sured with the vapors being contained within the tank. Since oxygen and an ig-
nition source are less available inside the tank, the closed cup flash point is
lower than the open cup flash point.
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, its availability, and its excel-
lent temperature control characteristics. Water has very low viscosity and good
thermal conductivity and heat capacity which makes 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.
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 but with higher viscosity. Use caution
with ethylene glycol since this fluid is very toxic. Ethylene glycol must be dis-
posed 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 a fume
hood or adequate ventilation system should be used. As with most oils, mineral
oil expands as temperature increases. See Section 7.3.1, Fluid Expansion Res-
ervoir, for information on the fluid expansion reservoir use. Since the viscosity
and thermal characteristics of mineral oil is poorer than water, temperature sta-
bility is not as good. Mineral oil has very low electrical conductivity. Use cau-
6331 Calibration Bath
User’s Guide
30
