Calibration baths

What is a calibration bath?

To ensure that temperature sensors provide consistently accurate readings, they must be calibrated at regular intervals. For temperature calibration, calibration baths are used which automatically measure temperature points in the temperature range of the probe with the help of a liquid. For measurement, the probes are immersed in the bath, which is filled with water or certain types of oil, depending on the temperature to be measured. The water or oil in the calibration bath circulates to maintain the desired temperature evenly throughout. Following the measurement, the results are then compared with a reference instrument. Calibration baths are used for a higher degree of temperature uniformity.

Calibration baths come in various forms, either as portable equipment or as large, permanently installed laboratory equipment. For on-site applications, WIKA offers the CTB9100 series of micro calibration baths. The lowest calibration uncertainties are achieved by a stable and uniform temperature environment, generated by the calibration bath. Another advantage is that, regardless of the size and shape of the probe, any probe can be calibrated as long as it can be fully immersed. Depending on the liquid and also the temperature controllers used for heating and cooling, operators often use several baths with different temperature ranges, in order to calibrate the sensors at different temperatures.

Calibration bath or dry-well calibrator?

For temperature calibration, depending on the environment, application and industry, calibration baths or dry-well calibrators can be used. Both instruments can reach different levels of uncertainty and act as a stable temperature source. The decision which of the two to use depends, among other things, on the following criteria:

• temperature range,
• portability,
• handling of liquid media
• and temperature stability.

When are calibration baths used?

Calibration baths are used for calibration in laboratories and measurement and control laboratories. They are also used in the pharmaceutical and food industries for the calibration of short temperature probes. The calibration bath can also be used to calibrate several probes. Since the test item is immersed in water or oil for calibration, the calibration bath cannot be used in sterile environments. Sterile industries usually rely on dry-well calibrators to calibrate their temperature probes.

What should I look out for in the heat transfer fluid used?

When selecting the temperature control fluid, attention must be paid to high thermal conductivity and low viscosity with regard to homogeneous temperature distribution. In addition, the liquid should be inert, have a low vapour pressure, should not decompose chemically, nor burn, and should maintain its properties over a wide temperature range. In practice, silicone oils are usually used. The temperature range lies between -45 °C and +300 °C, depending upon the design. The calibration baths are fitted with a temperature-controlled liquid tank with a usable depth of 200 mm. The maximum immersion depth for the test items of 200 mm reduces heat conduction errors and achieves smaller measurement uncertainties.