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a viscosity in the molten state similar to the viscosity of
water. For your convenience, it is shipped in a granular form,
making it easy to fill your bath.
Between its melting point of 150°C and 550°C, this salt has
the highest temperature stability and uniformity available in
a bath fluid. It does not smoke like oils or give off dust
like “sand” or fluidized alumina baths. Check with your bath
manufacturer before using this salt to make sure your
equipment is compatible. Hart offers three standard bath
models and custom-designed units for use with this salt.
Fluid Information
It’s important to understand a few specifications before
selecting a bath fluid. We’ve seen bath fluids advertised
with a temperature range that spans from the freezing point to
the flash point or beyond.
For example, type 710 silicone oil has a freezing point of
–22°C, but freezing point has nothing to do with the point at
which the oil becomes so thick it cannot be properly stirred.
Type 710 oil should really only be used down to about 80°C.
It’s a viscosity issue, not a freezing-point issue. Usable
range is the question. Suitability for calibration work is the
specification that counts.
The usable viscosity range is determined by your bath’s
stirring or pumping design. Hart baths can be operated using
fluids with up to 50 centistokes viscosity. This gives you
additional usable range in the lower temperature levels of the
fluid.
Some baths advertised as calibration baths require fluids with
10 centistokes or less viscosity to operate properly. The
usable ranges in our table on the previous page assume the use
of a Hart bath.
In addition to range and viscosity issues, there are a number
of other issues to consider when choosing a bath fluid. The
other considerations are:
- Thermal characteristics.
- Lifetime.
- Change in characteristics due to temperature cycling.
- Absorption of water from the air.
- Vaporization—fumes and fume hood requirements.
- Expansion due to heat.
- Contamination—mixing oils or introducing contamination
with unclean probes.
- Conductivity properties.
- Effects of using fluids outside of their range—fire,
explosion, polymerization.
- Effects of altitude on boiling point.
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