Fluke 7341 User Manual page 41

7341 Calibration Bath
User's Guide
36
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) pro-
viding 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 heat-
ing rate to reach the higher temperatures as quickly as possible. The cooling re-
mains off at temperatures above 60°C.
The automatic cooling mode may not be ideal for all circumstances. The auto-
matic mode may be turned off permitting manual control. These controls are
available through the front panel as described in Section 9.13, Operating Pa-
rameters, or through the digital interface as described in Sections 10.3, Inter-
face Commands and 10.4, Cooling Control. 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 ei-
ther in automatic as described above, always on (low cooling capacity) or al-
ways 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 by-
pass would normally be on.
• When using the bath at temperatures below 0°C in an ambient of less than
23°C, a small benefit to stability may be realized by using the hot gas by-
pass.
• The refrigeration may be beneficially used for short times (less than one
hour) above 60°C but less than 100°C.
This list is not intended to be complete, but only suggests some of the situa-
tions when automatic modes may not be best. Most of the time, the automatic
functions are adequate and should be used.