Breathing resistance is usually defined by work of breathing per volume respiratory ventilation. It is divided into internal and external resistance and also into an elastic and a resistive component. Total breathing resistance is an important factor, limiting a diver's physical capacity. In unmanned tests of breathing equipment, only external breathing resistance is measured. The ultimate and ideal requirement for breathing resistance is that the equipment adds no extra resistance to breathing, but instead assists respiratory ventilation so that the density dependent increase of internal resistance is reduced. Realistic requirements, however, must be set so that total cost to fulfil them is justified by gain in safety and efficiency. To determine this, breathing resistance must be weighted against other important properties of the life support equipment such as complexity, fail frequency, maintenance, mobility, entanglement, noise, vision, temperature, pCO2, pCO2, and humidity. Requirements for breathing resistance are included in different proposed performance goals for breathing equipment. At a workshop held at NUTEC in 1981, Morrison's performance goals proposed for Department of Energy gained most support. NEDU also presented performance goals which were about equal on theimportant points. Main differences were: NEDU proposed only recommended, not also acceptance levels like Morrison, NEDU was more stringent for low respiratory ventilations and less stringent for high ventilations, hydrostatic imbalance (HI) was treated differently. The main problem is to interpret ‘recommended’ and ‘acceptable’ levels for breathing resistance. There should be no justification to use equipment not within the acceptance limits. Justifications to use equipment within acceptance levels, but not within recommended levels are: it is impossible to reach recommended levels and at the same time fulfil other requirements, important for safety and efficiency, the depth is shallow and working conditions are specially safe and easy. Today, with improved equipment and much deeper depths, ‘realistic’ requirements have come much closer to ‘recommended’ levels than they were in 1981.
WOB Work of breathing
UBA Underwater breathing apparatus
HI Hydrostatic imbalance; the pressure difference between relaxation or reference pressure at mouth opening for UBA/lung and a specified pressure fixed to a location of the body, e.g. lung centroid.
msw Pressure as depth in metres of sea water
The old acceptance criterion for breathing equipment was to put it on a diver and see if it ‘worked’. If the diver was experienced and used to an inferior equipment, the acceptance was granted. For a long time this method was good enough, mostly because other aspects of diving techniques were more in need of improvement than breathing resistance. For several years now, however, the breathing equipment and its resistance have been recognized as one of the most critical factors in deep diving. A natural consequence of this was that a more precise and objective method for evaluating breathing equipment was needed.