Viscosity; Specific Heat; Thermal Conductivity; Thermal Expansion - Fluke 7103 User Manual

7103 Micro-Bath User's Guide
Bath Fluid
8.3.2

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 ten centistokes. Twenty
centistokes is about the upper limit of allow able 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.
8.3.3 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.
8.3.4

Thermal Conductivity

Thermal conductivity measures how easily heat flows through the fluid. Ther mal
conductivity of the fluid affects the control stability, temperature unifor mity, and probe
temperature settling time. Fluids with higher conductivity distribute heat more quickly
and evenly improving bath performance.
8.3.5

Thermal Expansion

Thermal expansion describes how the volume of the fluid changes with temper ature.
Thermal expansion of the fluid used must be considered since the in crease in fluid
volume as the bath temperature changes may cause overflow. Excessive thermal
expansion may also be undesirable in applications where constant liquid level is
important. Many fluids including oils have significant thermal expansion.
8.3.6

Electrical Resistivity

Electrical resistivity describes how well the fluid insulates against the flow of electric
current. In some applications, such as measuring the resistance of bare temperature
sensors, it may be important that little or no electrical leakage oc cur through the fluid.
In such conditions choose a fluid with very high resistivity.
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