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Choosing a Constant Temperature Bath Liquid



Choosing a Constant Temperature Bath Liquid

In order to accurately measure kinematic viscosity in glass
capillary viscometers, the viscometers must be suspended in a
constant temperature bath. The ideal bath fluid would possess
low viscosity, high heat capacity, and low vapor pressure over
a wide range of temperatures. In addition, the liquid should
have a very high flash point and be relatively low in cost.
Because the viscometer must be viewed through the bath
liquid, it is important for the liquid to clear and without color.
Unfortunately, no single fluid meets all these requirements.
When selecting a fluid, keep the following guidelines in mind.

Viscosity: Should be very low so that moderate stirring can
effectively eliminate temperature gradients in the bath.

Heat Capacity: Temperature changes in the bath are less rapid when bath liquids with a
high heat capacity (thermal conductivity) are used. With the exception of water, most
choices for bath fluids will have about the same heat capacity.

Volatility: A liquid which is relatively volatile will require frequent replenishment.
Furthermore, rapid evaporation at the bath surface produces a cooling effect, making
control more difficult.

Because no single fluid can be used at all possible bath temperatures, the choice of a
suitable fluid must begin by establishing the temperature range over which the bath will
be operated. The following is a list of operating temperature ranges and some liquids
suitable for use in these ranges.

TEMPERATURE RANGE RECOMMENDED BATH FLUID
-100 to 10C (-148 to 50F) Methyl Alcohol
-100 to 10C (-148 to 50F) Ethyl Alcohol
-10 to 20C (14 to 68F) Isopropyl Alcohol
Low Viscosity Silicones
5 to 60C (41 to 140F) Water, Low Viscosity Oils
Silicones (Dow CorningAE200 fluid, 1 cSt)

60 to 100C (140 to 212F) White Oil with oxidation inhibitor (IBF Bath Oil)
Silicones (Dow CorningAE200 fluid, 10 cSt)

80 to 135C (212 to 275C) White Oil with oxidation inhibitor (IBF Bath Oil)
Silicones (Dow CorningAE200 fluid, 20 cSt)
100 to 150C (212 to 302C) Silicones (Dow CorningAE200 fluid, 50 cSt)
135 to 200C (135 to 392C) Silicones (Dow CorningAE200 fluid, 200 cSt)
200 to 300C (392 to 572F) Silicones (Dow CorningAE550 fluid)

Methyl alcohol is quite volatile, has a low flash point, and has a degree of toxicity. Ethyl
alcohol is also volatile and has a low flash point, but unless ingested is relatively non-


toxic. Some halogen-based fluids are marketed as low temperature bath fluids, but these
also have a high volatility, may be slightly toxic, and are quite expensive.

Isopropyl alcohol is less toxic than methyl alcohol and somewhat less volatile. However,
it becomes highly viscous at low temperatures and is therefore unsuitable for use at very
low temperatures.

Silicone fluids are available in a number of viscosities and can be used over a wide range
of temperatures if the proper selection of viscosity is made for the temperature range of
interest. A bath containing silicones requires extra care when used for capillary
viscometry. If silicones enter inside a viscometer, its calibration factor may be altered by
a significant amount. Silicones are also relatively expensive liquids.

Water is almost the ideal fluid in the temperature range for which it is suitable. Because
in some cases there is a tendency for algae formation, some degree of water treatment
may be necessary. Water can be used at temperatures close to the boiling point, but water
replenishment to offset evaporation becomes a nuisance and the hot vapor can make
working above the bath uncomfortable. Also, it may be difficult to establish optimum
control at elevated temperatures because of the rapid cooling resulting from surface
evaporation.

Refined white oils (paraffin oils) of relatively low viscosity can be used at temperatures
above the level at which water becomes unsatisfactory. Because these oils will turn
faintly yellow and continue to darken with prolonged exposure to heat, we recommend
using an oil like CANNONís IBF Bath Oil which contains an oxidation inhibitor to retard
discoloration. Even with the addition of an inhibitor, after several months the oil will
darken and must be replaced with new oil. Nevertheless, over the long term IBF Bath Oil
will prove to be less expensive than silicones. It is also easily cleaned from viscometers
without affecting the calibration factor.

The search for more suitable bath fluids is unending. Hydrogenated vegetable oils,
coconut oil, synthetic oils, and certain chemical compounds have been used with some
success at various temperatures. However, at CANNON, long experience has shown that
the fluids listed in the above table will provide the best results.
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