Fluke 7008, Руководство пользователя, Страница: 36 / 78

7008
Users Guide
8-2
8.1.3 Specific Heat
Specific heat is the measure of the heat storage ability of the fluid. Specific heat,
though to a lesser degree, also affects the control stability. It also affects the
heating and cooling rates. Generally, a lower specific heat causes slightly better
control stability and quicker heating and cooling. With fluids with higher specific
heat the controller may require a decreased proportional band to compensate for
the decrease in sensitivity of the bath temperature to heat input.
8.1.4 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
more quickly and evenly improving bath performance.
8.1.5 Thermal Expansion
Thermal expansion describes how the volume of the fluid changes with
temperature. 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. Oils typically have significant thermal
expansion.
8.1.6 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 occur
through the fluid. In this case consider a fluid with very high electrical resistivity.
8.1.7 Fluid Lifetime
Many fluids degrade over time because of vaporization, water absorption, gelling,
or chemical breakdown. Often the degradation becomes significant near the
upper temperature limit of the fluid.
8.1.8 Safety
When choosing a fluid always consider the safety issues associated. Obviously
where there are conditions of extreme temperatures there can be danger to
people and equipment. Fluids may also be hazardous for other reasons. Some
fluids may be considered toxic. Contact with eyes, skin, or inhalation of vapors
may cause injury. A proper fume hood must be used if hazardous or bother some
vapors are produced.