General Operation; Heat Transfer Fluid; Temperature Range; Viscosity - Fluke 7341 User Manual

8

General Operation

8.1

Heat Transfer Fluid

Many fluids will work with 7341 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 (see Section
9.10, Cutout) or the high limit (see Section 9.16, Calibration Parameters) 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. This creates a more uniform
temperature throughout the bath. This improves the bath response time allow-
ing it to maintain a more constant temperature. For good control the viscosity
should be less than 10 centistokes. 50 centistokes is the practical upper limit of
allowable viscosity. Viscosity greater than this causes very poor control stabil-
ity because of poor stirring and may also overheat or damage the stirring motor.
Viscosity may vary greatly with temperature, especially with oils.
When using fluids with higher viscosities the controller proportional band (see
Section 9.9, 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 and the heating and cooling
rates. Generally, a lower specific heat means quicker heating and cooling. The
8 General Operation
Heat Transfer Fluid
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