Temperature control and process instrumentation are widely used in many food and manufacturing industries and laboratory environments. Accurate temperature measurements are essential for providing process optimization, preserving product quality, and ensuring health and safety. The temperature devices and sensors commonly used include thermocouples, RTD’s, thermistors, PRTs, and bi-metal or liquid-in-glass thermometers.
To achieve the best performance, highly accurate and stable temperature sources are required for checking and calibrating these devices by generating a wide range of reference temperatures that the devices can be compared with.
Two of the most common types of temperature sources are Dry Block Calibrators and Liquid Temperature Baths. Although they perform similar tasks, each has advantages over the other and one may be more appropriate for specific applications.
Dry Block Calibrators
Dry Block Calibrators are versatile temperature sources that control the heating and cooling of a metal block to the desired temperatures. Dry Block Calibrators utilize a range of interchangeable inserts that are available with hole diameters to fit the most common sizes of probes and devices. Dry Block Calibrators come in various models with ranges from about –10 to 1100°F.
Liquid Temperature Baths
Liquid Temperature Baths are temperature sources in which a liquid, typically water or an oil, is heated or cooled to a specific temperature. The liquid is usually stirred to enable a uniform temperature differential between the various depths of the liquid.
Liquid Temperature Baths that use water are restricted to a temperature range between 32°F and 212°F. Liquid Baths using various viscosities of oils can achieve temperatures from about –20°F to 570°F. Certain baths containing specialized materials such as sand or salts can reach temperatures up to 1300°F.
Choosing the Best Temperature Bath
Choosing the best Temperature Bath for your needs involves taking a look at several of the factors below and determining the impact each one will have in your environment.
Probe Size and Shape
Liquids Baths are the best choice for larger or irregular shaped probes or devices. They can test virtually any shape and size of device instead of being restricted to the standard sizes of the metal inserts in Dry Block Calibrators. They are also capable of testing a number of devices at a time depending on the opening of the well.
Dry Block Calibrators are best suited for standardized probe sizes. Selecting the proper insert to match the temperature sensor is important. A close fit between the insert and the temperature sensor is required to obtain the proper heat transfer for accurate measurements. Non-standard sized inserts can be ordered however, a separate insert may be needed for each specific size of probe required, which could become expensive.
Because the liquid is usually being stirred, Liquid Baths do not suffer from temperature differentials sometimes experienced in Dry Block Calibrators and they also offer excellent stability over the entire temperature range. Accuracy with Liquid Baths can be as high as ≤0.4°F, better than that which can be achieved by most Dry Block Calibrators.
Dry Block Calibrators are generally more portable than Liquid Baths and are useful when performing field testing or on-site calibrations. The liquid in Liquid Baths may need to be removed if they are shipped or transported over long distances.
Initially, because Dry Block Calibrator inserts have a small surface area, they are capable of heating up to the temperature setpoint faster than Liquid Baths. The Liquid Baths have a larger total surface area so it takes longer to heat up. In many cases, laboratories will keep their Liquid Baths running continually to avoid the lengthy startup time, especially if the testing is always completed at one temperature.
Once heated, Liquid Baths provide a better contact surface area for heat transfer than Dry Block Calibrators. Liquid Baths also heat more evenly and there are very little temperature fluctuations during the cycling of the bath controller.
Liquid Baths are prone to contamination so they should be avoided if the environment needs to be free from contaminants, such as in clean rooms or some medical laboratories. The removable inserts in Dry Blocks can easily be cleaned or sterilized, which makes them a better choice for clean environments.