Dave buys a new calibration bath because of a requirement to calibrate to –80°C. His boss notices the temperature range on the bath is from –80 °C to 110 °C. Sensing a new source of revenue, the boss says, “Great, now we can finally calibrate thermometers to 110 °C too.”

“No”, Dave says, “we can’t set the temperature higher than 70°C”

“Why, what’s wrong with the bath?”

“Nothing, it’s working perfectly.”

Confused and agitated, the boss leaves while mumbling something about temperature calibration and the black arts, while a now very amused Dave can get back to work in his lab.

How can a bath that is working perfectly not be capable of reaching a temperature inside its own operating range? The answer lies in the properties of the fluid media chosen for the calibration bath. As usual, these properties depend on temperature.

High temperature limitations of bath fluids

Bath fluid properties that could limit the upper temperature range of a calibration bath are:

• flash point
• evaporation rate
• boiling point

Because of safety concerns, the flash point is one property that puts an upper limit on temperature range. This old War Department video explains what the flashpoint is and its relationship to temperature.

The vapor pressure of a fluid is an indication of a liquid’s evaporation rate. Fluids with high vapor pressure evaporate quickly. For example ethyl alcohol evaporates more quickly than water at the same temperature. Ethyl alcohol has a vapor pressure of 0.08 atm at 25°C while water has a vapor pressure of 0.03 atm at the same temperature. Rapid evaporation is expensive because the fluid has to be replaced. Rapid evaporation also causes heat losses that impact the performance of the temperature controlled bath.

Vapor pressure increases with temperature, and when the vapor pressure is equal to the surrounding environmental pressure (usually atmospheric) the fluid boils. Higher altitudes have lower atmospheric pressures and so boiling will occur at lower temperatures at higher altitudes. When the fluid is boiling the temperature of the fluid is unable to rise any higher even until all of the fluid has been turned into vapor.

Low temperature limitations of fluids

Fluid properties that could limit the lower temperature range are:

• fluid viscosity
• freezing point

Fluid viscosity is a measure of the fluids resistance to flow. We usually think of it as the thickness of the fluid. This short video illustrates that concept.

When kinematic viscosity is higher than 10 centistokes it will create a strain on the stirring and pumping mechanisms. Also, the fluid will not transfer heat uniformly as required during a calibration.

Usually the viscosity of a fluid will become a problem long before the fluid freezes (i.e. Ethlene Glycol) or reaches the pour point (i.e. silicone oil). But that is not always the case. For example, water will freeze at 0 °C but the viscosity of the water will not be a problem until slush starts to form.

A single fluid may not cover your bath’s entire range

At the beginning of this article, the boss wanted to use a calibration bath over its entire operating range of –80°C to 110°C. It seems like a reasonable request that it should, and a bit of a paradox that it would not.

A clearer picture forms when we realize that we have to consider the useable temperature range of the fluid media in the calibration bath too. It’s a bit disappointing to find out that physics has spoiled our fun. Flashpoints, viscosity and evaporation may frustrate our plans to cover the entire operating range of the bath with a single fluid. Although a fluid with the desired properties may not yet exist, at least we can say we have a better explanation for the behavior of calibration baths than "black magic."

For more detailed information on how to choose a bath fluid for your application, see the application note: Choosing a bath fluid.