Temperature calibration equipment: 4 key bath specifications

Liquid temperature calibration baths are the most stable and uniform for performing comparison calibration of temperature probes and sensors in a laboratory. They use a large fluid mass (heat transfer fluid) to maintain a stable and uniform test environment that provides the flexibility necessary to calibrate probes and sensors of various sizes, shapes, and lengths.

Technician calibrating a temperature probe in a calibration laboratory

Selecting the right temperature calibration bath takes good research and analysis since many temperature calibration equipment suppliers provide incomplete or confusing specifications. Some examples include:

• Performance may only be given for a single temperature point, not for the bath’s range
• Specifications don’t indicate which heat transfer fluid was used in testing
• There’s no indication whether the specifications apply for the entire working volume

Since a calibration bath is a significant investment that will last many years, you’ll want to get the right information to make an informed decision. You must get comprehensive and clear specifications so that you can have the confidence your calibration bath will provide the performance you need for your application.

You will want to evaluate four key specifications when buying a calibration bath: temperature range, stability, uniformity, and tank size.

1. Temperature range

Temperature calibration equipment suppliers normally publish and advertise a temperature range for each calibration bath model. However, often no single heat transfer fluid works well across the entire temperature range. For example, take a temperature calibration bath that has a range of -45 °C to 150 °C. Ethanol is a good choice for this bath below 0 °C, but at temperatures above 0 °C, another heat transfer fluid such as silicone oil would be required. As a result, calibration labs must choose between changing heat transfer fluids or using multiple calibration baths to cover the full temperature range of their application.

At temperatures below 0 °C, Halocarbon, HFE, methanol, ethanol, ethylene glycol, Dynalene, and some silicone oil types are all candidate heat transfer fluids. At temperatures above 0 °C, there are several silicone oil types available, plus water and mineral oil that could be used. For extremely hot temperatures above 300 °C, salt is a preferred option.

Viscosity is a measure of a fluid’s resistance to flow — we often think of it simply as “thickness.” It is commonly measured in “centistokes” (cSt). The higher the number of centistokes, the more viscous (or thick) a fluid is. Heat transfer fluids that are too viscous strain stirring and pumping mechanisms and don’t adequately transfer heat uniformly from temperature sources to thermometers. We recommend using fluids with a viscosity of 50 centistokes or less at the desired control temperature. A homogeneous temperature within the “calibration zone” of a bath is required to achieve a calibration with low uncertainty. Low viscosity heat transfer fluids reduce bath temperature gradients and contribute to better calibration uncertainties.

Please refer to the How to Select a Calibration Bath Fluid guide for more information on heat transfer fluid selection.

2. Stability

Stability is the ability of a calibration bath to maintain a constant temperature over time. A bath’s stability will vary at different temperatures. Many temperature calibration equipment suppliers only provide one spec at or near ambient. Some give a single stability spec and never mention that it only applies to one temperature or a narrow range with a specific heat transfer fluid. Ask about stability over the whole range that interests you.

Heat transfer fluid also affects stability. The higher a fluid’s viscosity and the lower its heat capacity, the larger the effect on stability will be. In addition to asking about the temperature range, ask what fluid was used when the specification was defined. For example, at 37 °C, a bath will be more stable with water as the medium. If you’re going to use oil, expect somewhat greater instability. If your oil is highly viscous at 37 °C, expect even greater degradation in stability. Oils can also heat from friction of stirring.

3. Uniformity

A temperature calibration bath can be highly stable but lack uniformity. The bath must be homogenous in temperature throughout the test zone where you’ll make your comparison measurements. When you place two or more probes in the fluid, they should be at the same temperature during your measurement. The uniformity spec defines the peak value for this error source. The more probes you’re testing, the larger the test zone and the more important uniformity becomes.

Uniformity mostly depends on mixing. Does the calibration bath use a circulator pump to mix the heat transfer fluid? If so, are there thermal flow patterns in the bath that interfere with uniformity? Be sure to check both vertical and horizontal temperature gradients.

A calibration bath may not have a horizontal temperature gradient but still have a vertical gradient between different depths. This is a problem if your reference probe and the probes under test are different lengths. For example, you may be testing 3-inch long probes and your reference is a 19-inch SPRT. You can only immerse the test probes to 3 inches, but if you immerse the SPRT to only 3 inches you lack the necessary depth to avoid stem effects that will cause measurement errors. If you properly immerse the SPRT and your bath suffers from vertical gradients, you won’t be measuring the temperature at the 3-inch depth of your probes under test.

4. Tank size

You should consider how many temperature probes and sensors you plan to calibrate. A bath with a tank size that allows large batches to be calibrated may be appropriate for laboratories that calibrate many probes and sensors each year. On the other hand, a bath with a smaller tank would be a better fit for laboratories with a lower temperature calibration volume.

When longer SPRTs, PRTs, and liquid-in-glass thermometers require calibration, a bath with a generous immersion depth should be a consideration. Tank opening size is an important factor when odd-shaped sensors (e.g. tri-clamp sanitary sensors) and sensors with large transmitter heads need calibration. When analyzing the tank size required for your application, be sure to allow for proper sensor immersion depth and fluid space below the sensors, as well as between the sensors being tested and the tank wall.

Deviations from a central reference temperature taken in water with a 1/4-inch diameter PRT at 25 °C

Selecting a temperature calibration bath: Questions to ask

• What is the temperature range of the bath?
• What are the recommended heat transfer fluids and their operating ranges?
• What’s the bath stability and uniformity at temperature points in the range that interests you?
• What heat transfer fluids were used to measure stability and uniformity specs?
• Is the bath tank size adequate for the number and size of probes and sensors to be calibrated in a batch?

Looking for more specifics on temperature calibration baths before you select one? See our guide, “How to a choose a calibration bath”, for more detailed information to ensure you make the right choice.