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By Wayne McLerran
A PID, sometimes referred to as a three-term controller, is a device that compares
a set point (SP) or set value (SV) to the actual process value (PV) as measured by
a sensor over a period of time.  When used to control temperature, the sensor is
usually some form of a thermocouple.  In simplistic terms, the set point or set
value is what you want and the process value is what you get.  Using a feedback
system and a unique algorithm, the PID supplies power to the pot until the process
or actual value is equal to the set or desired value.  PID stands for Proportional-
Integral-Derivative.  For a somewhat simplified discussion of PIDs, go to
//www.csimn.com/CSI_pages/PIDforDummies.html.  For much more information
than you need to know, run an Internet search for “PID controller”.

Casting pot thermostats wait until the temperature is below the set point, then full
power is applied until the temperature is above the set point.  It then shuts off and
the cycle starts over, resulting in temperature overshoot and undershoot.  When
used with a casting pot a PID quickly cycles the power on and off to make small
adjustments to the amount of heat being generated in order to maintain the desired
temperature.  After it was “trained” on my pot and once the alloy temperature
increased to within 20 degrees of the set value the PID started cycling the power on
and off at approximately a 1-second rate.  It continued the 1-second cycle once the
set temperature was reached, adjusting the applied power as necessary to maintain
the temperature.  I understand the cycle rate may be different for different pots.

So, does using a PID result in better bullets?  The answer is it may or may not
help.  If you have an inexpensive pot that does a less than stellar job of controlling
the alloy temperature, a PID will likely be a benefit.  Some contend that the Lee
pots fall into this category.  It may also help to some degree if you’re unable to
maintain a steady casting cadence.  If you’ve developed a good casting technique
and your bullets consistently drop out of the mould to within +/- 1gr. than using a
PID will most likely not improve the weight variance.  As an example, prior to
installing the PID, I was able to consistently cast 400 gr. bullets to within +/- 2gr.  
Using the PID for the 1st time, the weight variance of 300 bullets dropped to +/-
1.5gr.  Since then the spread has been reduced to +/- 0.5gr.  I can't say for sure
that all the weight variance reduction is due to the PID, but it sure has helped.

I found that the PID made casting a little easier and more accurate.  Once it was set
up and “trained” I did not have to guess what temperature the thermostat setting on
the pot was set at; and changing the temperature an exact amount is easy.  What
was especially nice was seeing the exact alloy temperature at all times and how it
was affected when the ladle is submersed and when the sprues are returned to the
pot.  By the way, when submersed in the full 22 lb pot, a cold (room temperature)
RCBS ladle dropped the alloy temperature 25 degrees, and it was simple to
determine when the temperature recovered prior to beginning the casting sequence.

Many have built their own units.  It’s a relatively simple design and all the
necessary parts are readily available on eBay, Amazon and/or several suppliers for
$50 to $100.  You’ll need the main controller, solid state relay (SSR), heat sink,
appropriate thermocouple, on/off switch, fuse/fuse holder, terminal strip, power
cord and small project box.  All of this is discussed on numerous Internet sites and
in several threads on the Cast Boolits forum
http://castboolits.gunloads.com/forum.php).  Run a search for “PID controller”.  
BTW, the controller pictured in the photo was purchased from a company which is
no longer in business.  For those that don’t want to bother with building one, I
understand there is a fellow that frequents the Cast Boolits forum that sells PID

In closing I’ll leave you with a few comments.  The benefits of using a PID
depends on what you’re casting for.  If your pot has a bottom pour feature and you
are using it for casting handgun bullets I doubt a PID will provide any noticeable
benefits.  But if you're ladle/dip casting for match grade bullets I think it's an
excellent idea based on how well it works for me.  Most of my casting has been for
.40 cal (400 gr.) bullets using a RCBS bottom pour pot, but I don’t use the bottom
pour feature.  I “dip cast” with a ladle.  I also have a 20 lb lee pot and use it with a
ladle and different alloy for .45 cal bullets.  The PID is switched from pot to pot.    
Not only does the PID control the temperature very well within a few degrees it
also indicates when your pot is up to casting temperature.  A Lee pot uses a very
basic temp switch that allows relatively large swings in the alloy temperature.  It
works for many but in my estimation there’s room for improvement when casting
match grade bullets.

BTW, if you are considering purchasing an expensive pot, you can save a bunch of
bucks by going with a Lee pot and using a PID.  The combination will result in a
casting setup that provides much better temperature control than available with
expensive pots.  Plus you gain the benefit of precise visible feedback of the alloy
temperature while casting.  But don’t expect the addition of a PID to significantly
improve results if your casting technique is poor.  Precisely controlling the alloy
temperature is only one of several factors affecting cast bullet quality.

Wishing you great shooting,
Updated 6/10/18

Since writing the article below RCBS and Lyman have introduced casting
furnaces with PID controllers built in.  The RCBS ProMelt 2, RCBS Easy Melt
and the Lyman Mag 25 feature easy to program and use PID feature.  The RCBS
ProMelt 2 and Lyman Mag 25 are bottom pour designs but can also be used for
ladle dipping.  The RCBS Easy Melt model for ladle dipping only is an
especially good buy.  All hold 25 lbs of alloy.  To read a full evaluation of the
RCBS Easy Melt furnace click on:
Evaluation of the RCBS Easy Melt Casting
Furnace with PID.  For more details on how PIDs work, read on.

I've been using a PID controller when casting since Aug. 2015.  It’s an electronic
device that is installed in-line with the AC power to the casting pot.  No
modifications are required to the pot.  The PID controller (hereon referred to as a
PID) plugs into AC power and the pot plugs into the PID.  A thermocouple
connected to the PID is either installed in the pot prior to adding alloy or inserted
into the alloy after it has sufficiently melted.  The thermostat on the pot is turned
to maximum temperature and the PID takes over control of the power to the pot
to finely manage the temperature.  After it was calibrated (“trained”), which was
a simple step, it held the alloy temperature of my 22 lb capacity RCBS Pro-Melt
casting pot to within +/- 5 degrees of the set temperature during the normal
casting sequence.

Following is a photo of my setup.  You will note the PID in the center of the
photo and the thermocouple inserted into the alloy.  Also note the temperature is
set for 750 degrees (green readout) and the actual alloy temperature is 751
degrees (red readout).