General Operation; Bath Fluid; Temperature Range; Viscosity - Fluke Calibration 6020 Series User Manual

8

General Operation

8.1

Bath Fluid

Many fluids work with the 6020 series baths. Choosing a fluid requires consid-
eration of many important characteristics of the fluid. Among these are temper-
ature range, viscosity, specific heat, thermal conductivity, thermal expansion,
electrical 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. 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. 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 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, 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.
8 General Operation
Bath Fluid
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