Selecting the right calibration bath takes good research and analysis since many bath 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 bath 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.
Bath suppliers normally publish and advertise a temperature range for each model. However, often no single bath fluid works well over the entire temperature range. For example, a Fluke Calibration 7341 Deep-Well Compact Bath has a range from -45 °C to 150 °C. Ethanol is a good fluid for this bath below 0 °C, but at temperatures above 0 °C another fluid such as silicone oil would be required. As a result, calibration labs must choose between changing bath fluids or using multiple 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 bath 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. Bath fluids which are too viscous create strain on stirring and pumping mechanisms and don’t adequately transfer heat uniformly from temperature sources to thermometers. We recommend using fluids whose viscosity is 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 fluids reduce bath temperature gradients and contribute to better calibration uncertainties.
Stability is the ability of a bath to maintain a constant temperature over time. A bath’s stability will vary at different temperatures. Many suppliers give you only one spec at or near ambient. Some give a single stability spec and don’t ever mention that it applies only to one temperature or a narrow range. Ask about stability over the whole range that interests you.
Bath 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 larger instability. If your oil has high viscosity at 37 °C, expect even greater degradation in stability.
A bath can have good stability but poor 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 depends mostly on the mixing of the bath fluid. Does the bath use a circulator pump for mixing? If it does, are there thermal flow patterns in the bath that interfere with uniformity? Be sure to check both vertical and horizontal temperature gradients.
The number of temperature probes and sensors to be calibrated should be considered. A bath with a larger 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 size would be a better fit for laboratories with a lower calibration volume.
When longer SPRTs, PRTs, and liquid-in-glass thermometers need to be calibrated, a bath with plenty of immersion depth should be evaluated. 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 and between the sensors being tested and the tank wall.