pressure in a casing string may build up due to fluid flow and thermal expansion a known rupture disk can resolve problems of excessive pressure and the well can continue to function normally however a further occurrence of such excess pressure cannot be dealt with moreover it is sometimes difficult to ascertain whether the excess pressure was caused by such a manageable event or whether it is indicative of a more serious problem especially if repeated occurrences of the excess pressure cannot be detected nor alleviated in known systems embodiments of the present invention mitigate these problems for some embodiments a number of different casing subs 401 may be used in one string of casing fig2 shows a transmitting portion 250 of the safety mechanism the portion 250 comprises a transmitter not shown powered by a battery not shown a transducer 240 and a thermometer not shown an analogue pressure signal generated by the transducer 240 passes to an electronics module 241 in which it is digitized and serially encoded for transmission by a carrier frequency suitably of 1 hz 10 khz preferably 1 khz 10 khz utilizing an fsk modulation technique the resulting bursts of carrier are applied to a magnetostrictive transducer 242 comprising a coil formed around a core not shown whose ends are rigidly fixed to the well bore casing not shown at spaced apart locations the digitally coded data is thus transformed into a longitudinal sonic wave the transmitter electronics module 241 in the present embodiment comprises a signal conditioning circuit 244 a digitizing and encoding circuit 245 and a current driver 246 the details of these circuits may be.