Choosing a liquid calibration bath and its fluid
The fluid sets the range, the stirring sets uniformity, and depth sets immersion. Ethanol, water, oil and salt each own a range; safety and cleanliness decide among the overlaps.
- Ethanol (−80 to +10 °C): low viscosity cold, flammable, hygroscopic; absorbs water over time which raises the freezing point.
- Water (5 to 95 °C): cheap, non-toxic, excellent uniformity; evaporation and biological growth are the maintenance issues.
- Silicone oil (−40 to +250 °C depending on grade): low vapour pressure; viscosity rises sharply at the cold end, degrading uniformity.
- Molten salt (200 to 550 °C): high uniformity, hazardous, corrosive to some sheaths.
- Fluidised alumina baths: 50 to 700 °C, dry, poorer uniformity than liquids.
Whatever the fluid, characterise uniformity and stability with the fluid, stirrer speed and depth you will use, and record it. A bath's brochure numbers apply to the manufacturer's test conditions.
Frequently asked questions
- Can one bath cover −30 to +150 °C?
- Not with one fluid. Two baths (alcohol and oil or water) are usual; changing fluid in a single bath is possible but laborious and risks contamination.
References
- [1]ISO/IEC 17025:2017 — General requirements for the competence of testing and calibration laboratories
General technical guidance written against the cited sources. It is not regulatory or legal advice and does not replace the applicable standard, guideline or a qualified reviewer's judgement.
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