We describe an open circuit method for measuring O2 consumption and CO2 production suitable for use in hyperbaric atmospheres and designed to yield continuous data while allowing the critical instrumentation to remain outside the hyperbaric chamber. The subject wears a light, loosely fitting, full face mask of clear plastic, through which ambient chamber gas is drawn at either 100 l/min for rest or sleep, or 200 l/min during exercise. Samples of the chamber gases and of the mixed expired gases are bled out of the chamber, then compared in differential analyzers. The analog signals from the analyzers are conditioned, proportioned to the mask ventilation rate, totalized and integrated to yield gas volumes, then printed every 10 min. A method of calibrating the system using inflows of pure CO2 was devised for use at 1 atm absolute and also at hyperbaric pressures. Data taken by this method during a 17-day saturation dive compared well to the data obtained using conventional Douglas bag techniques.

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