8
General Operation
8.1
Bath Fluid
Many fluids will work with 7009 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 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 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. The lower tempera-
ture range of the fluid is determined by the freeze point of the fluid or the tem-
perature at which the viscosity becomes too great. The upper temperature is
usually limited by vaporization, flammability, or chemical breakdown of the
fluid. Evaporization 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 cutout 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 stability of the bath. With low vis-
cosity, fluid mixing is better which creates a more uniform 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 upper limit of allowable
viscosity. Viscosities greater than this cause very poor control stability 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. It also affects
the heating and cooling rates. Generally, a lower specific heat causes slightly
better control stability and quicker heating and cooling. With fluids with higher
specific heat the controller may require a decreased proportional band to com-
pensate for the decrease in sensitivity of the bath temperature to heat input.
21
General Operation
