User manual

General Operation
8.3 Bath Fluid 8
8-3
8.3.5 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
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 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 occur
through the fluid. In such conditions choose a fluid with very high resistivity.
8.3.7 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, substantially reducing the fluid’s lifetime.
8.3.8 Safety
When choosing a fluid always consider the safety issues associated. Obviously
where there are conditions of extreme hot or cold 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 bothersome
vapors are produced.
 Warning:
Fluids at high temperatures may pose danger from BURNS,
FIRE, and TOXIC FUMES. Use appropriate caution and safety
equipment.
Fluids may be flammable and require special fire safety equipment and
procedures. An important characteristic of the fluid to consider is the flash point.
The flash point is the temperature at which there is sufficient vapor given off so
that when there is sufficient oxygen present and an ignition source is applied the
vapor will ignite. This does not necessarily mean that fire will be sustained at the
flash point. The flash point may be either of the open cup or closed cup type.
Either condition may occur in a bath situation. The closed cup temperature is
always the lower of the two. The closed cup represents the contained vapors
inside the tank and the open cup represents the vapors escaping the tank.
Oxygen and an ignition source will be less available inside the tank.
Environmentally hazardous fluids require special disposal according to applicable
federal or local laws after use.
8.3.9 Cost
Cost of bath fluids may vary greatly, from cents per gallon for water to hundreds
of dollars per gallon for synthetic oils. Cost may be an important consideration
when choosing a fluid.