Abstract: In underwater diving a record is made on an electronic recorder of the instantaneous pressure to which the diver is subjected at successive time intervals to give complete time-versus-pressure records for any one diver for a given time period, for example, twelve hours. A pressure transducer delivers an electronic signal to the recorder, and this pressure data is recorded at successive time intervals, for example, every minute of the twelve-hour period. The recorded data is read digitally and is read in analogue in the form of an x-y graph, either permanent or fleeting. The recorded data is preferably kept in permanent form on electronic chips also referred to as programmable read-only memories. The entire recording device can be carried by the diver, and the readout apparatus may be located at the surface of the water. Continuous topside monitoring is also provided.