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8
General Operation
8.1
Bath Fluid
Many fluids will work with 7060/7061 bath. Choosing a fluid requires con-
sideration of many important characteristics of the fluid. Among these are
temperature range, viscosity, specific heat, thermal conductivity, thermal ex-
pansion, electrical conductivity, fluid lifetime, safety, and cost.
8.1.1
Temperature Range
One of the most important characteristics to consider is the temperature
range of the fluid. Few fluids work well throughout the complete tempera-
ture range of the bath. The temperature at which the bath is operated must
always be within the safe and useful temperature range of the fluid used.
The lower temperature range of the fluid is determined by the freeze point
of the fluid or the temperature at which the viscosity becomes too great.
The upper temperature is usually limited by vaporization, flammability, or
chemical breakdown of the fluid. Vaporization of the fluid at higher temper-
atures may affect temperature stability because of cool condensed fluid
dripping into the bath from the lid.
The bath temperature should be limited by setting the safety cut-out so that
the bath temperature cannot exceed the safe operating temperature limit of
the fluid.
8.1.2
Viscosity
Viscosity is a measure of the thickness of a fluid, how easily it can be
poured and mixed. Viscosity affects the temperature uniformity and stability
of the bath. With lower viscosity, fluid mixing is better therefore creating a
more uniform temperature throughout the bath. This uniform temperature
improves the bath response time which allows it to maintain a more con-
stant temperature. For good control the viscosity should be less than 10
centistokes. 50 centistokes is about the practical upper limit of allowable
viscosity. Viscosities greater than this cause very poor control stability be-
cause of poor stirring and may also overheat or damage the stirring motor.
With oils, viscosity may vary greatly with temperature.
When using fluids with higher viscosities the controller proportional band
may need to be increased to compensate for the reduced response time.
Otherwise the temperature may begin to oscillate.
8.1.3
Specific Heat
Specific heat is the measure of the heat storage ability of the fluid. Specific
heat, though to a lesser degree, affects the controller stability and the heat-
ing and cooling rates. Generally, a lower specific heat means quicker heat-
ing and cooling. Different specific heats may require an adjustment to the
8
General Operation
27
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