General Operation; Heat Transfer Fluid; Temperature Range; Viscosity - Fluke 7008 User Manual

8.1 Heat Transfer Fluid

Many fluids will work with 7008 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, 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 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 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 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 stability of the bath. With low viscosity,
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.

General Operation

Chapter 8
8-1