Dry-well calibrators are portable instruments for the easy on-site calibration of temperature measuring instruments. A temperature dry-well calibrator consists of an electrically heated or cooled metal block, a precise temperature control and an interior reference with indicator for determining the block temperature. In the metal block is really a bore for accepting the insert. This exchangeable insert is supplied with one or more bores with individual diameters and serves for the adaption of the test items.
With on-site calibration, the thermometer to be tested could be taken directly out of its thermowell in the plant, without needing to interrupt the electrical connections to the evaluation system, and inserted in to the appropriate bore of the insert. Here it should generally be ensured that the bore diameter, bearing in mind the thermal expansion of the calibration item, is selected to be as tight as possible, since any air gap between your test item and the insert impairs heat transfer. Therefore, the bore diameter should be a maximum of 0.5 mm greater than the external diameter of the test item.
Structure of a dry-well calibrator
The required temperature with which the test item should be calibrated is regulated with the calibrator. Frequently temperature probes are calibrated at the respective process temperature. Once a thermal equilibrium has been set between the thermometer being tested and the calibrator, the measuring deviation of the test item can be recorded. This corresponds to the temperature difference between its temperature and the reference temperature displayed on the calibrator. The total measurement uncertainty is made up of the accuracy of the calibrator and the short-term stability of the test item. With respect to the temperature range and the properties of the calibration item, calibration is carried out with a measurement uncertainty of 0.1 K to 3 K.
What else should be taken into account?
The minimum immersion depth for dry well calibrators is 70 mm. That is driven by the axial gradient, since heated air rises from underneath up. Should digital pressure gauge not be reached, an external reference thermometer must be used for calibration. This is then placed on the same level because the object to be calibrated and the outcomes are compared with each other.
Recommended guideline for the immersion depth of a test item
Diameter of the test item x 15 or for a known sensitive length: sensitive length + diameter of the test item x 10
More info on our portable temperature calibrators and services are available on the WIKA website.