Dry-well calibrators are portable instruments for the simple 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 internal reference with indicator for determining the block temperature. In the metal block is really a bore for accepting the insert. This exchangeable insert comes 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 will generally be ensured that the bore diameter, considering the thermal expansion of the calibration item, is selected to be as tight as possible, since any air gap between the test item and the insert impairs heat transfer. Therefore, the bore diameter should be no more than 0.5 mm higher 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 could 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. Based on 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 ought to be taken into account?
pressure gauge 10 bar for dry well calibrators is 70 mm. This is driven by the axial gradient, since warm 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 because 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
Further information on our portable temperature calibrators and services are available on the WIKA website.