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 in-
side 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 applica-
ble federal or local laws after use.
8.1.9
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.
8.1.10
Commonly Used Fluids
Below is a description of some of the more commonly used fluids and their
characteristics.
8.1.10.1
Water
Water is often used because of its very low cost, availability, and excellent tem-
perature control characteristics. Water has very low viscosity and good thermal
conductivity and heat capacity which makes it among the best fluids for control
stability at low 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 pre-
vent its use in some applications. Water has a limited temperature 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
softened to prevent mineral deposits. Consider using an algicide chemical in the
water to prevent contamination.
8.1.10.2
Ethylene Glycol
The temperature range of water may be extended by using a solution of 1 part
water and 1 part ethylene glycol (antifreeze). The characteristics of the ethylene
glycol-water solution are similar to water. Use caution with ethylene glycol
since the fluid is very toxic. Ethylene glycol must be disposed of properly.
8.1.10.3
Mineral Oil
Mineral oil or paraffin oil is often used at moderate temperatures above the
range of water. Mineral oil is relatively inexpensive. At lower temperatures
mineral oil is quite viscous and control may be poor. At higher temperatures
vapor emission becomes significant. The vapors may be dangerous and use of a
fume hood is highly recommended. As with most oils, mineral oil expands as
temperature increases so be careful not to fill the bath too full that it overflows
when heated. The viscosity and thermal characteristics of mineral oil is poorer
31
8 General Operation
Bath Fluid