottom will never get close to that temperature -- even when it gets down to an inch left.

However, that final inch IS still molten.

Therefore, the alloy is one which remains liquid at a level that far below 600°...or the thermometer isn't in lead deep enough to get a good reading.
The PID sensor seems to bear out the reading on the thermometer, so I guess we can assume that alloy near the bottom of a pot never gets as hot as we expect...even when the level is very shallow.

Yet, pure lead...requiring 621° to melt...will turn liquid, even if it's only a teaspoonful in the bottom of the pot.

There is something in all of that which is hard for me to grasp.

If I was out in the shop (where the temperature is now below zero) and casting bullets...instead of sitting at this keyboard...I might not care so much.
But, while dipping puddles of logic, instead of lead, this discussion is very interesting to me.

I hope others can shed more light on the subject while I go out to feed cows...
CM