Figure 6 Bath Temperature Fluctuation At Various Proportional Band Settings - Fluke 7341 User Manual

7341 Calibration Bath
User's Guide
46
The temperature stability of the bath depends on the width of the proportional
band (see Figure 6). If the band is too wide, the bath temperature deviates ex-
cessively from the set-point due to varying external conditions. This is because
the power output changes very little with temperature and the controller cannot
respond very well to changing conditions or noise in the system. If the propor-
tional band is too narrow, the bath temperature may swing back and forth be-
cause the controller overreacts to temperature variations. For best control
stability the proportional band must be set for the optimum width.

Figure 6 Bath Temperature Fluctuation At Various Proportional Band Settings

The optimum proportional band width depends on several factors among which
are fluid volume, fluid characteristics (viscosity, specific heat, thermal conduc-
tivity), heater power setting, operating temperature, and stirring. Thus the pro-
portional band width may require adjustment for best bath stability when any of
these conditions change. Of these, the most significant factors affecting the op-
timum 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 control-
ler 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 propor-