RCBS Pro Melt heating issue , bottom half of pot.

[Case Stuffer]

My RCBS PM is 40 years old and has served me well and is still being used but I have noticed something strange . I added a PID (external) around a month ago and it works great and most times I never cast down more than 1/2 pot before adding more alloy. Now that I have a 4" long K type probe and constant temperature display I have noticed when alloy drops to 2-1/2" below full even with RCBS control set to max.(850F) and PID set to 730F PID full on (100%) alloy temperature drops and keeps droping until I add more alloy to raise the level.

Any ideas why furnace thinks alloy is 850F when in fact it is less than 600F but only when level is below approx. 1/2 full?


See post #8 for solution.

Turns out the K type thermocoupler was going bad.

[Mike W1]

Interesting question for which I have no answer. Where is the tip of the TC located in the pot?

[Case Stuffer]

Pot is 5-1/2" deep so tip of probe is 1-1/2 " off of bottom and approx. 3/8" from the side wall at the 8 o clock position. I have read that the 4" K probe only uses approx. 3/16" of the end of the probe for readings and thus it is still submerged in around 1-1/2" of alloy.

I have no idea where the OEM bi- metal gets its' reading of the pot / alloy from as I have never had mine apart or seen photos of one striped down.

Remeasured and threads of probe is 1/2" below top rim so end is 4-1/2 " fro top thus 1" from bottom.


I pulled 8 pounds fast by dropping into 1 pound stick molds and alloy temp dropped to 600 according to PID probe and with 100% power available to furnace it remained off even when set to 850F.

I used a fan to cool furnace some and it started heating but then cut off again at 685F according to PID. Added 2 pound back to pot and set OEM Dial to 725F and furnace stayed on util this temperature was reached and PID reading agreed with it.

This is truely strange and use have something to do with where OEM gets its' readings from.

I just remembered the This Old Pot Thread . Time to post a link to this thread there .



.

[Mike W1]

I was under the impression usually that the tip is about 1/2" from bottom and also 1/2" from sides. Not sure where I picked that up but likely on this forum so that's where I put mine. However here's part of what Auber says about things below. That probably wouldn't work in my little Lee pots. FWIW

For liquid and air measurement in a container, the length of the probe is also important factor
if the sensor is mounted on the wall of the container. Normally, the temperature by the wall
is lower than in the center. If the sensor is very short, the mounting on wall will function as a
heat sink, it will drain more heater than the probe tip can pick up, making the reading constant
lower. Normally, the probe should be at least 4 in (100mm) from the wall. If possible, an
insulated mounting, such as plastic mount instead of metal mount, will help.

[Case Stuffer]

I was under the impression usually that the tip is about 1/2" from bottom and also 1/2" from sides.

OK but standard K type probe is only commonly available is 4" length . Mine is mounted so threads of mounting area is approx. 1/2" below top rim of pot so no way to go deeper and be able to fill the pot.

I do not believe the issue has anything to do with the probe or PID anyway. THe PID only made me aware of this no furnace heating when approx. 1/2 full or less. Furnace will not heat if less than 1/2 full even if PID is not used and furnace is plugged directly into 1210V AC source. Heating element seems fine as furnace is rated 800 watts and a Kill - A -Watt placed in-line indicated voltage at 120V and 758 watts when red switch light on furnace indicates that it is heating.

Added:

Let me change no heating at 1/2 or below to only low heating as in 620 to 650 or so. Alloy does remain fluid. It just so happens that 620F as indicated by PID is the point where Alloy shows siigns of going fluid at surface when pot is mostlly full.

[Mike W1]

I'd be tempted to call RCBS and get their take on it being it does the same thing without using your PID. My Auber probe is
6" BTW and a 10" is available from them. Not that knowing that is gonna help you with this problem.

[Hardcast416taylor]

RCBS 1-800-533-5000 They probably can answer your question.Robert

[Case Stuffer]

Mystery solved and was not what i expected to find. YTurns out the thermocoupler developed issues. I have read that only about 3/16" of the tip is used but in my case level had to be p to within 2" of the threads on the mounting area or readings were not accurate.

I had a spare hermocoupler here,installed it and t reads correctly with only 1/2" submerged in the alloy. All is good again now and my 40 year old RCBS Po Melt was never the issue after all.

Lesson learned, probes fail and perhaps should be one of the first things to check. I just ordered a couple of spares for $12.50 delivered from a state side Ebay vendor so no big deal money wise and it only took a few minutes to swap them out.

[Mike W1]

Must have misread your earlier post as I thought it was doing it even when you weren't using the PID at all. Apparently that wasn't the case. TC's can go bad for sure.

It is the tip of the TC doing the "work" though. Two dissimilar metals producing a small voltage which the PID reads. I noticed on a Auber Q&A they only recommend 3" submersion on my 6" TC.

[leadman]

Glad you found the problem.

Just to put it out there they thermostat is not a bi-metal type. It has a bourdon bulb with a substance inside it (usually freon or ammonia) that is mounted to the bottom of the pot and connected via capillary tube (copper) to the thermostat. The substance expands and contracts depending on the temperature to adjust the thermostat.

[Case Stuffer]

Must have misread your earlier post as I thought it was doing it even when you weren't using the PID at all. Apparently that wasn't the case. TC's can go bad for sure.



It is the tip of the TC doing the "work" though. Two dissimilar metals producing a small voltage which the PID reads. I noticed on a Auber Q&A they only recommend 3" submersion on my 6" TC.


Well it was and wasn't, furnace was not coming on but that was due to alloy being above et point but PID reading was way low and was messing with my mind.
I have read that numerious times and that is why I was fooled. Simple fact is this one reads 100 to 125F low if only the bottom 3-1/2" is submerged and reads close to 500F low if only 1/2"of the tip is submerged. As long as over 2-1/2" of it is submerged it read fine. It would work fine in an oven where thee whole length is exposed to the same hot air flow..

[Case Stuffer]

leadman thank you so very much. I had no idea as I have never seen one opened up.

[Case Stuffer]

I noticed on a Auber Q&A they only recommend 3" submersion on my 6" TC.

I wonder if that is based on providing better accuracy or long life ?

My guess is that most submerge most of their probe's length in the molten alloy and also expose the full length in their powder coat ovens.

Perhaps others have experienced failure after a relative short time and just not posted about it. My new probe now has approx. 20 hours submerged in 735F alloy spread over the last 5 days and all remains well.

[Beagle333]

Pics from the super secret inside of a Pro Melt! :grin:

http://castboolits.gunloads.com/showthread.php?41055-Here-are-some-PICS-of-the-inside-of-an-RCBS-Pro-Melt

[Case Stuffer]

Thanks for that link Beagle333 , now I know.

[Mal Paso]

Auber has a FAQ that covers a lot. There is a section on Thermal Drain. I had the TC Tip just inside the bottom of the pot and was reading 40+F low. Remounting so 5/8 of the shaft was inside the pot corrected the problem. It may be because the stainless steel of the probe is more heat conductive than lead and wicks heat away from the Thermocouple Junction??????