Eight divers performed an addition test and a screwplate test of manual dexterity in the open sea under four conditions#x2014;-breathing either air or an oxy-helium mixture and working at a depth of either 10 or 200 ft. Speed of addition was impaired at depth for both air (19.9 per cent) and helium (14.8 per cent), while errors increased only on air (from 5.9 to 21.1 per cent). The manual dexterity test also showed a decrement in speed for both air (46.7 per cent) and helium (31.8 per cent), and air divers lost more screws at depth (11.1 per cent) than at 10 ft (4.7 per cent). While a decrement at depth was expected in the air dives, the considerable impairment shown on oxy-helium dives was not. A further experiment was therefore run in a dry pressure chamber to study the effects of breathing oxy-helium at pressure when the additional stresses associated with deep diving in the open sea were absent. At a pressure equivalent to 200 ft of water, there was a 10 per cent impairment in speed on both the screwplate (p0.01) and the addition test (p0.05). It is concluded that the diver's efficiency on either breathing mixture is impaired at depth, but that at 200 ft the helium diver works slightly faster and considerably more accurately than the air diver.
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