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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 scan-
ning 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.
8.5
Cooling
The BACK PRESSURE control valve limits the cooling capacity of the unit. It
will normally be open all the way (full CCW) for temperature slewing and op-
eration. If during operation the front panel meter indicates excessive cooling,
this valve is closed partially (turn CW) until the percentage of heating to cool-
ing is brought into line. It is necessary to wait a few minutes after each adjust-
ment until the system settles.
Set the COOLING TEMPERATURE to 5-10°C below the SET TEMPERA-
TURE for near ambient and below operation. For temperature above 16C set
the COOLING TEMPERATURE at 7°C. The cooling temperature should NOT
be set above 7°C or 90 psig. About 45°C the refrigeration will not be required
as there is sufficient cooling to the room. Wait a few minutes after each adjust-
ment until the system establishes itself. Readjust if required after settling for a
while.
For maximum cooling for slewing to lower temperatures, the BACK PRES-
SURE valve should be fully open and the COOLING TEMPERATURE set to
0°C. Readjust higher or lower as required after set temperature is reached.
User’s Guide
26