What are the differences between dry well calibrators and calibration baths, and what are their advantages and disadvantages?
Dry well calibrators and calibration baths are both temperature calibrators. As their names imply, one uses a dry medium (air) for calibration, and the other uses a liquid medium. Dry well calibrators, also called dry block calibrators, have an insert with individual bores in which temperature sensors are placed. Calibration baths have an insert with a leakproof cover and a basket for the test sensor. Both types use a thermally insulated metal block, which is heated. For instruments that work with Peltier elements, the block can also be cooled. The reference, which the calibrator controls, is mounted directly into the metal block. The working range of a calibration bath is limited due to the liquid – usually silicone oil, although distilled water can also be used. The liquid must be sufficiently viscous and stable at certain temperatures, thus limiting the upper temperature range of silicone oils to approximately 480°F (250°C), to remain below their flash point. Dry block calibrators do not have such limitations. Liquids do have the advantage of better homogeneous mixing using a magnetic stirrer at the bottom of the bath. This means axial and radial gradients are not an issue. The working range of commercially available temperature calibrators, using Pt100 or Pt1000 resistance thermometers (RTDs) as a standard, extends from approximately −50°F to 1,200°F (−45°C to 650°C). Calibrators that work with Peltier elements are typically used from about −30°F to 330°F (−35°C to 165°C). There are also high-temperature dry well calibrators, which can be used up to about 2,370°F (1,300°C) and work with precious-metal thermocouples as standard and control thermometers. In these cases, the measurement uncertainties are higher than in calibrators that use an RTD as a standard.
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