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Micro-Bath
General Operation
Viscosity
Viscosity is the thickness measurement of a fluid and how easily it can be poured and mixed. Viscosity
affects the temperature stability of the bath. Fluid mixing is better with low viscosity and this creates a
more uniform temperature throughout the bath. If the bath temperature is more uniform, the bath
response time is better. Viscosity should be <10 centistokes for good control. The upper limit of
allowable viscosity is about 20 centistokes. Viscosities greater than this cause poor control stability.
With oils, viscosity varies greatly with temperature.
With higher-viscosity fluids, the controller proportional band could need to be increased to compensate
for the reduced response time. Otherwise, the temperature could 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 affects the heating and cooling rates. Generally, a lower specific heat
means quicker heat and cool down times. Adjust the proportional band, depending on the specific heat
of the fluid, when necessary.
Thermal Conductivity
Thermal conductivity measures how easily heat flows through the fluid. Thermal conductivity of the
fluid affects the control stability, temperature uniformity, and probe temperature settling time. Fluids
with higher conductivity distribute heat faster and more evenly which improves bath performance.
Thermal Expansion
Thermal expansion is how the volume of the fluid changes with temperature. Thermal expansion of all
fluids used must be considered since the increase 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, oils included, have significant thermal expansion.
Electrical Resistivity
Electrical resistivity is how well the fluid insulates against the flow of electric current. In some
applications, measuring the resistance of bare temperature sensors, for example, it is important that
little or no electrical leakage occurs through the fluid. In such conditions, choose a fluid with a high
resistivity.
Fluid Lifetime
Many fluids degrade over time because of evaporation, water absorption, gelling, or chemical
breakdown. Often the degradation becomes significant near the upper temperature limit of the fluid and
this substantially reduces the lifetime of the fluid.
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