Cannon Instrument CT-600 (Contant Temperature Bath) - Manual (Page 29)

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CANNON CANNON CANNON CANNON CANNON ® CT-500/600 Series II CONSTANT TEMPERATURE BATH
Version 3..3—March, 2012; CANNON CANNON CANNON CANNON CANNON ® Instrument Company
2139 High Tech Road • State College, PA • 16803 • USA
CHAPTER
D
APPENDIX D
—
CHOOSING A
BATH LIQUID
The ideal bath liquid 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. If the fluid is to be used in a kinematic viscosity bath where it is
necessary to view the instruments through the bath liquid, then it is
important for the liquid to be clear and without color. Unfortunately, no
single fluid meets all these requirements. When selecting a fluid, keep
the following guidelines in mind.
Viscosity:
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 with a high heat capacity.
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 more frequent replenish-
ment. 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
temperature ranges and bath liquids suitable for use in these ranges:
Temperature Range (°C) Suitable Bath Liquids
+5 to +60 Water, Low Viscosity Oils
Silicones (Dow 200 fluid, 1 cSt)
+60to +135 White Oils with oxidation inhibitor
Silicones (Dow 200 fluid, 20 cSt)
CAUTION NEVER USE FLAMMABLE BATH LIQUIDS.
Silicone fluids
Silicone fluids are available in a wide range of viscosities and can be
used over a wide range of temperatures if the proper selection of viscos-
ity is made for the temperature range of interest. However, silicones are
relatively expensive, and a bath containing silicones requires extra care
when used for capillary viscometry. If silicones are inadvertently intro-
duced into a viscometer capillary, its calibration factor will be altered by
a significant amount.