7037/7038/7040/7041
User's Guide
30
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. Wait a few minutes after each adjustment until the system settles.
The cooling temperature should NOT be set above 90 psig. About 45
°
C the
refrigeration is not 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.
Temperature Controller
The bath temperature is controlled by Fluke Calibration’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 platinum resistance 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 automatically turns 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 shuts 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 the
Controller Operation
section. Operation using
the digital interfaces is discussed in the
Digital Communications Interface
section.
When the controller is set to a new set-point the bath heats or cools 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.