Free-flow diving helmets avoid the dead space problem by supplying far more air than the diver can use, this makes the whole interior of the helmet effectively fresh air at the cost of significantly higher gas usage in open circuit systems.
Free-flow diving helmets avoid the dead space problem by supplying far more air than the diver can use, and eliminating the oro-nasal compartment. This makes the whole interior of the helmet effectively fresh air, as it is adequately flushed during and after each exhalation at the cost of significantly higher gas usage in open circuit systems.