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III.
The Effect of Power Level and Tuning on Control Stability
Your J-KEM temperature controller is capable of controlling the temperature of virtually any application
with great precision. But, it’s important to know how the controller works and how to adjust it when the controller
needs to regulate
non-traditional
heaters, or heating applications. For any heating application, there are two critical
considerations:
1) The maximum amount of power that should be applied to the heater.
2) The PID tuning parameters the controller is using.
All examples in this discussion use the same reaction set up:
10 ml of oil in a 25 ml round bottomed flask being heated with a 25 ml heating mantle.
Consideration 1: The maximum amount of power to apply to the heater
Most heaters used in research labs are greatly over-powered for the applications they are typically used for. For
example, in our setup a 25 ml heating mantle has 60 watts of power at 120Vac, which is over 10 times too much
power to heat a solution to typical (20 to 150C) reaction temperatures. One of the purposes of J-KEM Power
Reduction Circuit (the silver knob on the front of the controller associated with volumes) is to precisely regulate
the maximum power applied to the heater. The plot below shows the temperature profile that results when the
power reduction circuit is set to the proper setting of “10-100 ml”, which limits the heating mantles output power
to about 5 watts, the correct amount of power needed in this application.
Consideration 2: The PID tuning parameters.
A thorough discussion of the PID algorithm is beyond the scope of this discussion, but a good explanation can be
found in Wikipedia. Basically, PID parameters match the control characteristics of the controller to the heating
characteristics of the heater. For example, consider the difference in heater characteristics between a gas and an
electric kitchen range. When a gas range comes on, it comes on at 100% power instantaneously, and when it is
turned off, 100% of the power is removed instantaneously.
In comparison, when an electric range is turned on, it may take 30 seconds before the burner even begins to
warm, and maybe 3 minutes to get up to full heater power. Also, when it is turned off it may take 5 minutes to
remove all heater power (i.e., cool). Your J-KEM controller can regulate both of these heater styles with high
precision, but it needs to know which of the two heater styles it is powering. The PID parameters are used to
describe the heat style.
Every heater has a unique set of tuning parameters, but it’s not necessary to use the
perfect
set every time, which
would require a lot of unnecessary setup of the controller. The J-KEM controller is highly adaptive so the
perfect
set of tuning parameters don’t need to be loaded for stable temperature controller, the set just needs to be
