Dry-well calibrators are portable instruments for the simple on-site calibration of temperature measuring instruments. A temperature dry-well calibrator includes an electrically heated or cooled metal block, an accurate temperature control and an internal reference with indicator for determining the block temperature. In Kill is a bore for accepting the insert. This exchangeable insert is supplied with a number of bores with individual diameters and serves for the adaption of the test items.
With on-site calibration, the thermometer to be tested can 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 you possibly can, since any air gap between the 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 ought to 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 full total measurement uncertainty comprises 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 completed 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 the bottom up. Should the minimum immersion depth not be reached, an external reference thermometer can be used for calibration. That is then placed on the same level as the object to be calibrated and the results are compared with one another.
Recommended rule of thumb 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
Note
Further information on our portable temperature calibrators and services are available on the WIKA website.