Chapter 3: Use
20
Stirring the electrolyte helps a great deal.
The heater should have a large area of contact with the cell. Water jackets are good in this regard. Cartridge
type heaters are poor.
Insulation surrounding the cell may minimize inhomogeneities by slowing the loss of heat through the walls of
the cell. This is especially true near the working electrode, which may represent the major pathway of escaping
heat. It is not unusual to find the electrolyte temperature near the working electrode 5
–
20
C lower than that of
the bulk of the electrolyte.
If you cannot prevent thermal inhomogenieties, you can at least minimize their effects. One important design
consideration is the placement of the RTD used to sense the cell temperature. Place the RTD as close as
possible to the working electrode. This minimizes the error between the actual temperature at the working
electrode and the temperature setting.
A second problem concerns the rate of temperature change. You would like to have the rate of heat transfer to
the cell
’
s contents high, so that changes in the cel
l’
s temperature can be made quickly. A more subtle point is
that the rate of heat loss from the cell should also be high. If it is not, the controller risks gross overshoots of the
set point temperature when it raises the cell temperature. Ideally, the system actively cools the cell as well as
heats it. Active cooling can consist of a system as simple as tap water flowing through a cooling coil and a
solenoid valve. Temperature control via an external heater such as a heating mantle is moderately slow. An
internal heater, such as a cartridge heater is often quicker.
Tuning the TDC5 Temperature Controller: Overview
Closed-loop control systems such as the TDC5
must
be tuned for optimal performance. A poorly tuned system
suffers from slow response, overshoot, and poor accuracy. The tuning parameters depend greatly on the
characteristics of the system being controlled.
The temperature controller in the TDC5 can be used in an ON/OFF mode or a PID (Proportional, Integral,
Derivative) mode. The ON/OFF mode uses hysteresis parameters to control its switching. The PID mode uses
tuning parameters. The controller in PID mode reaches the set-point temperature quickly without much
overshoot and maintains that temperature within a closer tolerance than the ON/OFF mode.
When to Tune
The TDC5 is normally operated in PID (proportional, integrating, derivative) mode. This is a standard method
for process-control equipment that allows for rapid changes in the set parameter. In this mode the TDC5 must
be tuned to match it to the thermal characteristics of the system that it is controlling.
The TDC5 is shipped in a default for PID-control mode configuration. You must explicitly change it to operate
in any other control mode.
The TDC5 is initially configured with parameters appropriate for a Gamry Instruments FlexCell
™
heated with a
300 W jacket and cooled using solenoid-valve controlling water-flow through a cooling coil. The tuning settings
are described below:
