Electronic temp control

[jsizemore]

Does the bulb for the thermostat in a RCBS pot come in direct contact with the molten lead or is it in direct contact with the outside of the pot? If it's on the outside of the pot, how come it's OK for the RCBS and not OK for the thermocouple for the PID? If I read correctly, regardless of the length of a thermocouples probe, the bimetal strip is contained in the tip of the probe that generates the voltage to signal the PID. Sounds like it's just a matter of calibration which I assume an RCBS would require over it's lifetime. What's the big deal?

[lurch]

AFAIK, the thermostat bulb in the RCBS is probably not in direct contact with the pot - at least not screwed to it to provide a solid contact for optimum heat transfer... Don't own one and haven't taken one apart to know for sure. I know the Lyman Mag 20 is not screwed down in this manner. I would assume both are mounted very close to the pot, probably lightly touching it. The reason that they work pretty well is because the area around the pot is insulated. The heat flux across the insulation, and by extension the area where the bulb is (between the insulation and the pot), is pretty small because of that and hence the temperature in the vicinity of the bulb is darn close to the melt temperature.

The thermocouple placed on the outside of the Lee pot is not in an insulated environment and therefore has a heat flux across it much higher than the thermostat bulb in the RCBS/Lyman. The junction is buried in it somewhere and is not directly at the tip, and the contact point at the pot is not a perfect thermal conductor either. Consequently the actual junction of that thermocouple can be significantly lower in temperature than the melt. Insulating the thermocouple reduces the heat flux through it and gets the junction temperature closer to the melt temperature.

Calibration would work (throwing in an offset and gain adjustment - most of the cheap ones have at least an offset but not necessarily a gain adjustment) well in the case of a relatively constant ambient. When the ambient varies, the amount of heat flux across the thermocouple varies as well and the calibration is off (unless the ambient is close to the ambient where the calibration was done). If the variation in ambient is significant then the repeatability of melt temperature vs. indicated temperature degrades. The RCBS/Lyman pots don't have a calibrated knob anyway so calibrating those is a moot point (if you want to correlate a knob setting to a given temperature on your own, then that might require fiddling with over time). The PID controller has a readout and having it display the correct temperature is nice, but not strictly necessary if you don't need it to display an accurate temperature. Personally, I was using my setup just fine with the temperature reading off. It gave me perfectly repeatable performance in my reasonably controlled ambient - for a given set point and temperature reading, the melt temp was the same from one time to the next. I finally got around to insulating the thermocouple when I decided I wanted the temperature display to be better - a nicety, not a necessity.

[Zbench]

Sizemore,

As I understand it, the RCBS pot has neither. It has a bimetallic strip in the temperature control which is no where near the pot or the lead. As current flows through the circuit, and the strip, the strip warps due to the current (not unlike the way a light bulb filament gets hot) and breaks the circuit. When the circuit breaks, the strip cools and then starting carrying current again.

The fact that the lead is getting warmed and melted is a secondary consequence...the strip is calibrated with the dial to give approximate temperature readings.

The thermocouples we are talking about actually get hot, directly in the lead or the bottom of the pot, and therefore directly provide feedback to the PID.

So, there is quite a bit of difference in any of the commercial pots and the PID set up we are talking about.

Pete

[Lee W]

The RCBS pro melt is a thermostat. There is a stainless steel bulb clamped to the bottom of the pot.
Here is a link to the instruction sheet. Read the introduction on page 4.

http://www.rcbs.com/downloads/instru...structions.pdf

[evan price]

There is a big difference between a THERMOSTAT and a THERMOCOUPLE.

A thermostat is usually some form of bi-metallic strip that the two metals making up the strip expand at different rates as heated. The difference in expansion causes the strip to curl (or uncurl) and this is used to either directly turn on and off an electrical circuit (by the curling metal strip touching across an electric contact), or (such as in your home thermostat for the furnace) move a mercury switch to turn on a circuit.

A thermocouple is a device which is very simple. It is made from two dissimilar pieces of wire that are joined at one end. When the two wires are heated they will create a voltage difference. This voltage difference is linear depending on the rate that the heat changes. The temperature controller looks at the voltage and performs a calculation depending on what type of thermocouple it is to get the current temperature. The different types of thermocouples (designated with letters, such as J, K, T etc) have a specific chart of voltage VS temperature that is a standard worldwide.

The only difference between thermocouple types is the type of dissimilar wires used to make the junction. For instance, a Type J thermocouple is made of Iron and Constantin. A Type K thermocouple is made of Chromel and Alumel. A Type T thermocouple is Copper and Constantin.

The temperature range for a thermocouple is dependant on the construction materials (type of wire) used- if your wire melts at 800 degrees, it is no good for a furnace flame. You need a type that allows a higher temperature. However that 800 degree melt point wire would be OK to measure the temperature in a swimming pool.

Please note that this describes an ACTUAL thermocouple. There are devices that are used for example to control the pilot gas supply for a gas-fired appliance that are called "thermocouples" but are actually a tube filled with wax that expands or contracts as it warms; this is a safety device to shut down the gas if the pilot light blows out. They are not the same sort of thing.

The temp sensor in a car engine works the same way. There is a rheostat (a sliding scale made of wire, it varies in resistance by position, also called a "varistor") that has a contact which moves across it. There is a wax pellet in the bottom. As the wax pellet heats up, it expands, pushing the contact up. The rheostat changes the resistance based on the position of the contact on the wires. The guage measures the resistance and shows the temp.

The way a process controller (PID is a type of process control) works is it looks at the value supplied by the temperature sensor. It compares that real-time value with the "setpoint" which is the desired temp you program. IF the temp is at or above that temp, the controller does nothing. If the temp is below that setpoint, the controller turns on the heat source. Better controllers are able to calculate how fast the device it is controlling loses heat, and how fast the heat supply is capable of raising the temperature, and they will balance the temp as close to the setpoint as possible with the least amount of off-and-on as possible. You can also program the hysteresis so that it will stay much more closely to the setpoint. Obviously for something that is not considered "process critical" such as a simple lead melting furnace, if you go up or down ten degrees from setpoint it's not a big deal. But let's say you are manufacturing construction adhesives by chemical reaction. It may be critical that you hold to a certain temperature within one degree. You can then program the controller to be more aggressive at keeping the temperature at setpoint and not allowing drops. The only problem is this leads to more on-and-off and short-cycling, which can damage the heat source or the contacts in the device due to arcing or wear. A more sophisticated controller with PID looping will monitor the rate of heat loss and know that if it turns on the heat at a certain point before the temperature falls below setpoint, it will prevent the temp from dropping below setpoint in the first place. When used with a proportional control heat source (one which is analog or scalable- it can turn on 10% or 100% or anything in between) it can hold setpoint temp +/- less than one degree.
With an ON/OFF sort of control (such as the electric heat element in the lead pot), you will have more overshoot and undershoot and you have to balance the setpoint with being a little hotter or cooler sometimes.


That was more than I originally meant to say- but hope it is useful.

[RoyRogers]

Please note that this describes an ACTUAL thermocouple. There are devices that are used for example to control the pilot gas supply for a gas-fired appliance that are called "thermocouples" but are actually a tube filled with wax that expands or contracts as it warms; this is a safety device to shut down the gas if the pilot light blows out. They are not the same sort of thing.

Evan - thanks for the excellent post. However, this it is not the way the thermocouples used in many gas appliances are utilized. Typically they are there to prove pilot or main burner flame. Ovens also use thermocouple(s) to signal a thermostat as to when to light a main burner or to send current to an electric heating element. All these thermocouples generate a small (mv) current when heated. When used as safety devices on a gas appliance this current is employed to hold a control magnet in operating position. When in this position, the internal passageways that allow gas flow through the gas valve/thermostat are open. If no voltage is generated by the thermocouple, the magnet releases which in turn closes the gas passageways inside the gas valve/thermostat. I believe you may be thinking of the way expanding contracting wax in a gas water heater's thermostat's immersion bulb signals for heat?

[x_il_towman]

YOU OLD GUYS WITH YOUR ANCIENT SAECO AND LYMAN LEAD POTS NEED TO DO WHAT I DID.
(Plastic Process Equipt. Inc--8303 Corporate Park Dr.---Macedonia, Oh. 44056). IS THE PLACE TO CONTACT FOR A REPLACEMENT THERMOSTAT. I JUST COMPLETED REPLACEING MINE, AND WITH A LITTLE MINOR ENGINEERING(?) MADE IT FIT. WORKS GREAT, JUST LIKE HAVING MY OLD POT BACK. YOU WANT MODEL B-200. SORRY, DON'T HAVE ADDRESS OR WEB, BUT GOOGLE IT.. WORTH A TRY. $23.00 PLUS UPS/// Mac

Anyone here ever use this thermostat???

I too am rebuilding an old Lyman 61 pot & this thread has been very helpful.
I am concerned that the temp setting listed for the b-200 is not within operating range. ( 100-550F see enclosed picture).

Thanks in advance for the help...

[Lee]

If I understand correctly, the TC for a furnace or hot water heater is just that. It is a device that generates a small millivolt voltage. That is supplied to a small coil in the gas valve. THAT opens or closes the main gas valve. No wax at all. Or am I missing something in the definition of what we are trying to control??
(Never had to replace either, always thought they "were all alike" You know, kinda like ..........Lee

[RaymondMillbrae]

Just curious to know HOW folks are "fine tuning" their PID temperature controllers.

I just finished making my PID devise the other day, and I am still working with it to stabalize the molten lead temperatures.

I know that if I set my PID controller for 700-degrees, it will shut the power OFF before the molten lead hits 700-degrees. But since the coils retain so much heat, the temperatures will keep rising until it goes 20-degrees higher than the preset 700-degrees.

My home-made PID devise has a "self optimization" function...but is that what I need?

I was also looking at playing with the "differential" (hysteresis), but I am not sure if this is the way to go, as I do not completely understand how to set the differential parameters.

I'll keep my ear to the ground in the meantime.

In Christ: Raymond

[lurch]

Raymond,

The "auto tune" or "self optimization" works well enough in my experience to get to a very workable solution. Unless you are versed in PID theory, I would definitely suggest filling your pot about half or a little more full and let the controller decide what it's parameters are going to be. Without knowing what the "differential" parameter on your controller really is, I would assume that it is a dead band adjustment, essentially meaning that the sensed temperature has to be off from set point by a given amount before any correction is applied to the on-off duty cycle of the power to the heating element. Messing around with the PID parameters can lead to an unstable or oscillating temperature if not done carefully. When I let mine auto tune, I found that the initial startup gives me a decent initial overshoot but once that has subsided, temperature is maintained very nicely. Any significant deviation is due to adding metal to the pot in such quantity that the power of the heating element is insufficient to kick the temperature up any quicker.

[jeff423]

I use the auto tune or self optimization feature. I have 3 pid's on various types of units. Aquarium, coffee maker and lead pot. One thing that I noticed is that the better pid's had a longer self tuning cycle. My real cheapie went through one cycle and that was it. My better unit went through 3 cycles and my best one has a continuous learn feature that is always monitoring. It listed for $300 but I got it used on ebay for $75.
One thing you might try for the self tune is to let it get to the correct temperature, then turn off the controller. Restart it and go back through the auto-tune. This will reset the PID for casting conditions - as opposed to bringing it from cold all the way to casting temp.
I would also use as large a solid state relay as you can afford and a heat sink. They last much longer if kept cool. I mounted a cheap computer cooling fan to blow on my 25 amp SSR on my lead pot. I have not had any problems with it so far but would probably get a 40 amp next time. I have a 50 on my espresso machine, but no fan.

YMMV

Jeff

[RaymondMillbrae]

Lurch,

below is a picture of the PID temperature controller I recently finished.

Are you telling me that you keep your casting pot half full (or a little over half)...and when the "self optimaization" is done, you fill it up normally? Or are you telling me you ALWAYS keep your casting pot half full?

By the way, I am using a Lee Pro 4-20. Shoulld I keep the knob turned all the way up to the HI position, or should it be kept lower? What are your observations?

Jeff423,

I will try what you said above.

QUOTE: One thing you might try for the self tune is to let it get to the correct temperature, then turn off the controller. Restart it and go back through the auto-tune. This will reset the PID for casting conditions - as opposed to bringing it from cold all the way to casting temp.

I am using a 25 amp SSR, with a large aluminum heat sink cannibalized from an old mother board. I believe it should do the trick.

Thanks, and if someone has something else to add...add right on!

In Christ: Raymond

PS: I'll be doing a tutorial soon...so stay tuned.

[blikseme300]

2c added to the mix.

My experiences with temperature stability due to experimentation and observation made me conclude these things:

1. Use a pot that is large enough. The level must not change rapidly when volume casting larger boolits, ie using 2x .45 6-banger molds in tandem.

2. The pot needs to be properly insulated. Use kaowool for this.

3. Use a PID controller and let the temperature stabilize before starting to cast. Initial overshoot of the temperature is normal. Once stable it will stay very close to the set temperature even when cold ingots are added.

See my solution to my needs here: http://bliksemseplek.com/boolits.html

Bliksem

[lurch]

Lurch,

below is a picture of the PID temperature controller I recently finished.

Are you telling me that you keep your casting pot half full (or a little over half)...and when the "self optimaization" is done, you fill it up normally? Or are you telling me you ALWAYS keep your casting pot half full?

By the way, I am using a Lee Pro 4-20. Shoulld I keep the knob turned all the way up to the HI position, or should it be kept lower? What are your observations?

Jeff423,

I will try what you said above.

QUOTE: One thing you might try for the self tune is to let it get to the correct temperature, then turn off the controller. Restart it and go back through the auto-tune. This will reset the PID for casting conditions - as opposed to bringing it from cold all the way to casting temp.

I am using a 25 amp SSR, with a large aluminum heat sink cannibalized from an old mother board. I believe it should do the trick.

Thanks, and if someone has something else to add...add right on!

In Christ: Raymond

PS: I'll be doing a tutorial soon...so stay tuned.

Your setup looks very nice. Your questions are valid, so here goes. Fill your pot half or so full (or about at the halfway point of where you intend on running the fill while casting) and let the thing autotune. The lead level in the pot does not need to remain there when casting. The amount of lead in the pot acts like a "lag" or time delay to the temperature we see vs. the amount of heat energy we put in. More lead is more lag and less is less. That time delay is an integral part of the tuning process and getting it somewhere in the middle of the range of where it will operate is a way to get a compromise on the overall system response for a range of pot fill levels. I have not tried this, but tuning with a full pot might improve the response when adding ingots, with possibly reduced performance (oscillation...?) at low fill levels. Try it and see if you are so inclined - can't really hurt as you can always just re-tune...

Jeff gives good advice about letting the pot get to temperature and then starting the autotune. That's what I did, but neglected to mention. The controller I have tunes itself based on what is called the step response of the total system. Without getting too technical, the step response is just what the system does when given an input disturbance that looks like, well, a step - raise (or lower) the input (power from the heating element) from one level to another and observe the system response over some time period. From that with a little math it is possible to figure out what the "optimum" PID parameters are. Optimum can be defined several ways with fastest time to temperature with no overshoot and fastest time to temperature within a certain window with over and undershoot not exceeding x probably being the most relevant. Also, the rate at which heat leaves the system at casting temperature is greater than the rate at which it leaves at room temperature. This also affects how the final "ideal" PID tuning will turn out. If the controller you have tunes itself once the temperature has reached the "set point" of the controller then you can just establish your set point and start the tuning process and be done with it. If the controller does not tune at its set point, then get things to temperature and then start the tuning to take care of the heat loss factor. For a pot like the Lee, the heat loss at temperature is a lot higher than say for a Lyman or RCBS due to the insulation of the latter two as opposed to the essential lack of for the Lee.

For the Lee pot, definitely keep the control turned up all the way if you keep it in the circuit as it appears you will be doing from the pictures. That is what I did. Think of this as sort of a backup system to keep things from getting out of hand should the SSR or thermocouple fail. The SSR will fail shorted when it does and this would just turn on the heat full blast with no secondary stop to keep things in line if the Lee control is bypassed. Keeping the Lee control at max also keeps it out of the way of the PID controller. One point to mention here is that if the Lee control actuates during the autotune, the autotune results will be off. You definitely want that control out of the PID controller's way when running, but still able to keep the lid on in the event of any failures.

[RaymondMillbrae]

Thanks,

good advice! (Especially about NOT bypassing the Lee heat controls, which I was about to do).

In Christ: Raymond

[mac1911]

Been poking around CB for ideas on upgrading and building new things.
Any up dates to available PID controllers for upgrading old lee pot

Seems many of the links are long dead

[Mal Paso]

These guys have good quality products and support. https://www.auberins.com/index.php?m...=index&cPath=1

FAQs here. https://www.auberins.com/index.php?main_page=page&id=6

[jsizemore]

I see a lot of builds that use a sealed box. The box needs to stay relatively cool to operate correctly. My original Auberins instructions stated this. I've used an ONEAC power conditioner case, computer bower supply perforated box and a Sony camera power supply carry case. All have plenty of ventilation. The case for the ONEAC/Auberins build is made of aluminum so the SSR is attached directly to the case bottom with silicone grease. All run cool to the touch.

[Mike W1]

Don't remember reading this thread when I built my units but it's kind of interesting to see how things evolved since it was started. By the time I got around to doing mine the consensus seemed to be mount the TC 1/2" from bottom and sides and that seems to have worked out for me. Mine are removable ones.

Somewhere along the way I read to just do the autotune when the PV comes to within about 20° of the SV. Haven't done any casting for quite awhile now but I don't remember a lot of overshoot on mine. Wired a neon to light when the SSR is shooting power to the pot and believe it stays on till SV is reached without cycling like it does when all it's trying to do is maintain a steady temperature. Will have to monitor that a little more closely when I get back at casting.

Personally I use 2 REX C100s on the pots and an Auber on the hotplate. You do have to know what to look for when buying a REX but have had fine luck with mine. I didn't have that good luck with the Mypins.

[mac1911]

These guys have good quality products and support. https://www.auberins.com/index.php?m...=index&cPath=1

FAQs here. https://www.auberins.com/index.php?main_page=page&id=6

Thanks for the link,
i have not read it yet but found this.
https://drive.google.com/file/d/0B14...80VGdlNEk/view