Lee pro 4 pot and thermocouple installation under the melting pot

[Larry7293]

Would drilling a small hole through the bottom side of the Lee housing{missing the coil} and putting a two inch long thermocouple through the hole and across the length under the melting pot be a good idea? Any drawbacks to this?
Does the thermocouple read the temperature along the whole length of the rod or only from the tip?

[JonB_in_Glencoe]

Just the tip. A TC should be in the melt. Part of why Lee's original equipment temp control is so poor, is because the part sensing temp is not in the melt.

[Conditor22]

A friend used a short thermocouple and crimped a round electrical on the end then bolted it around the spout in the 4-20.

This makes sense to me because that is where you want the temperature when pouring and it frees up real estate in the pot.

Leaving the thermocouple "open" under there would let it pick up heat from the coils.

You might try holding the thermocouple against the bottom of the pot with high temp insulation and screwing the bottom cover on.
then would need to adjust the temperature setting to get the desired temperature in the pot. (or just figure out what temperatures work best for you and go from there)

I have thought about cutting a long nut in half, drilling and tapping a few holes in it then tacking it to the bottom of my pot, slide the thermocouple in the 1/2 nut and use set screws to lightly secure it to the bottom of the pot.


Those of you that have newer Lyman and RCBS pots, how/where do they put the thermocouple?

[joe leadslinger]

Here is a vid on how to install a TC on the bottom of a Lee pot.
https://youtu.be/wsthzWoVi6U

[AlHunt]

I don't see why not. Even if it's not exactly in sync with the alloy temp, it still gives you a data point. If a thermometer in the pot says 750 when you're casting the way you want and the thermocouple reads it as 725, who cares? I'd assume you're running a PID so your target temp would be 725. Or whatever your numbers work out to.

As others have said, screwing it to the bottom night be easier,

[oley55]

I am in the process of installing a thermocouple in my brand new 4/20 right now. Well actually waiting for my thermocouple/s to arrive. There are a number of youtube videos of folks installing them, but I have no confidence a solid probe type thermocouple will get us accurate readings, especially a short 2" one. The end of that thermocouple must be in solid contact with the pot's metal. Otherwise you are measuring the ambient air under the pot (or so me thinks).

I ordered thermocouples from an ebay seller out of St. Paul Minnesota which I had previous dealings with. I'm using the open type: (K-Type Thermocouple Wire Sensor for Digital Thermometer Probe Fiber Glass PK-400 for only $8.99 ea) and plan to Dremel a slot/groove in a large SS flat washer around the bottom nipple to hold it against the bottom of the pot without crushing the tip of the thermocouple, or will possibly use an eye crimp type connector. I bought two thermocouples just in case I mess one up. Also these are max rated at 752F and wonder if it will be adequate over the long run, but I thought the kiln super high temp thermocouple was probably overkill. I just got shipping notification and should have them in-hand in 5-7 days.

I'm also going to insulate my pot with some 1/8" kaowool ceramic paper around the inside and a couple layers in the bottom to provide more protection for the fiberglass wrapped wire of the thermocouple.

That's my plan for now and if I remember I'll take some pics and post results.

[Larry7293]

I would think you would want a thermocouple that was rated for a higher temperature??
What PID are you using?

[oley55]

I would think you would want a thermocouple that was rated for a higher temperature??
What PID are you using?

You may very well be correct.

"This is a reliable thermocouple wire with a K-Type sensor. The k-type sensor is at the tip of a 3ft long wire probe, which has a medium temperature fiberglass insulation, good up to 752 degree Fahrenheit °F or 400 degree Celsius °C. The fiberglass is secured with thin thermocouple wire, not with a plastic shrink tube (which could melt at the high temperature)."

I may be wrong, but as described above I believe the thermocouple tip carries a higher range and the the limitation is the wire insulation. Hence my intention to use kaowool to provide protection to the insulation. The following is from a different source, but the thermocouple appears to be the same:

"This type K thermocouple has a bare tip and glass braided insulation. It can be used for measuring air temperatures due to the fast response time of the bare tip. It can also be fit into a thermowell because of its small outer dimensions.

Compatible type K thermocouple mini connectors can be found on this page.
Specifications:
Length 1.5 meter (59 inch).
Wire diameter 24 gauge, 0.020 inch (0.5 mm)
Insulation Fiber glass braided
Outer dimension 0.09 x 0.05 inch (2.3 x 1.3 mm)
Maximum temperature 500°C (900°F)

If it fails and/or doesn't meet my expectations, I'll just find a different thermocouple.


I put the PID together a couple years ago for my salt-bath annealing. I bought most of my components from Auber Instruments.

[Mal Paso]

I built up a pad of brass on the bottom of 2 Lee 4-20s, drilled holes at an angle, and Silver Brazed them in the hole right next to the spout. They need to be 1/2 inch inside the pot to read accurately.
my first try was just inside the pot and it suffered from thermal drain. (read low) They worked great, wildfire got them.

I used the 1/4" threaded thermocouple from Auber, took the threaded part off.

[oley55]

Well I got my thermocouple and discovered trying to bend/curl that big flat washer was way more difficult than expected. I ended up using an eye crimp connector cut at 12:00 o'clock and then folded the tabs in twice to create enough space to prevent crushing the thermocouple's tip.

Haven't loaded it up with lead yet to see what my temp variance will be, but I think getting the thermocouple a little ways from the nipple would give a more accurate reading of the whole pot versus the nipple temp.

[Mal Paso]

You will need to calibrate the PID as you won't get the actual lead temperature from the outside of the pot.

[Stephen Cohen]

I built up a pad of brass on the bottom of 2 Lee 4-20s, drilled holes at an angle, and Silver Brazed them in the hole right next to the spout. They need to be 1/2 inch inside the pot to read accurately.
my first try was just inside the pot and it suffered from thermal drain. (read low) They worked great, wildfire got them.

I used the 1/4" threaded thermocouple from Auber, took the threaded part off.

hat is the best trick I have seen to date. regards Stephen

[Gunslinger1911]

I'll second "in the pot".
My ProMelt 2 has the probe too close to the coils. Reads 750 on the PID when the Lyman dial in the alloy is reading ~450. Really throws off the heating potential because the PID thinks it's up to temp so it cuts the juice to the coil.

[Conditor22]

I built up a pad of brass on the bottom of 2 Lee 4-20s, drilled holes at an angle, and Silver Brazed them in the hole right next to the spout. They need to be 1/2 inch inside the pot to read accurately.
my first try was just inside the pot and it suffered from thermal drain. (read low) They worked great, wildfire got them.

I used the 1/4" threaded thermocouple from Auber, took the threaded part off.

I really like that concept. I'm afraid I would destroy the thermocouple fluxing my lead and having the thermocouple down there would make cleaning the bottom of the pot problematic while casting.

A+ for ingenuity. Please let us know how it holds up.

I also agree that you won't get the actual lead temperature with the PID outside the pot, but as mentioned a little adjustment on the PID should get you there and I'd like to know the temperature at the spout not the middle of the pot.

Yes, you may have to relearn favorite casting temperatures having the thermocouple on the outside. Putting a thermometer in the melt at first could/would get you close.

Lyman lead (dial type) thermometers have been known to be a moving target. test it with boiling water.



My 2¢

[Mal Paso]

I really like that concept. I'm afraid I would destroy the thermocouple fluxing my lead and having the thermocouple down there would make cleaning the bottom of the pot problematic while casting.

A+ for ingenuity. Please let us know how it holds up.

I also agree that you won't get the actual lead temperature with the PID outside the pot, but as mentioned a little adjustment on the PID should get you there and I'd like to know the temperature at the spout not the middle of the pot.

I think I ran those pots for 4 years before the Sobeanes Fire got them. 50,000 or so boolits. The thermocouples were stainless, never had a durability problem.

56% Silver brazing rod was my solution. The melting point of the brass brazing rod would kill the thermocouple so I used it to build up thickness and used Silver Brazing to bond the stainless thermocouple to the hole in the brass and pot.

None of the commercially made pots have the thermocouple inside the pot. They all recalibrate to represent the actual lead temperature.

[oley55]

I was just playing around and heated up 3 pounds of pewter in my new thermocouple pot, The analog thermometer read 9-10 degrees cooler than the PID (600 degrees vs 610 degrees). Pretty surprised I only had 10 degrees difference, but with only three pounds in the pot, probably not a valid test. Hope to be able to do a test with a normal pot of alloy in the next few days (watching young grand kids this weekend and messing with hot stuff while they are awake/around is a Bozo-No-No). If it stays in that close of a range I doubt I'll bother with the calibration settings, since I use this PID controller with my salt bath annealing as well.

[44magLeo]

Mounting the thermocouple next to the spout has been tried. They found that when you pour the temp reading goes high then when you stop pouring it drops back. This causes the PID to vary more than they liked.
One guy drilled a 1/4 " hole in the center of the bottom of his hot and used a thermocouple with the short threaded section on it. Held it in place with a chrome acorn nut. This read the lead temp with much less variation.
I may try this myself but Find having the thermocouple hanging in the pot work just fine.
Leo