Dry well calibrators/micro calibration baths consist of a thermally insulated metal block which is heated, and for instruments which work with Peltier elements, which can also be cooled. The reference, which the calibrator controls, is mounted directly into the metal block. The working range of commercially available temperature calibrators, using Pt resistance thermometers as a standard, extends from approximately -45 °C to 650 °C. Calibrators that work with Peltier elements are typically used from -35 °C to 165 °C, and those that are fitted with resistance heating from 35 °C to 700 °C. In addition, there are high-temperature dry well calibrators which can be used, depending on the model, up to 1300 °C. They work with precious-metal thermocouples as standards and control thermometers. In these cases, the measurement uncertainties are higher than in calibrators which use a resistance thermometer as a standard. The working range of micro calibration baths is (unlike the dry block calibrators) strictly limited, due to the use of liquids (usually silicone oil) instead of inserts. In order to be able to calibrate using these liquids, they must be sufficiently viscous at ambient temperature. This requirement then limits the upper end of the temperature range to approximately 250°C. They do have the advantage of homogeneous mixing of the liquid due to a magnetic stirrer at the bottom, meaning no axial nor radial gradients are observed.