THE EFFECT OF STANDARD WAVE CONDITIONS ON DRY SUIT INSULATION WHEN MEASURED ON HUMANS
Testing of immersion suits is commonly conducted in calm or circulatory water. Several investigators have argued that this is an unrealistic test for a suit that is designed to protect a human from hypothermia in open ocean conditions. In support of this argument, Steinman et al. (1) and Romet et al.(2) reported a significant reduction of suit insulation in turbulent water when compared to still water by an average of about 30% when measured on humans. Such differences were found for loose-fitting wet suits but were not investigated for dry suits. Recently, Sowood et al. (3) reported a reduction of dry suit insulation by about 30% in turbulent water (wave height of 60 cm) compared to still water when tested on a thermal manikin.
HOW SHOULD IMMERSED DRY SUIT INSULATION BE MEASURED
In order to position the manikin in the water in an attitude representative of that adopted by a floating survivor wearing a lifejacket (immersed to the neck facing on coming waves at approximately 30" with the arms and hands just below the water surface), the Manikin was secured to a buoyant frame which was allowed to float loosely tethered in a large wave tank (National Research Council Institute for Marine Dynamics, St John's Newfoundland, Canada). Once immersed in the water, excess air from inside the dry suit was allowed to vent naturally using a tube inserted under the dry suit neck seal. This method was used to simulate the venting that most wearers induce to provide a more upright flotation angle in the water and to ensure that the same conditions were recreated each time the manikin was immersed. Measurements were made in still water over a 12 hour period, in water made turbulent by bubbling air underneath the TIM also over a 12 hour period, and in waves with sipficant wave heights whch ranged from 0.01m to 0.6m. The spectra of the waves used were those recorded by the Joint Offshore North Sea Wave Project (JONSWAP) and periods were chosen of maximum energy for the various sipficant wave heights as set out by Bhattacharyya'. The insulation provided by Assembly C (long cotton underwear, RAF Inner Knitted Coverall Mkl , Must0 Ocean Dry Suit, Aircrew Helmet, Gloves, Socks and Boots, Full coverage Anti-G trousers and Chest Counter-pressure Garment) was measured in still water and in water made turbulent by bubbling air underneath the TIM at 50, 120, and 2401.d'. RESULTS The immersed insulation of Assembly A was unaffected by 12 hours of immersion in still water (0.59~10) or in water made turbulent by bubbling (0.55~10). Increasing wave heights reduced the insulation to 70 - 75% of the still water value for sigmficant wave heights of O.6m (Figure 1). This was primarily due to the effect of waves on the insulation covering the head which reduced to 20% of the still water value. Waves also had simcant effects on the insulation covering the hands, chest, back and legs. The abdomen, buttocks, arms and feet were least affected. The insulation provided by Assembly B (0.09~10) showed a similar reduction in waves (Figure 2). A 0.3m si@cant wave height resulted in a 50% reduction in insulation largely due to reductions in the insulation covering the head, chest, arms, legs and hands. The buttocks, back and abdomen were also af€ected. Assembly C was tested in still water and in water made turbulent by bubbling air at merent flow rates. Immersed insulation decreased to 75% of the still water value (0.70~10) as air flow increased to 240 l.min-' (Figure 3). The head, chest, back, hands and legs showed the biggest decrements in insulation.
Predicting survival time for cold exposure
The prediction of survival time (ST) for cold exposure is speculative as reliable controlled data of deep hypothermia are unavailable. At best, guidance can be obtained from case histories of accidental exposure. This study describes the development of a mathematical model for the prediction of ST under sedentary conditions in the cold. The model is based on steady-state heat conduction in a single cylinder comprised of a core and two concentric annular shells representing the fat plus skin and the clothing plus still boundary layer, respectively. The ambient condition can be either air or water; the distinction is made by assigning different values of insulation to the still boundary layer. Metabolic heat production (M) is comprised of resting and shivering components with the latter predicted by temperature signals from the core and skin. Where the cold exposure is too severe forM to balance heat loss, ST is largely determined by the rate of heat loss from the body. Where a balance occurs, ST is governed by the endurance time for shivering. End of survival is marked by the deep core temperature reaching a value of 30° C. The model was calibrated against survival data of cold water (0 to 20° C) immersion and then applied to cold air exposure. A sampling of ST predictions for the nude exposure of an average healthy male in relatively calm air (1 km/h wind speed) are the following: 1.8, 2.5, 4.1, 9.0, and >24 h for −30, −20, −10, 0, and 10° C, respectively. With two layers of loose clothing (average thickness of 1 mm each) in a 5 km/h wind, STs are 4.0, 5.6, 8.6, 15.4, and >24 h for −50, −40, −30, −20, and −10° C. The predicted STs must be weighted against the extrapolative nature of the model. At present, it would be prudent to use the predictions in a relative sense, that is, to compare or rank-order predicted STs for various combinations of ambient conditions and clothing protection.