Cost; Commonly Used Fluids; Water; Ethylene Glycol - Fluke 7341 User Manual

7341 Calibration Bath
User's Guide
8.1.9
8.1.10
8.1.10.1
8.1.10.2
8.1.10.3
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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.

Commonly Used Fluids

Below is a description of some of the more commonly used fluids and their
characteristics.

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.

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.

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. Be careful not to fill the bath too full to
avoid overflows when heated (see Section 7.4.1, Fluid Expansion Reservoir).
The viscosity and thermal characteristics of mineral oil is poorer than water so
temperature stability will not be as good. Mineral oil has very low electrical
conductivity. Use caution with mineral oil since it is flammable and may also
cause serious injury if inhaled or ingested.