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.
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.
43
9 Controller Operation
Proportional Band
Fluid
Temperature
Heater Setting
Proportional
Band
Stability
Water
30.0°C
Low
0.04°C
±
0.0004°C
Water
60.0°C
Low
0.04°C
±
0.001°C
Eth-Gly 50%
35.0°C
Low
0.05°C
±
0.0005°C
Eth-Gly 50%
60.0°C
Low
0.05°C
±
0.001°C
Eth-Gly 50%
100.0°C
High
0.4°C
±
0.007°C
Oil
35.0°C
Low
0.1°C
±
0.003°C
Oil
60.0°C
Low
0.2°C
±
0.002°C
Oil
100°C
Low
0.2°C
±
0.003°C
Table 3
Proportional Band — Fluid Table
Figure 8
Bath temperature fluctuation at various proportional band settings
