is a calculation of the time at which the diver must begin the ascent in order to reach the surface before his air supply is depleted thus a warning signal may be generated based upon the actual air consumption of the diver and the microprocessor 39 s calculation of the decompression cycle while computing the nitrogen partial pressure updates for each simulated tissue the actual air consumption can be measured continuously by monitoring the pressure drop of the air tanks at fixed intervals of time therefore referring to fig4 pressure sensor 84 is coupled to the air tanks not shown worn by the diver to develop a voltage proportional to the air pressure of the diver 39 s air tanks the voltage from pressure transducer 84 is coupled to analog to digital converter 54 via analog multiplexer 88 under the control of i o bus 58 to generate a digital value usable by cpu 56 after the simulated nitrogen partial pressures for each tissue e g the individual p 1 have been calculated and stored in ram 52 the microprocessor can then calculate the time required to reach the surface by varying the hydrostatic pressure p once the calculation of time required for each tissue is made these times are compared to find the maximum amount of time which in turn is compared to the value proportional to the air pressure in.