The minimum immersion depth for dry well calibrators is 70mm. This is driven by the axial gradient, since warm air rises from the bottom up. Should this minimum immersion depth of 70mm not be reached, an external reference thermometer must be used for calibration. This is then placed on the same level as the object to be calibrated and the results are compared with each other. The other alternative is to use a micro calibration bath, since with the thorough mixing of the liquid, there is no requirement for a minimum immersion depth. Rule of thumb for recommended immersion depth: 1. Diameter of the test item x 15 or 2. for a known sensitive length: sensitive length + Diameter of the test item x 10