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An experimental pressure compensated wet suit
Ocean Engng, Vol. 9, No. 3, pp. 281-290, 1 9 8 2 . Printed in Great Britain.
0029-8018/82/030281-10503.00/0 © 1982. Pergamon Press Ltd.
AN E X P E R I M E N T A L PRESSURE COMPENSATED WET SUIT* D.R. BURTONand M.P. NORTON~ Division of Energy Technology, Commonwealth Scientific and Industrial Research Organization, Highett, Victoria, Australia 3190 Abstract--Personal thermal insulation by means of neoprene foam wet suits provides adequate temporary control of body heat loss only at the shallow end of the air diving depth range, but it constitutes by far the most popular approach to diver thermal support. However, compression of the trapped gas phase in neoprene foam seriously reduces its insulation performance on exposure to high ambient pressure. With conventional wet suits equipped with heating, the necessary level of power required at depths greater than about 30 m is too high, and also unsafe without the back up that increased insulation would provide. One approach to the goal of depth-insensitive insulation is to use a wet suit with a continuous internal gas space pressurized nominally to ambient pressure, so that its thickness remains substantially constant at all depths. The composite material properties required are: outer skins that are tough, flexible and free of pin holes; an open foam internal structure capable of resisting, without significant dimensional change, the relatively small pressure changes that occur over the height of a man; and a high bond strength. Samples of a composite material that meets these requirements have been developed, and a prototype suit has been successfully fabricated by conventional techniques.
1.
INTRODUCTION
FOR THE great majority of divers, who work at present without the benefits of supplementary heating, a high body heat loss situation is normal. A condition of hypothermia develops with increasing duration of diving and, particularly in, the temperate and cold water areas of the world, it is a problem of great practical importance. Often, restrictions must be placed both on the safe duration of exposure and the amount of work accomplished per dive. It is widely accepted that complete solutions to this problem will involve two complementary approaches. Improved external thermal insulation is required to limit heat loss through the skin, and supplementary heat addition is also necessary. In applications such as modern commercial diving, a satisfactory heat balance situation can be maintained with very little body insulation by means of the free flooding hot water suit (Long and Smith, 1972). However, very large amounts of power must be transmitted to the diver by umbilical connections to surface installations. In some applications such as lockout diving from submersibles, scientific work, police, military and recreational diving, it may be inconvenient or impossible to obtain direct thermal support from the surface. In such applications a preferred approach is to use an improved thermal insulation performance and-if necessary a reduced supplementary power supply carried by the diver. * Based on a paper presented at the Underwater Technology lntBrnational Conference, DIVETECH "81, London, November 1981. t Present address: Department of Mechanical Engineering, University of Western Australia, Nedlands, Western Australia 6009. 281
282
D.R. BURTONand M.P. NORTON
Earlier work by CSIRO explored the feasibility of a simple supplementary heating system based on the use of magnesium as a fuel. Initial applications in actively-heated gloves and also in heating of the torso, have been decribed previously (Burton and Chan, in press; Chan and Burton, 1981). Also, theoretical models have been developed, and an experimental program established, to determine the insulation performance both of conventional wet suit materials and composite materials suitable for pressure compensated wet suits (see later). The analytical models of Norton (1981) predict changes in gas phase volume and thermal conductance for closed cell, open cell, and composite foamed structures under hydrostatic pressure with and without pressure compensation. The thermal conductance measurements of Norton and Chan (1981) confirm these predictions. 2.
DRY SUITS
At present, dry suits offer the highest available insulation performance, but because they are of necessity a fairly loose fit, the enclosed gas tends to migrate to the top of the suit. This may cause an attitude instability. Dry suits also present possibilities of inboard water leakage through wrist seals and closures, and the diuretic response to combined immersion and cold poses effectively similar problems within 2 hr (Wattenbarger, 1977). 3. WET SUITS Neoprene foam wet suits provide adequate thermal insulation at the shallow end of the air diving depth range. Their low cost, comfort, flexibility, tolerance to damage and freedom from flooding problems are factors which have made them by far the most popular type of garment for diver thermal support. However, compression of the trapped gas phase in neoprene foam seriously reduces its insulation performance on exposure to high ambient pressure (Beckman, 1963). For conventional wet suits equipped with supplementary heating, the necessary power required would become too high at depths greater than about 30 m, and also unsafe without the backup that increased insulation would provide. 4.
DEPTH-INSENSITIVE WET SUITS
Two approaches to the goal of depth-insensitive thermal insulation pose exciting engineering challenges. The use of flexible foam incorporating rigid gas filled microspheres, in attempts to provide wet suit insulating materials of greatly reduced compressibility, have been described by Beckman and Frey (1967), Rocco (1972), Mylotte (1973) and Ohsawa et al. (1979). Ultimate success in this approach would considerably enhance the depth capability of the wet suit concept. Another approach is to use a material with a continuous internal gas space pressurized nominally to the ambient pressure existing around the diver. In this way, suit thickness is maintained substantially constant at all depths. Prior to the work reported here, gas pressure-compensated wet suits were described by Beckman a n d ' F r e y (1967), Barth616my (1970) and Bramham et al. (1972). The relative scarcity of recent published work in this field, together with a need to enhance the depth capability of the CSIRO system of supplementary heating, have prompted this preliminary investigation into the feasibility of pressure-compensated flexible insulation.
An experimental pressure compensated wet suit 5.
283
MODE OF ACTION OF PRESSURIZED SUIT
In order to achieve its primary function as an insulator the garment, like all diving suits, retains a relatively immobile envelope of gas around the body. Unlike a conventional wet suit, however, most of the gas space of each garment portion is continuous, and its volume is actively maintained by a gas pressure control system. In normal situations where gas flow is very small, the gas phase pressure within each suit portion is therefore almost constant throughout, whereas the external surfaces rare subject to the vertical hydrostatic pressure gradient of approximately 10 kPa/m in sea water. Whilst gross pressure forces at depth are cancelled by compensation, local pressure forces still generally exist between the gas retaining outer skins and must therefore be resisted by distortion of the internal structure. Depending on the chosen compensation pressure, this force may be entirely positive, teriding to expand the internal retaining structure, and incidentally ensuring that small leaks are outboard leaks of gas. Alternatively, a lower compensation pressure tends to collapse the retaining structure. This may be bad for tolerance of leaks, but necessary for coping with delamination damage. In general it was thought that such pressure forces should be minimized, and thus a compensation pressure for each suit portion was chosen such that the force was zero at its mid-height when vertical. 6.
D E S I R A B L E MATERIAL PROPERTIES
In view of the modest nature of the compensated suit programme, a rigorous a-priori quantitative specification of desirable properties, with subsequent massive screening of candidate materials and manufacturing techniques, could not be attempted. It was decided, however, to specify the essential requirements, and to use readily available materials. These essential requirements for the composite are: outer skins that are tough, flexible and free of pin holes; an open foam internal structure capable of resisting, without significant dimensional change, the pressure changes that occur over about half the height of an immersed man; and a high bond strength adhesive treatment. Materials chosen for testing consisted of a core of open cell polyurethane flexible foam of d e ~ i t y 50-53 kg/m 3, void fraction 0.95, and of nominal thickness 6.3 mm. To each side of this material were bonded skins of closed cell neoprene foam of thickness 2 mm, void fraction 0.7 with the sealed face directed outwards. 7.
ADHESIVE TREATMENTS
Four adhesive treatments were available for test. Two adhesive formulations were chosen, b o t h of the s o l v e n t - b a s e d contact type. Formulation 1 was applied semi-automatically by a coated steel roller normally used for laminating the stretch fabric reinforcement to wet suit material. Formulation 2 was applied manually by means of a paint roller. Each formulation was either cold-cured under light mechanical pressure, or subjected to treatment in a steam-heated laminating press. Square samples of the polyurethane core material, nominally 0.4 × 0.4 m, were laminated to neoprene skins approximately 20 mm larger all round, so that at the perimeter the outer skins could be directly bonded together to form an airtight envelope. Burst strength tests, performed several weeks after curing, are shown in Table 1. This shows for six samples, the internal gas pressures at which delamination occurred within 2 min at normal laboratory temperatures for the four adhesive treatments described.
284
D.R. BURTONand M.P. NORTON
Pressure was applied to the samples by means of a sharpened hypodermic tube inserted through the neoprene outer skin. Delamination was detected visually by the appearance of a large blister. A photomicrograph of the cross-section showing the white open foam core with black neoprene skin attached is shown in Fig. 1. From Table 1 it appears that the highest bond strength was obtained with adhesive 1 cured at room temperature, the next highest was also adhesive 1 subjected to hot pressing. Unfortunately it was not possible to obtain sufficient samples for a full replication, and the statistical significance of this result has not been tested. TABLE 1.
DELAMINATION
PRESSURES
OF SIX COMPOSITE
FOAM
SAMPLES BY ADHESIVE TREATMENT
Adhesive
8.
1
2
44 kPa 40 kPa
36 kPa
76 kPa
8 kPa 16 kPa
Hot pressed Cold cured
STRESS STRAIN CHARACTERISTICS
Small pressure changes applied to the internal compensating layer of the composite foam are resisted by changes of thickness. For positive internal pressures, the expansion was proportional to the applied pressure, and was approx. 2% of thickness for every increase of 10 kPa or 1 m of sea water. Pressures were measured both by liquid m a n o m e t e r and bourdon gauge, and strain was measured by means of a dial gauge applied to a glass plate resting on the sample. The purpose of the plate was to give a space average of deflection over a large area. For internal pressures below atmospheric pressure, the response of the samples was more complex. It was found, for instance that compressive strain was dependent on the history of the sample. Generally the strain was smaller for the first application of decreased pressure than it was for subsequent applications, and several cycles of compression and recovery were necessary before reproducible results could be obtained. Also, the material structure generally responded very slowly to such forces, and it was often necessary to wait for several hours for the thickness change to be completed after a change of pressure. Thirdly, the amount of strain depended on whether the sample was being further compressed or relaxed. Figures 2 and 3 show the results of these tests. The compression strain bears a non-linear relationship to pressure, and the difference between compression and recovery, or hysteresis, is more marked in Fig. 2, which was for a sample cured at room temperature. The hysteresis effect and compressive stiffness are less in Fig. 3, which was for a sample given the hot press treatment. For the purposes of selecting a suit material, adhesive formulation 1 with the room t e m p e r a t u r e cure is to be preferred, and is satisfactory for a prototype. The limits of internal pressure are bounded by the delamination pressure of +76 kPa or 7.6 m sea water, and by compressive strain at about - 7 kPa ( - 0 . 7 m sea water). Although the
A n experimental pressure compensated wet suit
CLOSED CELL NEOPRENE SKIN
OPEN
CELL
CORE
4~i ~
Fro. 1.
Photomicrograph of the celt structure of composite foam. x 20.
285
286
D.R. BURTONand M.P. NORTON _z
20
DECREASE
IN
INTERNAL kPa
20
< =:
J
PRESSURE
10
~..-.--.-=--=--'-'--'~
20
INCREASE
IN
S
80
PRESSURE
k Pa
10
2O
INTERNAL
70
z
30
u~
40
uJ
0 50
60
FIG, 2.
DECREASE
IN
Stress strain characteristics of cold cured composite material.
INTERNAL
PRESSURE z
kPa 20
.o
10 INCREASE
IN
INTERNAL
PRESSURE
kPa
10
20
20
z
<
3O
40
~ o. 0
50
60
FIG. 3.
Stress strain characteristics of hot pressed composite material.
30
An experimental pressure compensated wet suit ADJUSTABLE
PIN
ORIFICE
INLET
SILICONE
ORIFICE
AND
BAYONET LOCKING
CONNECTOR
DISC
EXHAUST
FIG. 4.
FOR
TYPE
287
NON - RETURN
VALVE
Suit gas pressure control system.
initial resistance to compression of this material was better than that for the hot pressed material, a sharp drop in stiffness occurred at an internal pressure decrease of 0.75-m sea water (Fig. 2). Further work is required to select a core material more resistant to buckling. In addition, tests of tear strength, elasticity, resistance to fatigue, etc., would also assist in material selection. 9.
PRESSURE C O N T R O L SYSTEM
Accurate pressure control is required for the gas space of a c o m p e n s a t e d suit, for the same reasons as it is required for the lungs. A feasible solution to the problem could involve a demand regulator, as used in underwater breathing, positioned about midway along each suit portion. H o w e v e r , it was decided that the expiratory valves of such devices do not seal reliably enough, and the inconvenient shape of the regulators would present a snagging hazard. Accordingly, an exhaust non-return valve was designed with a low profile, and configured to seal completely under reverse pressure. Instead of a demand regulator, a simple flow control orifice was used. It was situated within the connector that attached the suit to its feed hose. In the first instance, the pressurizing gas was air, derived from a first stage tapping of the breathing regulator. A sketch of the supply orifice and non-return valve is shown in Fig. 4. The continuous air supply was adjusted in use to deliver 1-2 I./min, at atmospheric pressure. This small flow is not sufficient to keep the suit charged on rapid descent..Accordingly, an estimate of the charging time following an instantaneous descent from a pressure of n atm to n + 1 atm is presented here. Due to the design of the first stage reducing valve, the orifice upstream pressure is approximately 8 atm higher than ambient water pressure, and the feed orifice is considered to be sonic at all depths shallower than about 80 m. Its mass flow is
288
D.R. BURTONand M.P. NORTON
therefore proportional to upstream pressure only, upstream t e m p e r a t u r e variations having been neglected. Analysis proceeds for a suit having an air space of v 1. at n atm, and with all processes assumed isothermal. At n + 1 atm the initial suit air capacity decreases to (n/n+l)v 1. Volume of gas required to restore capacity is 1
n n+l
) vl.
(1)
At 1 atm the orifice mass flow is adjusted to 1 1./min STP (standard temperature and pressure), with upstream gauge pressure 8 atm. At n + 1 atm, upstream absolute pressure is n + 1 + 8, and the orifice mass flow is n+1+8
l./min STP
The actual volume flow delivered is
(1)(n+l+8),Jmin 8
,2,
Charging time is therefore given by the ratio of expression (1) to expression (2) which is:
(1
n )
n + 1 v 8 ( n + 1) n + 9
min
8v . . n. +.9
min
For a suit portion containing 6 1. of air (v = 6) the charging time following an instantaneous descent of 10 m is shown in T a b l e 2. The charging time is also the time required to restore full buoyancy• T h e initial b u o y a n c y loss following the sudden depth increase is also shown, and is derived f r o m expression ( i ) .
TABLE 2. CALCULATED INITIAL BUOYANCY DECREASE AND CHARGING TIME FOLLOWING A SUDDEN DEPTH INCREASE OF I 0 m SEA WATER (EQUILIBRIUM VOLUME 6 1. CHARGE RATE I l / m i n
STP)
Depth increase 0-10 m 10-20 m 20-30 m 30--40 m 40-50 m 50-60 m 60-70 m 70-80 m 80-90 m
Initial buoyancy drop 3.0 2.0 1.5 1.2 1.0 0.84 0.78 0.66 0.60
kg kg kg kg kg kg kg kg kg
Charging time 5.20 4.73 4.33 4.00 3.72 3.46 3.25 3.06 2.89
min min min min min min min rain min
An experimental pressure compensated wet suit 10.
289
SUIT M A N U F A C T U R E
A prototype suit was constructed using a composite material based on adhesive formulation 1 with the cold curing procedure. The core thickness, however, was reduced from 6.3 to 4.2 ram, and the outer surfaces were reinforced with stretch nylon in the manner of a conventional wet suit material. This resulted in a final suit thickness of 9.2 mm. Seams were butt-joined and then sewn on the outside. Final leak proofing of the seams proved to be difficult. Samples of seam could easily be perfectly sealed agaiffst 20 kPa air pressure with a one part moisture curing urethane adhesive ( M O R - A D 325), which is capable of flowing through the stretch nylon lining and forming a seal to the neoprene beneath. However, perfect sealing of samples is no guarantee that a complete suit can be successfully treated. In practice, small air leaks from seams were visible when the trouser portion of the suit was pressurized to 20 kPa and imhaersed. In immersion tests of several hours' duration the weight gain of the trouser portion of the suit was zero ( < 0.1 g), and in a subsequent test dive involving swimming and deliberate suit flexing for 30 min, the weight gain was approximately 1 g. In order to measure weight gains of this order, in a garment weighing 2.4 kg, it was necessary to expose the suit both before and after immersion to constant atmospheric conditions for about 12 h, and to reweigh in these conditions several times. Gram quantities of water may easily be removed by ventilation of the suit with dry air, for 1 or 2 h. The lap joined seams formed around the perimeter of the samples used for strain measurement have never shown any leakage, even after immersion for 10 h. It would be preferred, therefore, if suits could be assembled from individual panels pre-sealed by this method. 11.
DISCUSSION
The major disadvantages of the conventional wet suit are its limited depth capability resulting from gas phase compression, and its unacceptable expansion behaviour on decompression following exposure to a helium atmosphere (Penzias and G o o d m a n , 1973). These problems may be overcome if the insulating gas envelope can be pressure-compensated, as are all other gas spaces near and within the diver's body. In thermal conductance measurements on the composite air compensated material, it has been shown that insulation performance is practically independent of pressure between sea level and 30 atm, and is greatly superior to closed cell neoprene foam at that depth ( N o r t o n and Chan, 1981). T h e obvious advantages in p e r f o r m a n c e of compensating gases lower in thermal conductivity than air, for example carbon dioxide, should also be noted. In the present work it has been shown that such compensated materials as can be readily assembled from existing and easily available polymers are capable of resisting, without significant dimensional change, the vertical pressure gradient in water over half the height of a man. T h e y are also capable of withstanding up to 76 kPa of internal pressure before delamination. In addition it has been shown that complete suits may be manufactured from such materials by conventional techniques. Live tests of the present prototype in respect of its thermal performance have not been conducted because several suits and relatively deep w~iter exposures would be necessary in order to show a clear advantage over conventional wet suits. Such exposures exceed the diving capability of the laboratory, but some shallow water exposures in the sea have been performed. On the debit side, the major causes of concern are twofold. The response to
290
D.R. BURTON and M.P. NORTON
d e l a m i n a t i o n is b a l l o o n i n g of the o u t e r skins, which could result in a b u o y a n c y increase with fatal c o n s e q u e n c e s if allowed to h a p p e n . T h e o t h e r cause for c o n c e r n is leakage, which may follow p o o r quality control or suit d a m a g e . I n the p r o t o t y p e suit a n d the test samples, the o u t e r skins as supplied were c o m p l e t e l y free of pin holes. L e a k a g e n e v e r occurred from the p e r i m e t e r lap joins, but it was p r e s e n t to a slight d e g r e e in the butt seams of the p r o t o t y p e . Massive flooding of i m m e r s e d samples, even with u n s e a l e d edges, was quite difficult to achieve b e c a u s e w a t e r does not readily flow t h r o u g h the o p e n cell foam. A t the p r e s e n t stage of d e v e l o p m e n t it is not possible to m a k e definitive s t a t e m e n t s a b o u t diver acceptance. As a wet suit, the p r o t o t y p e was r e a s o n a b l y c o m f o r t a b l e to wear. D i v e r ' s acceptance of new e q u i p m e n t , h o w e v e r , also d e p e n d s critically on such factors as quality control, g a r m e n t life, safety, a n d total costs i n c l u d i n g those of inspection, m a i n t e n a n c e a n d repair. T h e s e factors can only be d e t e r m i n e d by f u r t h e r work u n d e r field conditions. Acknowledgement---Contributions to this programme were obtained from resources other than CSIRO by way
of material donations, manufacturing facilities, time and skilled help. In particular the participation of Messrs A. Wood of Dunlopillo, D. Dunne and A. Boutlis of Leggen Rubber Industries and I. Barany of Abalone Aquasuits is gratefully acknowledged.
REFERENCES BARTHf~L,~MV, L. 1970. DcSperditions calorique et protection thermique du plongeur. Travail Hum. 33, 195-216. BECKMAN,E.L. 1963. Thermal protection during immersion in cold water. Proc. 2tzd ,S~vmp. on L"tzderwater Physiology. Edited by C.J. Labmertson. BECKMAN,E.L. and FREY,HR. 1967. Electrically-heated pressure compensated wet suits for Sealab II. U.S Office of Naval Research, Report ONRACR 124, Chap. 37. Bramham, E., Dineley, J. and Wharmby, B. 1972. Heat loss compensation in deep diving. Hydrospace, 20. BURTON,D.R. and CttAN, C.Y.L. Experiments with chemically-heated diving gloves. Mar. Technol. Soc. J. (ill press). CHAN, C.Y.L. and BURTON,D.R. 1981. Local heating source for shallow water divers. J. Pwr Sources 6. 291-304. LONG, R.W. and SMm~, N.E. 1972. Hot water: an economical approach to increased diver performance and safety in the offshore oil industry. Offshore Technology Conference, Paper No. OTC. 1563, Dallas. MYLOrrE, J. 1973. Development and evaluation of deep sea swimsuit materials. U.S. Navy Clothing and Textile Research Unit, Technical Report No. 108, N.T.I.S. AD. 763378. NORTON,M.P. 198l. Theoretical considerations concerning prediction of diver wet suit insulation behaviour with and without pressure compensation. J. appl. Energy, 9, 85-105. NORTON, M.P. and CHAN, C.Y.L. 1981. On the insulation properties of composite cell foamed materials suitable for wet suits. Divetech '81, Society for Underwater Technology, London. OHSAWA,T., MIWA.N. and NAKAYAMA,A. 1979. A study of composite foams for diving suits subjected to high hydrostatic pressure. J. appl. Polvm. Sci. 23. 1233-1245. PENZIAS,W. and GOODMAN,M.W. 1973. Man Beneath the Sea - - A Review o[" Underwater Ocean Engineering. Wiley-Interscience, New York, pp. 586--631. Rocco, R. 1972. Passively insulated diving suit. Syrnp. Proc. The Working Diver, Marine Technolog~ Society. WATrENBARGER,J.F. 1977. A development program in diver thermal protection. A.S.M.E. Intersocietv Conference on Environmental Systems, San Francisco.
0029-8018/82/030281-10503.00/0 © 1982. Pergamon Press Ltd.
AN E X P E R I M E N T A L PRESSURE COMPENSATED WET SUIT* D.R. BURTONand M.P. NORTON~ Division of Energy Technology, Commonwealth Scientific and Industrial Research Organization, Highett, Victoria, Australia 3190 Abstract--Personal thermal insulation by means of neoprene foam wet suits provides adequate temporary control of body heat loss only at the shallow end of the air diving depth range, but it constitutes by far the most popular approach to diver thermal support. However, compression of the trapped gas phase in neoprene foam seriously reduces its insulation performance on exposure to high ambient pressure. With conventional wet suits equipped with heating, the necessary level of power required at depths greater than about 30 m is too high, and also unsafe without the back up that increased insulation would provide. One approach to the goal of depth-insensitive insulation is to use a wet suit with a continuous internal gas space pressurized nominally to ambient pressure, so that its thickness remains substantially constant at all depths. The composite material properties required are: outer skins that are tough, flexible and free of pin holes; an open foam internal structure capable of resisting, without significant dimensional change, the relatively small pressure changes that occur over the height of a man; and a high bond strength. Samples of a composite material that meets these requirements have been developed, and a prototype suit has been successfully fabricated by conventional techniques.
1.
INTRODUCTION
FOR THE great majority of divers, who work at present without the benefits of supplementary heating, a high body heat loss situation is normal. A condition of hypothermia develops with increasing duration of diving and, particularly in, the temperate and cold water areas of the world, it is a problem of great practical importance. Often, restrictions must be placed both on the safe duration of exposure and the amount of work accomplished per dive. It is widely accepted that complete solutions to this problem will involve two complementary approaches. Improved external thermal insulation is required to limit heat loss through the skin, and supplementary heat addition is also necessary. In applications such as modern commercial diving, a satisfactory heat balance situation can be maintained with very little body insulation by means of the free flooding hot water suit (Long and Smith, 1972). However, very large amounts of power must be transmitted to the diver by umbilical connections to surface installations. In some applications such as lockout diving from submersibles, scientific work, police, military and recreational diving, it may be inconvenient or impossible to obtain direct thermal support from the surface. In such applications a preferred approach is to use an improved thermal insulation performance and-if necessary a reduced supplementary power supply carried by the diver. * Based on a paper presented at the Underwater Technology lntBrnational Conference, DIVETECH "81, London, November 1981. t Present address: Department of Mechanical Engineering, University of Western Australia, Nedlands, Western Australia 6009. 281
282
D.R. BURTONand M.P. NORTON
Earlier work by CSIRO explored the feasibility of a simple supplementary heating system based on the use of magnesium as a fuel. Initial applications in actively-heated gloves and also in heating of the torso, have been decribed previously (Burton and Chan, in press; Chan and Burton, 1981). Also, theoretical models have been developed, and an experimental program established, to determine the insulation performance both of conventional wet suit materials and composite materials suitable for pressure compensated wet suits (see later). The analytical models of Norton (1981) predict changes in gas phase volume and thermal conductance for closed cell, open cell, and composite foamed structures under hydrostatic pressure with and without pressure compensation. The thermal conductance measurements of Norton and Chan (1981) confirm these predictions. 2.
DRY SUITS
At present, dry suits offer the highest available insulation performance, but because they are of necessity a fairly loose fit, the enclosed gas tends to migrate to the top of the suit. This may cause an attitude instability. Dry suits also present possibilities of inboard water leakage through wrist seals and closures, and the diuretic response to combined immersion and cold poses effectively similar problems within 2 hr (Wattenbarger, 1977). 3. WET SUITS Neoprene foam wet suits provide adequate thermal insulation at the shallow end of the air diving depth range. Their low cost, comfort, flexibility, tolerance to damage and freedom from flooding problems are factors which have made them by far the most popular type of garment for diver thermal support. However, compression of the trapped gas phase in neoprene foam seriously reduces its insulation performance on exposure to high ambient pressure (Beckman, 1963). For conventional wet suits equipped with supplementary heating, the necessary power required would become too high at depths greater than about 30 m, and also unsafe without the backup that increased insulation would provide. 4.
DEPTH-INSENSITIVE WET SUITS
Two approaches to the goal of depth-insensitive thermal insulation pose exciting engineering challenges. The use of flexible foam incorporating rigid gas filled microspheres, in attempts to provide wet suit insulating materials of greatly reduced compressibility, have been described by Beckman and Frey (1967), Rocco (1972), Mylotte (1973) and Ohsawa et al. (1979). Ultimate success in this approach would considerably enhance the depth capability of the wet suit concept. Another approach is to use a material with a continuous internal gas space pressurized nominally to the ambient pressure existing around the diver. In this way, suit thickness is maintained substantially constant at all depths. Prior to the work reported here, gas pressure-compensated wet suits were described by Beckman a n d ' F r e y (1967), Barth616my (1970) and Bramham et al. (1972). The relative scarcity of recent published work in this field, together with a need to enhance the depth capability of the CSIRO system of supplementary heating, have prompted this preliminary investigation into the feasibility of pressure-compensated flexible insulation.
An experimental pressure compensated wet suit 5.
283
MODE OF ACTION OF PRESSURIZED SUIT
In order to achieve its primary function as an insulator the garment, like all diving suits, retains a relatively immobile envelope of gas around the body. Unlike a conventional wet suit, however, most of the gas space of each garment portion is continuous, and its volume is actively maintained by a gas pressure control system. In normal situations where gas flow is very small, the gas phase pressure within each suit portion is therefore almost constant throughout, whereas the external surfaces rare subject to the vertical hydrostatic pressure gradient of approximately 10 kPa/m in sea water. Whilst gross pressure forces at depth are cancelled by compensation, local pressure forces still generally exist between the gas retaining outer skins and must therefore be resisted by distortion of the internal structure. Depending on the chosen compensation pressure, this force may be entirely positive, teriding to expand the internal retaining structure, and incidentally ensuring that small leaks are outboard leaks of gas. Alternatively, a lower compensation pressure tends to collapse the retaining structure. This may be bad for tolerance of leaks, but necessary for coping with delamination damage. In general it was thought that such pressure forces should be minimized, and thus a compensation pressure for each suit portion was chosen such that the force was zero at its mid-height when vertical. 6.
D E S I R A B L E MATERIAL PROPERTIES
In view of the modest nature of the compensated suit programme, a rigorous a-priori quantitative specification of desirable properties, with subsequent massive screening of candidate materials and manufacturing techniques, could not be attempted. It was decided, however, to specify the essential requirements, and to use readily available materials. These essential requirements for the composite are: outer skins that are tough, flexible and free of pin holes; an open foam internal structure capable of resisting, without significant dimensional change, the pressure changes that occur over about half the height of an immersed man; and a high bond strength adhesive treatment. Materials chosen for testing consisted of a core of open cell polyurethane flexible foam of d e ~ i t y 50-53 kg/m 3, void fraction 0.95, and of nominal thickness 6.3 mm. To each side of this material were bonded skins of closed cell neoprene foam of thickness 2 mm, void fraction 0.7 with the sealed face directed outwards. 7.
ADHESIVE TREATMENTS
Four adhesive treatments were available for test. Two adhesive formulations were chosen, b o t h of the s o l v e n t - b a s e d contact type. Formulation 1 was applied semi-automatically by a coated steel roller normally used for laminating the stretch fabric reinforcement to wet suit material. Formulation 2 was applied manually by means of a paint roller. Each formulation was either cold-cured under light mechanical pressure, or subjected to treatment in a steam-heated laminating press. Square samples of the polyurethane core material, nominally 0.4 × 0.4 m, were laminated to neoprene skins approximately 20 mm larger all round, so that at the perimeter the outer skins could be directly bonded together to form an airtight envelope. Burst strength tests, performed several weeks after curing, are shown in Table 1. This shows for six samples, the internal gas pressures at which delamination occurred within 2 min at normal laboratory temperatures for the four adhesive treatments described.
284
D.R. BURTONand M.P. NORTON
Pressure was applied to the samples by means of a sharpened hypodermic tube inserted through the neoprene outer skin. Delamination was detected visually by the appearance of a large blister. A photomicrograph of the cross-section showing the white open foam core with black neoprene skin attached is shown in Fig. 1. From Table 1 it appears that the highest bond strength was obtained with adhesive 1 cured at room temperature, the next highest was also adhesive 1 subjected to hot pressing. Unfortunately it was not possible to obtain sufficient samples for a full replication, and the statistical significance of this result has not been tested. TABLE 1.
DELAMINATION
PRESSURES
OF SIX COMPOSITE
FOAM
SAMPLES BY ADHESIVE TREATMENT
Adhesive
8.
1
2
44 kPa 40 kPa
36 kPa
76 kPa
8 kPa 16 kPa
Hot pressed Cold cured
STRESS STRAIN CHARACTERISTICS
Small pressure changes applied to the internal compensating layer of the composite foam are resisted by changes of thickness. For positive internal pressures, the expansion was proportional to the applied pressure, and was approx. 2% of thickness for every increase of 10 kPa or 1 m of sea water. Pressures were measured both by liquid m a n o m e t e r and bourdon gauge, and strain was measured by means of a dial gauge applied to a glass plate resting on the sample. The purpose of the plate was to give a space average of deflection over a large area. For internal pressures below atmospheric pressure, the response of the samples was more complex. It was found, for instance that compressive strain was dependent on the history of the sample. Generally the strain was smaller for the first application of decreased pressure than it was for subsequent applications, and several cycles of compression and recovery were necessary before reproducible results could be obtained. Also, the material structure generally responded very slowly to such forces, and it was often necessary to wait for several hours for the thickness change to be completed after a change of pressure. Thirdly, the amount of strain depended on whether the sample was being further compressed or relaxed. Figures 2 and 3 show the results of these tests. The compression strain bears a non-linear relationship to pressure, and the difference between compression and recovery, or hysteresis, is more marked in Fig. 2, which was for a sample cured at room temperature. The hysteresis effect and compressive stiffness are less in Fig. 3, which was for a sample given the hot press treatment. For the purposes of selecting a suit material, adhesive formulation 1 with the room t e m p e r a t u r e cure is to be preferred, and is satisfactory for a prototype. The limits of internal pressure are bounded by the delamination pressure of +76 kPa or 7.6 m sea water, and by compressive strain at about - 7 kPa ( - 0 . 7 m sea water). Although the
A n experimental pressure compensated wet suit
CLOSED CELL NEOPRENE SKIN
OPEN
CELL
CORE
4~i ~
Fro. 1.
Photomicrograph of the celt structure of composite foam. x 20.
285
286
D.R. BURTONand M.P. NORTON _z
20
DECREASE
IN
INTERNAL kPa
20
< =:
J
PRESSURE
10
~..-.--.-=--=--'-'--'~
20
INCREASE
IN
S
80
PRESSURE
k Pa
10
2O
INTERNAL
70
z
30
u~
40
uJ
0 50
60
FIG, 2.
DECREASE
IN
Stress strain characteristics of cold cured composite material.
INTERNAL
PRESSURE z
kPa 20
.o
10 INCREASE
IN
INTERNAL
PRESSURE
kPa
10
20
20
z
<
3O
40
~ o. 0
50
60
FIG. 3.
Stress strain characteristics of hot pressed composite material.
30
An experimental pressure compensated wet suit ADJUSTABLE
PIN
ORIFICE
INLET
SILICONE
ORIFICE
AND
BAYONET LOCKING
CONNECTOR
DISC
EXHAUST
FIG. 4.
FOR
TYPE
287
NON - RETURN
VALVE
Suit gas pressure control system.
initial resistance to compression of this material was better than that for the hot pressed material, a sharp drop in stiffness occurred at an internal pressure decrease of 0.75-m sea water (Fig. 2). Further work is required to select a core material more resistant to buckling. In addition, tests of tear strength, elasticity, resistance to fatigue, etc., would also assist in material selection. 9.
PRESSURE C O N T R O L SYSTEM
Accurate pressure control is required for the gas space of a c o m p e n s a t e d suit, for the same reasons as it is required for the lungs. A feasible solution to the problem could involve a demand regulator, as used in underwater breathing, positioned about midway along each suit portion. H o w e v e r , it was decided that the expiratory valves of such devices do not seal reliably enough, and the inconvenient shape of the regulators would present a snagging hazard. Accordingly, an exhaust non-return valve was designed with a low profile, and configured to seal completely under reverse pressure. Instead of a demand regulator, a simple flow control orifice was used. It was situated within the connector that attached the suit to its feed hose. In the first instance, the pressurizing gas was air, derived from a first stage tapping of the breathing regulator. A sketch of the supply orifice and non-return valve is shown in Fig. 4. The continuous air supply was adjusted in use to deliver 1-2 I./min, at atmospheric pressure. This small flow is not sufficient to keep the suit charged on rapid descent..Accordingly, an estimate of the charging time following an instantaneous descent from a pressure of n atm to n + 1 atm is presented here. Due to the design of the first stage reducing valve, the orifice upstream pressure is approximately 8 atm higher than ambient water pressure, and the feed orifice is considered to be sonic at all depths shallower than about 80 m. Its mass flow is
288
D.R. BURTONand M.P. NORTON
therefore proportional to upstream pressure only, upstream t e m p e r a t u r e variations having been neglected. Analysis proceeds for a suit having an air space of v 1. at n atm, and with all processes assumed isothermal. At n + 1 atm the initial suit air capacity decreases to (n/n+l)v 1. Volume of gas required to restore capacity is 1
n n+l
) vl.
(1)
At 1 atm the orifice mass flow is adjusted to 1 1./min STP (standard temperature and pressure), with upstream gauge pressure 8 atm. At n + 1 atm, upstream absolute pressure is n + 1 + 8, and the orifice mass flow is n+1+8
l./min STP
The actual volume flow delivered is
(1)(n+l+8),Jmin 8
,2,
Charging time is therefore given by the ratio of expression (1) to expression (2) which is:
(1
n )
n + 1 v 8 ( n + 1) n + 9
min
8v . . n. +.9
min
For a suit portion containing 6 1. of air (v = 6) the charging time following an instantaneous descent of 10 m is shown in T a b l e 2. The charging time is also the time required to restore full buoyancy• T h e initial b u o y a n c y loss following the sudden depth increase is also shown, and is derived f r o m expression ( i ) .
TABLE 2. CALCULATED INITIAL BUOYANCY DECREASE AND CHARGING TIME FOLLOWING A SUDDEN DEPTH INCREASE OF I 0 m SEA WATER (EQUILIBRIUM VOLUME 6 1. CHARGE RATE I l / m i n
STP)
Depth increase 0-10 m 10-20 m 20-30 m 30--40 m 40-50 m 50-60 m 60-70 m 70-80 m 80-90 m
Initial buoyancy drop 3.0 2.0 1.5 1.2 1.0 0.84 0.78 0.66 0.60
kg kg kg kg kg kg kg kg kg
Charging time 5.20 4.73 4.33 4.00 3.72 3.46 3.25 3.06 2.89
min min min min min min min rain min
An experimental pressure compensated wet suit 10.
289
SUIT M A N U F A C T U R E
A prototype suit was constructed using a composite material based on adhesive formulation 1 with the cold curing procedure. The core thickness, however, was reduced from 6.3 to 4.2 ram, and the outer surfaces were reinforced with stretch nylon in the manner of a conventional wet suit material. This resulted in a final suit thickness of 9.2 mm. Seams were butt-joined and then sewn on the outside. Final leak proofing of the seams proved to be difficult. Samples of seam could easily be perfectly sealed agaiffst 20 kPa air pressure with a one part moisture curing urethane adhesive ( M O R - A D 325), which is capable of flowing through the stretch nylon lining and forming a seal to the neoprene beneath. However, perfect sealing of samples is no guarantee that a complete suit can be successfully treated. In practice, small air leaks from seams were visible when the trouser portion of the suit was pressurized to 20 kPa and imhaersed. In immersion tests of several hours' duration the weight gain of the trouser portion of the suit was zero ( < 0.1 g), and in a subsequent test dive involving swimming and deliberate suit flexing for 30 min, the weight gain was approximately 1 g. In order to measure weight gains of this order, in a garment weighing 2.4 kg, it was necessary to expose the suit both before and after immersion to constant atmospheric conditions for about 12 h, and to reweigh in these conditions several times. Gram quantities of water may easily be removed by ventilation of the suit with dry air, for 1 or 2 h. The lap joined seams formed around the perimeter of the samples used for strain measurement have never shown any leakage, even after immersion for 10 h. It would be preferred, therefore, if suits could be assembled from individual panels pre-sealed by this method. 11.
DISCUSSION
The major disadvantages of the conventional wet suit are its limited depth capability resulting from gas phase compression, and its unacceptable expansion behaviour on decompression following exposure to a helium atmosphere (Penzias and G o o d m a n , 1973). These problems may be overcome if the insulating gas envelope can be pressure-compensated, as are all other gas spaces near and within the diver's body. In thermal conductance measurements on the composite air compensated material, it has been shown that insulation performance is practically independent of pressure between sea level and 30 atm, and is greatly superior to closed cell neoprene foam at that depth ( N o r t o n and Chan, 1981). T h e obvious advantages in p e r f o r m a n c e of compensating gases lower in thermal conductivity than air, for example carbon dioxide, should also be noted. In the present work it has been shown that such compensated materials as can be readily assembled from existing and easily available polymers are capable of resisting, without significant dimensional change, the vertical pressure gradient in water over half the height of a man. T h e y are also capable of withstanding up to 76 kPa of internal pressure before delamination. In addition it has been shown that complete suits may be manufactured from such materials by conventional techniques. Live tests of the present prototype in respect of its thermal performance have not been conducted because several suits and relatively deep w~iter exposures would be necessary in order to show a clear advantage over conventional wet suits. Such exposures exceed the diving capability of the laboratory, but some shallow water exposures in the sea have been performed. On the debit side, the major causes of concern are twofold. The response to
290
D.R. BURTON and M.P. NORTON
d e l a m i n a t i o n is b a l l o o n i n g of the o u t e r skins, which could result in a b u o y a n c y increase with fatal c o n s e q u e n c e s if allowed to h a p p e n . T h e o t h e r cause for c o n c e r n is leakage, which may follow p o o r quality control or suit d a m a g e . I n the p r o t o t y p e suit a n d the test samples, the o u t e r skins as supplied were c o m p l e t e l y free of pin holes. L e a k a g e n e v e r occurred from the p e r i m e t e r lap joins, but it was p r e s e n t to a slight d e g r e e in the butt seams of the p r o t o t y p e . Massive flooding of i m m e r s e d samples, even with u n s e a l e d edges, was quite difficult to achieve b e c a u s e w a t e r does not readily flow t h r o u g h the o p e n cell foam. A t the p r e s e n t stage of d e v e l o p m e n t it is not possible to m a k e definitive s t a t e m e n t s a b o u t diver acceptance. As a wet suit, the p r o t o t y p e was r e a s o n a b l y c o m f o r t a b l e to wear. D i v e r ' s acceptance of new e q u i p m e n t , h o w e v e r , also d e p e n d s critically on such factors as quality control, g a r m e n t life, safety, a n d total costs i n c l u d i n g those of inspection, m a i n t e n a n c e a n d repair. T h e s e factors can only be d e t e r m i n e d by f u r t h e r work u n d e r field conditions. Acknowledgement---Contributions to this programme were obtained from resources other than CSIRO by way
of material donations, manufacturing facilities, time and skilled help. In particular the participation of Messrs A. Wood of Dunlopillo, D. Dunne and A. Boutlis of Leggen Rubber Industries and I. Barany of Abalone Aquasuits is gratefully acknowledged.
REFERENCES BARTHf~L,~MV, L. 1970. DcSperditions calorique et protection thermique du plongeur. Travail Hum. 33, 195-216. BECKMAN,E.L. 1963. Thermal protection during immersion in cold water. Proc. 2tzd ,S~vmp. on L"tzderwater Physiology. Edited by C.J. Labmertson. BECKMAN,E.L. and FREY,HR. 1967. Electrically-heated pressure compensated wet suits for Sealab II. U.S Office of Naval Research, Report ONRACR 124, Chap. 37. Bramham, E., Dineley, J. and Wharmby, B. 1972. Heat loss compensation in deep diving. Hydrospace, 20. BURTON,D.R. and CttAN, C.Y.L. Experiments with chemically-heated diving gloves. Mar. Technol. Soc. J. (ill press). CHAN, C.Y.L. and BURTON,D.R. 1981. Local heating source for shallow water divers. J. Pwr Sources 6. 291-304. LONG, R.W. and SMm~, N.E. 1972. Hot water: an economical approach to increased diver performance and safety in the offshore oil industry. Offshore Technology Conference, Paper No. OTC. 1563, Dallas. MYLOrrE, J. 1973. Development and evaluation of deep sea swimsuit materials. U.S. Navy Clothing and Textile Research Unit, Technical Report No. 108, N.T.I.S. AD. 763378. NORTON,M.P. 198l. Theoretical considerations concerning prediction of diver wet suit insulation behaviour with and without pressure compensation. J. appl. Energy, 9, 85-105. NORTON, M.P. and CHAN, C.Y.L. 1981. On the insulation properties of composite cell foamed materials suitable for wet suits. Divetech '81, Society for Underwater Technology, London. OHSAWA,T., MIWA.N. and NAKAYAMA,A. 1979. A study of composite foams for diving suits subjected to high hydrostatic pressure. J. appl. Polvm. Sci. 23. 1233-1245. PENZIAS,W. and GOODMAN,M.W. 1973. Man Beneath the Sea - - A Review o[" Underwater Ocean Engineering. Wiley-Interscience, New York, pp. 586--631. Rocco, R. 1972. Passively insulated diving suit. Syrnp. Proc. The Working Diver, Marine Technolog~ Society. WATrENBARGER,J.F. 1977. A development program in diver thermal protection. A.S.M.E. Intersocietv Conference on Environmental Systems, San Francisco.