Fluke 7008 User Manual page 50

7008
Users Guide
If the proportional band is too narrow the bath temperature may swing back and
forth because the controller overreacts to temperature variations. For best control
stability the proportional band must be set for the optimum width.
The optimum proportional band width depends on several factors among which
are fluid volume, fluid characteristics (viscosity, specific heat, and thermal
conductivity), heater power setting, operating temperature, and stirring. Thus the
proportional band width may require adjustment for best bath stability when any
of these conditions change. Of these, the most significant factors affecting the
optimum proportional band width are heater power setting and fluid viscosity. The
proportional band should be wider when the higher power setting is used so that
the change in output power per change in temperature remains the same. The
proportional band should also be wider when the fluid viscosity is higher because
of the increased response time.
The proportional band width is easily adjusted from the bath front panel. The
width may be set to discrete values in degrees C or F depending on the selected
units. The optimum proportional band width setting may be determined by
monitoring the stability with a high resolution thermometer or with the controller
percent output power display. Narrow the proportional band width to the point at
which the bath temperature begins to oscillate and then increase the band width
from this point to 3 or 4 times wider. Table 4 lists typical proportional band
settings for optimum performance with a variety of fluids at selected
temperatures.
9-8
Figure 8. Bath temperature fluctuation at various proportional band settings