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7008
Users Guide
8-8
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 operation. If
during operation the front panel meter indicates excessive cooling, this valve is
closed partially (turn CW) until the percentage of heating to cooling is brought
into line. It is necessary to wait a few minutes after each adjustment until the
system settles.
Set the COOLING TEMPERATURE to 5-10
°
C below the SET TEMPERATURE
for near ambient and below operation. For temperature above 16
°
C 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 adjustment until
the system establishes itself. Readjust if required after settling for a while.
For maximum cooling for slewing to lower temperatures, the BACK PRESSURE
valve should be fully open and the COOLING TEMPERATURE set to 0
°
C.
Readjust higher or lower as required after set temperature is reached.
8.6 Temperature
Controller
The bath temperature is controlled by Hart Scientific’s unique hybrid
digital/analog temperature controller. The controller offers the tight control
stability of an analog temperature controller as well as the flexibility and
programmability of a digital controller.
The bath temperature is monitored with a linearized thermistor sensor in the
control probe. The signal is electronically compared with the programmable
reference signal, amplified, and then fed to a pulse-width modulator circuit which
controls the amount of power applied to the bath heater.
The bath is operable within the temperature range given in the specifications. For
protection against solid-state relay failure or other circuit failure, the
microcontroller will automatically turn off the heater with a second mechanical
relay anytime the bath temperature is more than a certain amount above the set-
point temperature. As a second protection device, the controller is also equipped
with a separate thermocouple temperature monitoring circuit which will shut off
the heater if the temperature exceeds the cut-out set-point.
The controller allows the operator to set the bath temperature with high
resolution, set the cut-out, adjust the proportional band, monitor the heater output
power, and program the controller configuration and calibration parameters. The
controller may be operated in temperature units of degrees Celsius or
Fahrenheit. The controller is operated and programmed from the front control
panel using the four key-switches and digital LED display. The controller may
also be optionally equipped with a serial RS-232 or IEEE-488 GPIB digital
interface for remote operation. Operation of the controller using the front control
panel is discussed following in Section 9. Operation using the digital interfaces is
discussed in Section 10.
When the controller is set to a new set-point the bath will heat or cool to the new
temperature. Once the new temperature is reached the bath usually takes 10-15
minutes for the temperature to settle and stabilize. There may be a small
overshoot or undershoot of about 0.5
°
C.
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