General Operation
Refrigeration 8
8-11
LED display. Remote digital operation with the controller is possible via the standard RS-
232 serial port. The controller may be optionally equipped with an IEEE-488 GPIB
digital interface. Operation of the controller using the front control panel is discussed
following in Chapter 9, Controller Operation. Operation using the digital interface is
discussed in Chapter 10, Digital Communication Interface.
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 15–20 minutes
for the temperature to settle and stabilize. There may be a small overshoot or undershoot
of about 0.5°C during this process.
Refrigeration
Cooling is provided by a compact refrigeration system utilizing the ozone safe R-507
HFC refrigerant. Refrigeration is often not needed when the bath is above 45 to 60°C.
The automated system automatically turns the system off when above 60°C. The
refrigeration system automatically changes modes depending upon bath temperature and
operation needs as described below. The refrigeration system is activated when the
switch on the front panel is turned on.
If the bath is below the 60°C cutoff point and is set to a temperature above 60°C, the
refrigeration shuts off to protect it from overheating and creating excessive internal
pressures. If the bath is above 60°C and is set to a new temperature below the cutoff
temperature, the refrigeration turns on again at 59°C.
When the bath is controlling at temperatures between the 60°C and 0°C, the bath operates
in a low cooling capacity mode with the hot gas bypass valve on (HGb on). The HGb
system reduces the cooling capacity and helps to improve the bath stability within that
temperature range. Since more cooling capacity is generally required below 0°C, the hot
gas bypass is turned off (HGb off) providing more cooling capacity.
When the bath is cooled from temperatures below the cutoff point to 0°C and if the new
set-point is at least 2°C below the current bath temperature, the cooling turns to
maximum capacity (HGb off) until the bath is within about 0.5°C of the new set-point.
This provides the fastest cooling possible to achieve the new set-point temperature. As
indicated, the cooling is always maximum below 0°C.
When heating the bath from temperatures of –40°C and the new set-point is at least 10°C
above the current bath temperature, the refrigeration turns off until the bath is about 1°C
below the new set-point. This permits the maximum heating rate to reach the higher
temperatures as quickly as possible. The cooling remains off at temperatures above 60°C.
The automatic cooling mode may not be ideal for all circumstances. The automatic mode
may be turned off permitting manual control. These controls are available through the
front panel as described in Operating Parameters in Chapter 9, or through the digital
interface as described in Interface Commands and Cooling Control in Chapter 10. The
cooling mode control may be set to automatic, on, or off. With these selections the
refrigeration is either in automatic as described above, always on, or always off. The hot
gas bypass modes are similarly selectable between automatic, on, or off. This system is
either in automatic as described above, always on (low cooling capacity) or always off
(high cooling capacity).
The following situations benefit by changing cooling modes.
• If the scan mode has been selected and the desired cooling scan requires maximum
cooling capacity in a range where the hot gas bypass would normally be on.
• The application requires maximum cooling capacity when the hot gas bypass would
normally be on.
• When using the bath at temperatures below 0°C in an ambient of less than 23°C, a
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