Fluke 7312, User Manual

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8 General Operation
8.1 Heat Transfer Fluid
Many fluids work with 7312 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 entire 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 temperature
range of the fluid is determined either 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 adversely 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 or 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 creating a more uniform tem-
perature throughout the bath. This improves the bath response time allowing it
to maintain a more constant temperature. For good control the viscosity should
be less than 10 centistokes. Fifty centistokes is about the practical upper limit
of allowable viscosity. Viscosity greater than this causes very poor control sta-
bility because of poor stirring and may also overheat or damage the stirring mo-
tor. Viscosity may vary greatly with temperature, especially with oils.
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, to a small degree, affects the control stability. It also affects the heating
and cooling rates. Generally, a lower specific heat means quicker heating and
cooling. The proportional band may require some adjustment depending on the
specific heat of the fluid.
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8 General Operation
Heat Transfer Fluid