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:
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
Effects of using fluids outside of their range—fire, explosion, polymerization
Effects of altitude on boiling point