General Operation
Heat Transfer Fluid 8
8-3
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, electrical resistivity, fluid lifetime,
safety, and cost.
Use the fluid chart in the manual to choose the type of fluid that is best suited for you
intended purpose.
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 Cutout in
Chapter 9) or the high limit (see Calibration Parameters in Chapter 9) so that the bath
temperature cannot exceed the safe operating temperature limit of the fluid.
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 allowing 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 stability because of poor stirring and may also overheat or
damage the stirring motor.
Viscosity may vary greatly with temperature, especially with oils. Viscosity increases as
temperature decreases. When fluid viscosity increases, stability performance of the unit
may decrease. Ensure that stirring occurs at all temperatures.
When using fluids with higher viscosities the controller proportional band (see
Proportional Band in Chapter 9) may need to be increased to compensate for the reduced
response time. Otherwise the temperature may begin to oscillate.
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 proportional band may
require some adjustment depending on the specific heat of the fluid.
Thermal Conductivity
Thermal conductivity measures how easily heat flows through the fluid. Thermal
conductivity of the fluid affects the control stability, temperature uniformity, and
temperature settling time. Fluids with higher conductivity distribute heat more quickly
and evenly improving bath performance.
Thermal Expansion
Thermal expansion describes how much the volume of the fluid changes with
temperature. Thermal expansion of the fluid must be considered since the increase in
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