Calibration Bath Fluids

Viscosity, volatility and other properties that change with temperature affect the performance of fluids in controlled baths and circulators. Fluke Calibration has tested and used each of the fluids we sell. Over the ranges recommended in the following table, each fluid remains at a low enough viscosity to be adequately pumped or stirred. Whether your application is industrial or critical lab calibration work, Hart fluids give you top performance and stability.

For temperature ranges too high for oils, we have a bath salt with 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. Fluke Calibration offers three standard bath models and custom-designed units for use with this salt.

Fluid specifications 

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. Fluke Calibration 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 Fluke 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