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8
General Operation
8.1
Bath Heat Transfer Fluid
Many fluids will work with 7100 bath. Choosing a fluid requires consideration
of many important characteristics of the fluid. Among these are temperature
range, viscosity, specific heat, thermal conductivity, thermal expansion, electri-
cal resistivity, 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 temperature 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 tem-
perature 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 temperatures 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 uni-
form temperature throughout the bath. This improves the bath response time
which allows it to maintain a more constant temperature. For good control the
viscosity should be less than 10 centistokes. 50 centistokes is about the practi-
cal upper limit of allowable viscosity. Viscosities greater than this cause very
poor control stability because 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, also affects the control stability and the heating
and cooling rates. Generally, a lower specific heat means quicker heating and
cooling. Different specific heats may require an adjustment to the proportional
band to compensate for the change in the sensitivity of the bath temperature to
heat input.
8 General Operation
Bath Heat Transfer Fluid
27
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