The Design of Oxygen Rebreather Equipment for Use in Foul-Air Speleology

The design of a lightweight oxygen rebreather'set suitable for short durati~n.exp~orations in foul-air caves is described, together with a discussion of its performance, l~m~tat~ons, and possible improvement. Excessive concentrations of carbon dioxide (C0 2) are not a common feature of British caves, and their occurrence is usually limited to a few unusual situations such as sump airbells and constricted passages beneath sewage inputs as at North Hill SWallet, Mendip (Barrington and Stanton, 1972). However, with the recent increase in explorations of tropical caves, British cavers are likely to encounter many more potentially dangerous accumulations of C02' The nature of these caves and their atmospheres has recently been described by James (1977) and need not be reiterated he~e. It will be sufficient to emphasise that the needs of the speleologist differ markedly from those of the high altitude mountaineer. The latter requires breathing apparatus purely to compensate for the small volume of oxygen inhaled at high altitude the foulair speleologist however, is concerned with removing or diluting an essentially toxic addition to his breathing mixture. Indeed, James (1977) has demonstrated that due to the different diffusion . rates of the two gases even dange~ously high C02 atmospheres may contain sufficient oxygen concentrations to sustain life. The speleologist has, in theory at least, a choice of three alternative life support systems: a non self-contained system based on the dilution of the toxic element to a safe level, or one of two self-contained systems. An examination of the data however, reveals that the former system is impracticable for the speleologist. If we accept a C02 concentration of 10% as normal for a foul-air cave of the type that would necessitate breathing apparatus, then allowing a 50% safety factor for upwards fluctuations leads us to consider the problem of diluting an atmosphere of 15% C02 to an acceptable level of perhaps 1.0% in the inspired mixture. since the only advantage of a dilution system is to reduce the volume of oxygen carried by supplementing it with a proportion of the surrounding atmosphere, the 15:1 dilution factor makes such a system redundant. In the past, foul-air cave exploration has relied on an 'open circuit' system employing standard SCUBA diving equipment (Fincham, 1977). The system has the advantage that the equipment is readily available, well tried, and by and large ~ery safe, but suffers from the not inconsiderable disadvantage of the weight and bulk of the compressed air cylinders. It therefore seems reasonable to suggest that the best solution is to employ a completely closed system in other ~ords. an oxygen rebreather in which the expired air is passed via a CO 2 absorbant and then returned to the lungs. The principle relies on the fact that the human body only uses a small l?roportion of the oxygen it inhales with some 95-97% of the inhaled air being exha~ed ag.ain sO .. _that in the SCUBA system the biggest proportion of the a~ailable oxygen (20.9% in normal air) is exhaled to the atmosphere and lost. 13ecause the oxygen rebreather is so much more efficient, the volume of gas (and ~ence the size and weight of the cylinders) required for a given length of time :is rela ti vely very small. EQUIPMENT AND TRIALS Recently, Life Support Engineering Ltd., a firm specialising in providing rni~itary underwater oxygen rebreathers, have designed and produced a small set :intended primarily for emergency mine rescue work. 'rhe apparatus was loaned to members of the British Speleological Expedition to Jamaica in 1977 with the :intention of testing its speleological potential in the further exploration of Riverhead Cave, where progress has been hindered by serious concentrations of C02 in the further reaches. Although high water conditions prevented its use