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Boiling Point: At or near the boiling point of the fluid, the temperature stabil-
ity is difficult to maintain. Fuming or evaporation is excessive. Large amounts
of heater power may be required because of the heat of vaporization.
Decomposition: The temperature may reach a point at which decomposition of
the fluid begins. Further increasing the temperature may accelerate decomposi-
tion to the point of danger or impracticality.
8.2
Stirring
Stirring of the bath fluid is very important for stable temperature control. The
fluid must be mixed well for good temperature uniformity and fast controller
response. The stirrer is precisely adjusted for optimum performance.
8.3
Power
Power to the bath is provided by an AC mains supply. See Section 3.1, Specifi-
cations for power details. Power to the bath passes through a filter to prevent
switching spikes from being transmitted to other equipment.
To turn on the bath switch the control panel power switch to the ON position.
The stir motor will turn on, the LED display will begin to show the bath tem-
perature, and the heater will turn on or off until the bath temperature reaches
the programmed set-point.
When powered on the control panel display will briefly show a four digit num-
ber. This number indicates the number of times power has been applied to the
bath. Also briefly displayed is data which indicates the controller hardware
configuration. This data is used in some circumstances for diagnostic purposes.
8.4
Heater
The power to the bath heater is precisely controlled by the temperature control-
ler to maintain a constant bath temperature. Power is controlled by periodically
switching the heater on for a certain amount of time using a solid-state relay.
The front panel red/green control indicator shows the state of the heater. The
control indicator glows red when the heater is on and glows green when the
heater is off. The indicator will pulse constantly when the bath is maintaining a
stable temperature.
The heater has two power level settings. The "HIGH" heater power setting is
used to quickly heat up the bath fluid to the desired operating temperature. The
"HIGH" heater power setting may also be required for control at high tempera-
tures. The "LOW" setting is used for control at lower temperatures and for
scanning at slower rates. When controlling at the "HIGH" heater power setting
instead of "LOW" the proportional band may need to be increased (typically by
a factor of four) to compensate for the increase in power gain. Otherwise the
temperature may oscillate.
8 General Operation
Stirring
31
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