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7011 Calibration Bath
User's Guide
8.5
8.6
32
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 valve to the nominal pressure indicated
by the chart (see table in Section 7.2 ). Interpolate values for temperatures be-
tween those shown. Above 38°C, set the pressure to 90 PSIG. Refrigeration
may not be required above 40°C. Actual pressures may be varied from chart
values in order to optimize performance, however, the pressure should not ex-
ceed 3 PSIG above chart values. Wait a few minutes after each adjustment until
the system establishes itself. Readjust if required after settling for a while.
The BACK PRESSURE valve should normally be open (fully CCW). The
valve may be partially closed to reduce cooling capacity.
The optimum controller output power is from 10 to 30 percent as monitored on
the controller duty cycle display (see Section 9.6).
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.
Temperature Controller
The bath temperature is controlled by Hart Scientific's unique hybrid digi-
tal/analog temperature controller. The controller offers the tight control stability
of an analog temperature controller as well as the flexibility and programmabil-
ity 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 micro
controller 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 resolu-
tion, 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 Fahr-
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