Chapter 3 : Key physical issues in the design and testing of immersion suits

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Specific Investigations into the Effects of Water Ingress (Leakage): Why it is So Important to Keep Dry?

Chapter 1 discussed the critical physical fact that water transfers heat away from the body approximately twenty five times more rapidly than air. However, because of the physiological responses evoked, humans only cool 2-5 times faster in water compared to air at the same temperature. Nevertheless, if the dry immersion suit leaks then there is a serious loss in its Clo or insulation value. In 1956, Hall and Polte (Reference 62) were the first people to demonstrate this using a thermal manikin. For an average man of 1.8m, a leak of 1620 grams would produce a 50% reduction in insulation.

In 1984, this work was extended by Allan (References 6 and 7) and Allan et al in 1985 (Reference 8). They demonstrated that a leakage of as little as 500 grams of water into a dry suit reduced the insulation by 30%! They then prescribed a water ingress test specifically for testing immersion suits which was modified from the original 23 minute test described by Ernsting in 1966. The original test required the subject to jump into the swimming pool followed by three minutes of swimming and twenty minutes of passive flotation in a life jacket. The acceptable leakage after this time was 500 grams. In 1982, the allowable leakage was reduced to 200 grams. The reason for this was that a 500 gram leakage was probably acceptable for survival for one hour at 5° C , but not for longer periods. Allan’s new test recommended a jump test followed by a twenty minute swim test or twenty minute test in a wave tank. The object being to ensure the water integrity of the closures of the suit and the wrist and neck seals (Reference 6). Unfortunately the manufacturers had still not grasped this important point; hence the quality control on the suits was still not good enough and suits continued to fail the thermal tests.

Why is it so Difficult to Keep the Fingers Warm?

The reasons for this have been superbly explained by Beckman et al in 1966 (Reference 18). in their review on the control of body heat loss in aircrew subjected to water immersion. This is quoted directly from their paper in Aerospace Medicine in April 1966 and summarized the pioneering work done by Newburgh, Spealman and Van Dilla in the 1940s (Reference 119).

Figure 10: Insulation of ideal fabric on a plane, cylinders and spheres.

 

(After Van Dilla, Day and Siple in Newburgh - Physiology of Heat Regulation. 1968, Hafner Publishing Co.)

Because of these physical facts, it is very difficult to insulate the fingers. Van Dilla produced a simple figure (Figure 11) to show the relative size of the mitten required to insulate the hands under different work loads.

Figure 11: Relative size of mittens needed for different exposure times at minus 20° F .

 

(After Van Dilla, Day and Siple in Newburgh Physiology of Heat Regulation. 1968, Hafner Publishing Co.)

Furthermore, Burton and Edholm (1955) (Reference 32), made the important comment that a fact know to ventilation engineers for many years was that insulating very narrow diameter cylinders actually caused a decrease in insulation value (Figure 12).

Figure 12: Regional Thermal Insulation

 

(After Burton, Edholm, Man in a Cold Environment)

Finally, Hall et al in 1954 (Reference 61) already noted that body insulation had little effect on hand cooling even when a maximum insulation of 4.7 Clo was worn. So bundling up has no effect unless one increases one’s level of heat production by exercise as is so beautifully demonstrated in Figure 11.

Why do Dry Suits Feel Uncomfortable for Constant Wear?

Each day, even at rest, a human loses approximately 500-850 mls of fluid through the skin. This is called insensible sweating. It has not been unknown for Canadian Air Force Tracker pilots flying over cold northern water off Newfoundland in June, to empty 1 litre of sweat out of their constant wear immersion suits on return from a six hour fishing patrol. As a result the Royal Navy Institute of Naval Medicine currently evaluates constant wear suits with a six hour air exposure, 20° C light intermittent exercise, then thirty minute immersion in 4° C – it is possible to accumulate more than 1 litre of sweat in the feet of impermeable suits, but vapour permeable suits remain almost dry inside.

Unless the suit is well ventilated with open cuffs, neck seals and openings at the feet to assist in the removal of this hot, humid layer close to the skin, the suit becomes hot and unbearable. Berglund (1966) (Reference 23) reviewed the topic of thermal comfort and the effect of clothing. He noted that humans are very good at sensing skin moisture and that their perception of skin wettedness between dry and soaking wet has a high correlation to measured skin wettedness. How skin wettedness is sensed is still unknown. Important to note is that skin wettedness above 30% increases the friction between skin and clothing contributing further to discomfort.

It is beyond the scope of this report to discuss the physics of clothing. For more details, the reader is directed to the excellent NATO Research Study Group 7, Handbook of Clothing: Biomedical Effects of Military Clothing and Equipment Systems with individual papers by Goldman, Lotens and Vangaard (Reference 117).

The Effects of Wave Motion on Immersion Suit Insulation

The majority of the early testing of immersion suits was done in cold water tanks in which the water was gently stirred. This was principally because the research was being conducted in physiology departments of universities which did not have access to wave making machines or large pools. Although it had been known for a long time, probably Goldman et al in 1966 were the first to note and accurately record that there was a difference in the insulation of clothing in turbulent water compared to still water. The decrease in insulation of a wet suit when measured on the manikin was from 0.76 to 0.71 Clo (Reference 58). Then Steinmann et al (1987) demonstrated that the core cooling rate and decline in skin temperature of human subjects were significantly larger in rough water than calm water. Such differences were found for loose fitting wet suits, but not tight fitting wet suits or dry suits (Reference 141).

Later, in 1991 Romet et al (Reference 134) confirmed the Steinmann study by reporting a significant reduction of wet immersion suit insulation in turbulent conditions compared to still water by an average of 29.7% when measured on humans. In 1994, Sowood et al (Reference 137) reported a 30% reduction in dry suit insulation when tested on a manikin in 60 cm waves compared to still water.

Then in 1995, Ducharme and Brooks (Reference 42) examined the effect of 70 cm waves on the dry suit insulation of suits worn by humans. They concluded that the loss of insulation ranged from 14 – 17% on humans and 36% on manikins. They recommended that future mathematical models should recognize this fact, that thermal manikin design should more closely match the floating position of a human in the water and investigation should take place at greater wave heights. This subsequently happened, with the Canadian Navy sea trials off Halifax harbour in 1996. Six subjects were immersed in 2.5° C sea water in waves of two metres height. At the end of the immersion, the dry suits had an average insulation of 1.24 immersed Clo which was not significantly different to values obtained with the same suits in 60-70 cm waves. Thus, to date, until anyone conducts experiments in greater wave heights, the hypothesis is that the loss of 15% in suit insulation plateaus at about a one metre wave height (Reference 30).

How Much Buoyancy is Allowable in a Helicopter Crew or Passenger Suit?

Unique to the helicopter crew and passenger flying over water is the potential for ditching and rapid inversion of the aircraft. The current immersion suits all depend on trapped air in the layers of the suit to provide the thermal insulation. This in turn makes the suit highly buoyant. If, however, the suit is too buoyant, then it will be impossible to make an escape from a downed, inverted, flooded helicopter.

This problem was addressed by Brooks and Provencher in three experiments at DCIEM in 1984 (Reference 27). The first experiment determined how to measure the inherent buoyancy of an immersion suit when inverted underwater. This led to the invention of the underwater weighing chair specifically for this purpose; this is now standard equipment used in the 1999 Canadian General Standards Board helicopter passenger suit standard (Reference 33). The second experiment was conducted in the DCIEM Deep Diving Facility. This was mocked up to represent a flooded Sea King Helicopter passenger seat and emergency exit. The objective was to determine what was the maximum added buoyancy that would overcome the ability of a human sitting inverted in a ditched helicopter from releasing the seat harness and pulling him/herself out of an emergency hatch. Seven male clearance divers conducted the escapes basically dressed in a T-shirt and cotton trousers. After each successful escape, further buoyancy was added until the diver could not escape and had to simply remain in the inverted seat breathing from the diver’s regulator. The results showed a very wide range of buoyancies, which caused problems. The failures occurred between 36 and 57 pounds of added buoyancy. It was established that the largest, strongest diver with the longest arm reach was physically pinned in the seat with 57 lbs of buoyancy. This set the absolute upper limit for buoyancy.

The third experiment was done in an open pool with the same divers (as controls) and also with nondivers. The objective was to investigate the effect of slightly more room to maneuver than in a diving chamber, and also to see if there was a difference for mixed gender non-divers of smaller stature and less upper body strength and shorter arms length. The divers did marginally better, the added buoyancy levels at which failure occurred ranged from 39 to 60 lbs . However, the non-divers were significantly more hampered by added buoyancy and failures occurred between 19 and 40 lbs . The principal difference being due to comfort level underwater, height, reach, upper body strength and shorter arms length. An initial limit of 20 lbs was established for the inherent buoyancy, but with this limit, the thermal requirement for the suit could not be met. Trials were then completed in the helicopter underwater escape trainer at Survival Systems Limited using the prototype suits built to the tentative new CGSB standard. All the students had no problem with escape with an inherent buoyancy of 35 lbs . To assist the manufacturers to meet the thermal requirement, the initial standard of 35 lbs (150 N) was finally established at 42 lbs (175N). This is a good example where groups involved in standards development resolved a practical issue.

Flotation Angle

As discussed previously, the ideal flotation angle is for the body to be resting at 45º to the oncoming waves. However, the additional buoyancy in the suits to protect from hypothermia prohibits this from happening. The majority of people adopt a horizontal position in the water (Figures 13 and 14). This problem has certainly been known since World War ll; it was alluded to by Smith (Reference 136), but was not formally recognized until a presentation made by McDonald at the Robert Gordon Institute ( RGIT ) in 1983: "The overall buoyancy of a very large percentage of thermal protective suits negate the self-righting characteristics of approved life jackets. Suits with inherent buoyancy also show no potential for self-righting, indeed most are equally stable with the wearer face down or face up." Therefore, with this in mind only by integrating the whole system from the basic design can the flotation angle be improved in the next generation of suits.

Figure 13: The problem of an incorrect flotation angle when wearing an immersion suit has been known at least since these tests at the RCAF Institution of Aviation Medicine, Toronto in 1944.

 

Figure 14: A group of subjects in the Bergen Fjord (1986) who have completed a swim away procedure from the helicopter prior to liferaft entry. Note their floating position in the water.

 

Measurement of Clothing Insulation

The measurement of insulation conceived by Gagge et al. in 1941(Reference 47) is the Clo value. This can be measured using humans or an immersion thermal manikin.

At its simplest, heat (H) flows from a place where the temperature is high (T₁) to a place where it is low (T₂) according to the relationship:

H = k(T₁-T₂)

where k is a constant called conductance that represents the ease with which heat flows. The reciprocal of conductance (1/k) therefore represents the thermal resistance to heat flow or the insulation (I) of a material. Insulation can therefore be estimated using the equation:

I = (T₁–T₂)/H

If T₁ is made to present skin/surface temperature and T₂ = suit surface/water temperature and H the heat being lost through the clothing, then the insulation of a clothing assembly can be calculated.

With a manikin, H is represented by the power supplied to the manikin. In humans metabolic heat production minus respiratory heat loss is assumed to represent the heat being lost from the body when body temperature is not changing (i.e. in steady state) (Tipton and Balmi, 1996) (Reference 159). If body temperature is falling this additional heat loss must be accounted for. Alternatively, sensors that measure heat flux can be placed on the surface of the body, under a suit, to measure the heat flowing from the body, through the clothing assembly and to the water (Bell et al, 1985) (Reference 21).

There are advantages and disadvantages associated with the use of both humans and manikins. For example, using humans carries medical and ethical responsibilities; failure to estimate or measure mean skin temperature, heat production and heat flux accurately introduces error, as does the estimation of changes in heat storage when deep body temperature is falling. In its favour, the human technique is more representative in terms of position in the water and fit of the suit; regional fluctuations in heat loss and insulation can be pinpointed subjectively ("it feels cold here") as well as objectively. Also, because a steady state is not required (falls in body temperature can be accounted for), the heat flux technique is quick and can be used to measure the effect of human movement such as swimming; the human technique also allows deep body temperature to be measured and thus insulation to be directly related to this variable.

The benefits associated with the use of manikins include avoidance of the medical and ethical consideration associated with human testing, easier logistics and greater reproducibility. Other advantages include:

1. there is no limit to the number of times the manikin can be immersed in the water
2. tests with manikins give accurate segmental insulation according to strict engineering principles:
3. there is no limit on the temperature of the water
4. the angle of the manikin in the water is consistent and so the Clo value for each suit is consistent and it is possible to do comparative tests between different suit designs
5. the suits can be tested in greater than Beaufort 3 sea conditions²
6. the cost of testing each suit is relatively inexpensive
7. subtle improvements in suit design to improve Clo value can be observed on the manikin where many consistent tests can be done. These improvements cannot be observed on small numbers of humans with different physiological responses to the same conditions.
8. All the cold thermal tests can be conducted on the manikin, yet the leak tests and ergonomic tests can still be done on the human in warm water.

Disadvantages of this method include the mistake that many people make of assuming that manikins react like humans. But, manikins do not react the same way as humans (they do not vasoconstrict, the generation and delivery, and therefore distribution of heat throughout the respective bodies differ). As a consequence, the results from manikins can be misinterpreted. Another weakness in the technique is that to relate the insulation measured on a manikin to alterations in deep body temperature requires the use of a mathematical model, with all the assumptions and limitations which that entails. More research is required to validate these assumptions.

Although we have come a long way in our knowledge, the three disadvantages to manikin testing are primarily related to the fact that the manikin is not articulated like a human and therefore does not ride the waves like a human. The manikin does not respire, nor need to keep the water clear of the oronasal cavity, and it does not vasoconstrict like a human. If these first two facts are examined closer, this means that the human in a flexible position at the surface of the water will tend to have more of the chest out of the water per unit time compared to the manikin. This results in less hydrostatic squeeze, particularly on the front of the suit, and to a lesser degree on the back of the suit. This in turn means that the results obtained from the manikin will be more pessimistic than for the humans. This in itself is not a particularly bad thing, because this means the manikin results will err on the safe side, but the downside to this is that a basically good suit which is close to the line on thermal protection may be failed when tested against a standard. Romet et al. (1991) (Reference 134) concluded there was no significant difference in Clo value of suits measured on humans or the manikin when in cold stirred water, but once waves were added, there were considerable inconsistencies.

This was recognized by Allan (1985) (Reference 5) when he originally persuaded the UK regulatory body to accept manikin testing over human testing. He was meeting resistance from the old die hards who did not wish to give up seeing six subjects sitting in 2° C water for six hours. He argued quite correctly that the results from a manikin test would in fact be more severe than the human test and err on the safe side. In other words, if the suit passed the test on the manikin, it would certainly pass the test on the thinnest human.

There is no doubt in the author’s mind that it was a very good decision to introduce manikin testing in the Canadian immersion suit standards. As a result, the second generation of suits are far better designed and manufactured than the first generation and when well maintained do not leak. However, when it was introduced, funding was expected to continue to refine the thermal link between the manikin and a vasoconstricted human, unfortunately this did not happen. In the current system the Clo value from each segment is summed to provide an overall average, and it is this average that is used in the various specifications and standards around the world. However, the use of such an average wastes the segmental data and can be misleading. The potential for error arises when the results for overall average external insulation obtained from a manikin are used to make decisions about the suitability of immersion suits to be worn by humans. With manikins, high average values for insulation can be most easily achieved by ensuring that an immersion suit assembly provides at least as much insulation, and preferably a little more, over the limbs compared to the torso. However, as noted earlier, on immersion in cold water, a human reduces heat loss from the extremities by vasoconstriction and the major pathway for heat loss is via conduction from the torso. As a consequence of the above, suits may gain approval on the basis of thermal manikin tests that are not necessarily of optimal design for human survival, where it is preferable to concentrate insulation over the torso. This problem could be most easily addressed by having different pass criteria for the insulation provided over the torso (higher) compared to the limbs (lower) (Tipton and Balmi, 1996) (Reference 159).

In response to the perceived problems associated with the use of manikins, some organisations (e.g. CEN , ISO ) have recommended cold water tests with humans. Instead of measuring insulation, deep body temperature is measured and in order to pass in its category a suit must prevent a given fall in deep body temperature in a given time. Whilst this approach is attractive because it involves the direct measurement of the impact of a suit on the important variable of deep body temperature, it also has some disadvantages. These include:

1. It is often difficult to get human subjects to sit in 2° C water for six hours. So, the subject pool to which statistics are applied can be small. This is one of the reasons why all the experiments so far have been conducted on small numbers of subjects.
2. Human subjects do not all behave in the same ways in cold water, i.e. some cool off quicker than others. So, selection of the "right" slow coolers may pass a suit, whereas selection of rapid coolers will fail a suit.
3. It is important not to choose cold acclimatized subjects.
4. It is very expensive to use humans because of the requirements for medical ethics approval, physician services at the pool etc.
5. For evaluation of suits that may fail the test, there is a likelihood of inducing non-freezing cold injury in the human subjects, so ethically and morally, human ethics committees are becoming increasingly unwilling to approve such experiments for pure suit testing to the standard. Alternatively, low peripheral temperature will result in subjects being removed from the water for medical/ethical reasons before a test has been completed.
6. The flotation angle for testing is inconsistent. The suit manufacturer can add a high Newton lifejacket (which may not been worn with the suit) to obtain better freeboard and hence less chance of neck seal leakage and less hydrostatic squeeze on the back of the suit. This results in better overall insulation.
7. The suits can only be tested in calm, stirred water or in a pool with a wave maker. Testing in the open ocean in a sea state greater than Beaufort 3 is not only cost prohibitive, but unlikely to be approved by an ethics committee.

It is concluded that no completely valid way exists of predicting the way a suit will perform in a real sea, during a real accident. However, several ways exist of comparing the performance of different suits in a standard environment. Of these, manikin tests are the easiest and most reproducible. The danger lies in the application of manikin test data to the real world; this danger is reduced as test specifications are more accurately defined (Sowood et al, 1994) (Reference 137).

Summary of Chapter 3

This chapter discusses the key physical issues of design and testing of immersion suits.

• Leakage of as little as half a litre of water into the suit reduces the insulation (immersed Clo value) by 30%. This is why a dry suit is required to protect from the long term effect of hypothermia.
• The insulation value of material on a flat surface is directly related to its thickness. Practically speaking, this means that one can achieve about 4 Clo of insulation per inch of clothing thickness. Increasing the thickness beyond this severely limits a human’s physical function. However, the insulative value of material on a cylinder, i.e. the fingers and toes does not increase linearly with added thickness; no significant improvement in insulative value occurs when over one inch in thickness is added. This is why it is so difficult to protect the hands and feet.
• The human produces (even at rest), approximately 500-850 mls of insensible sweat every 24 hours. Therefore, if a waterproof suit is to be worn, there has to be some method of removing this sweat from the skin surface. It is this skin wettedness that causes complaints that the suit is hot and unbearable.
• Early measurement of Clo value was conducted in stirred pool water. More recent work has shown that in open water the insulative value is reduced by 15% compared to pool water.
• The overall buoyancy of a very large percentage of immersion suits negates the self-righting ability of approved lifejackets.
• Care must be taken in the design of helicopter passenger immersion suits to ensure the inherent buoyancy does not preclude the ability to make an underwater escape from a rapidly sinking inverted helicopter.
• The pros and cons of using manikins and humans to measure insulation of a suit and for passing or failing a suit to a specific standard are discussed.

 

 

2) Beaufort 3 sea conditions (wind speed: 7-10 Knots, 8-12 mph , 13-19 km/h) Gentle Breeze: Leaves and twigs move around. Lightweight flags extend. Long wavelets, glassy sea crests.

 

 

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