Dry thimbles provide a common and convenient method of installing detectors in coolants The cost of installing dry thimbles is quite high, however, and, in addition, some installations may require cooling or cir­culating air to ensure dryness Then too, there is always some concern over possible leakage, particularly if the thimble is a penetration of a reactor

These difficulties tan be reduced by using detectors suitable for immersion m the particular medium involved Detectors with integral cable have been developed for immersion in water and hot gas, and a detector for immersion in liquid sodium is being developed

Direct immersion, while generally advantageous, has some disadvantages There must still be some sort of channel to restrict the path of motion of the detector Also, the difficulty of making adjustments in position is in creased Nevertheless, detectors for direct immersion are finding increased favor