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7102
User's Guide
8-4
8.3.10 Commonly Used Fluids
Below is a description of some of the more commonly used fluids and their
characteristics.
8.3.10.1 Water (Distilled)
Water is often used because of its very low cost, availability, and excellent
temperature 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 prevent 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 algaecide
chemical in the water to prevent contamination.
8.3.10.2 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 will expand 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 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.
8.3.10.3 Silicone Oil (Dow Corning 200.05, 200.10, 200.20)
Silicone oils are available which offer a much wider operating temperature range
than mineral oil. Like most oils, silicone oils have temperature control
characteristics which are somewhat poorer than water. The viscosity changes
significantly with temperature and thermal expansion also occurs. These oils
have very high electrical resistivity. Silicone oils are fairly safe and non-toxic.
Silicone oils are fairly expensive.
8.3.11 Fluid Characteristics Charts
Table 3 on page 25 and Figure 6 on page 26 have been created to provide help
in selecting a heat exchange fluid media for your constant temperature bath.
These charts provide both a visual and numerical representation of most of the
physical qualities important in making a selection. The list is not all inclusive.
There may be other useful fluids not shown in this listing.
The charts include information on a variety of fluids which often used as heat
transfer fluid in baths. Because of the temperature range some fluids may not be
useful with your bath.
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