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7 General Operation
por 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 open cup flash point is mea-
sured under the condition of vapors escaping the tank. The closed cup flash
point is measured with the vapors being contained within the tank. Since oxy-
gen and an ignition source is less available inside the tank the closed cup flash
point will be lower than the open cup flash point.
Environmentally hazardous fluids require special disposal according to applica-
ble federal or local laws after use.
7.1.9 Cost
Cost of bath fluids may vary greatly, from cents per gallon for water to hun-
dreds of dollars per gallon for synthetic oils. Cost may be an important consid-
eration when choosing a fluid.
7.1.10 Commonly Used Fluids
Below is a description of some of the more commonly used fluids and their
characteristics.
7.1.10.1 Water
Water is often used because of its very low cost, its availability, and its excel-
lent temperature control characteristics. Water has very low viscosity and good
thermal conductivity and heat capacity, which make it among the best fluids for
good control stability at lower temperatures. Temperature stability is much
poorer at higher temperatures because water condenses on the lid, cools and
drips into the bath. Water is safe and relatively inert. The electrical conductivity
of water may prevent its use in some applications. Water has a limited tempera-
ture range, from a few degrees above 0°C to a few degrees below 100°C. At
higher temperatures evaporation becomes significant. Water used in the bath
should be distilled or deionized to prevent mineral deposits. Consider using an
algicide chemical in the water to prevent contamination.
Note:
Water used at temperatures greater than 75°C (167°F) may have stability
problems.
7.1.10.2 Ethylene Glycol
The temperature range of water may be extended by using a solution of one
part water and one part ethylene glycol (antifreeze). The characteristics of the
ethylene glycol-water solution are similar to water but with higher viscosity.
Use caution with ethylene glycol since this fluid is very toxic. Ethylene glycol
must be disposed of properly.