Chemglass CG-1972-65 - Manual (Page 21)

21
Section 4: Application Notes
Supplemental application notes on the following topics are available by contacting J-KEM.
Application
Note
Subject
AN1 How to heat oil baths with your controller. (Included in Appendix)
AN2 Changing the controller’s thermocouple type.
AN3 Changing the heating outlet into a cooling outlet.
AN4 Using the controller for unattended fractional distillations.
AN5 Using the controller with inductive (motors, valves) loads.
4.1
Theory of How the Controller Works – Simply. For the purpose of explaining how the controller
works, the example of a solution heated with a heating mantle is used. The principles are the same for all
heater types.
Power
ON
J -KEM S cie ntific
Instruments for Science
Model 210 120 Vac, 840 watts
120 vac
Power to heater
Thermocouple
Heating mantle
Temperature of process
from the thermocouple
Output
Power Level
1-10 ml
10-100 ml 50-500 ml
300 ml-2 L
> 2 L Heat
Off
8 0. 0
o C
*
Figure 2
The controller, the heating mantle and the thermocouple form a closed loop feedback system. If
the process temperature is below the set point, the controller turns the heating mantle on and then
monitors the temperature rise of the solution. If a small rise results (indicating a large volume is being
heated) the controller sets internal parameters appropriate for heating large volumes. If a large rise in
temperature results, the controller responds by loading a set of parameters appropriate for heating small
volumes. For the controller to work ideally, information needs to travel instantaneously around the
feedback loop. That means that any power the controller applies to the heating mantle must reflect itself
in an instantaneous temperature rise of the solution and the thermocouple. Unfortunately, this type of
instantaneous heat transfer from the heating mantle to the solution to the thermocouple just doesn't occur.
The delay time between when power is applied to the heating mantle and when the solution rises in
temperature; and also the converse, when power is removed from the heating mantle and the
solution
temperature stops rising is the source of most controller errors. The reason for this can be seen in a
simple example.