Chapter 4 : Key Issues in the Construction of the Immersion Suit

PREVIOUS PAGE | NEXT PAGE

What is the State of our Immersion Suit Technology in 2002?

In 1986, Brooks (Reference 28) was depressed by the poor quality of suits provided to him for testing. Brand new ones straight from the manufacturers leaked, zippers seized up, ties on suits and gloves tore on initial donning and little attention had been made to sizing and fit. Furthermore, the lifejacket / survival suit interface was very poor. He therefore reviewed the whole topic and concluded that no suit in existence fulfilled all the criteria for an immersion suit and that in its present form had reached the peak of its development. All credit should be given to the pioneers since 1939 who had managed an uphill struggle against much adversity and resistance to produce a reasonably good immersion suit if manufactured correctly. However bluntly put, the one-piece immersion suit developed for the RCAF in 1945 could do nearly as good if not better job than the suits built in the 1980s (Figures 15 and 16). As stated above, the introduction of cotton ventile fabric later superseded by Gortex and introduction of the reliable waterproof zip in the 1980s, had made only marginal improvement in overall performance. He had hoped that the paper would stimulate government, industry and academia into looking at new concepts. This next section addresses the key issues in the design and construction of a good or a poor immersion suit.

Figures 15 and 16: RCAF Ferry Pilot suit, developed in 1945 (left), and the Canadian Forces CF18 pilots immersion suit developed in 1990 made from Nomex Gortex (right).

 

    

Water Integrity

If the suit is to be designed to protect from the four stages of the immersion incident, then until some creative idea is conceived, it must be waterproof. This brings up the problem of how to close the suit and how to seal it off at the hands and feet.

(a) The neck seal for quick don suits

The draw string method is very simple to manufacture and operate even with cold hands, but it leaks to some degree (Figure 17). This is made worse if the wearer has a poor freeboard. It will also leak on initial water entry. This can be ameliorated to a high degree if the drawstring is enclosed in a very soft rubber sleeve bonded to the collar of the suit. However, it is very good in its application for suits developed for mass abandonment; in this case protection from cold shock and swimming failure are the paramount threat and there will be a lifeboat or liferaft immediately available. It is very useful for suits donned quickly over existing clothing during abandonment. Whatever suit it is applied to must be used with a lifejacket. This system was well proven in the Falklands War.

Figure 17: Quick-don, once-only suit, note draw string to seal the neck. The advantage is that it is simple, cheap, and can be made for one-size-fits-all type suits.

 

(b) The neck seals for constant wear suits

This is achieved by bonding a wide rubber band around the neck. In order to make it a more comfortable suit, the rubber neck seal has been split in the center by a zip. Thus, in theory the neck can be left open for normal work and closed only prior to water immersion. However, this can tend to produce an uncomfortable lump under the chin as in Figure 18. Inserting a comfort flap is a good solution (Figure 19) but the flap must be well designed to avoid becoming snagged by the zip during rapid closure. An alternative to the central neck seal is an offset neck seal. This zip which closes at the side of the neck works reasonably well if the zip can be secured firmly in time (Figure 20).

Figure 18: An example of a central split neck seal. The end of the zip tends to fit uncomfortably on the larynx.

 

Figure 19: An example of a central split neck seal where an additional flap has been added for comfort.

 

Figure 20: An example of an offset split neck seal.

 

A modification to this idea is to extend the zip into the front (Figure 21) or the side (Figure 22) of the hood. Now the water integrity of the suit depends on the seal of the hood around the margin of the face rather than a seal around the neck. This type of seal must be closed in plenty of time before immersion because of the precision required to adjust the hood on the face, to get all the hair comfortably underneath it, and to ensure the zip is pulled right to the top and secured. It is also not very comfortable to wear this type of suit for any length of time when fully secured for a helicopter if the operators insist on it being secured during flight.

Figure 21: An example of a suit where the suit hood is utilized to protect the neck seal. The zip is positioned in the center and makes for an uncomfortable seal under the mouth and nose.

 

Figure 22: An example of a suit where the suit hood is utilized to protect the neck seal. The zip is positioned at the side to give better clearance for the nose and mouth and less discomfort when secured.

 

The problem with any suits that utilize the hood to protect the neck seal is that when in the water there is a limited field of vision and ability to hear vital orders. If it is undone in the water, or incorrectly secured in the first place, then the whole water integrity of the suit is compromised. It must also be noted that in a downed, flooded inverted helicopter escape the immediate hydrostatic squeeze on the suit during water entry can cause a sudden rush of air into the hood which simply blows it off. High volume, low working pressure relief valves in the shoulders or the hood work well are essential and prevent this. They however add to the cost and complications of the suit (Figures 23 and 24).

Figure 23: Students strapping into a TEMPSC show an example of a relief valve fitted in the hood to prevent the rapid escape of trapped air from blowing the hood of the face.

 

Figure 24: An example of a relief valve fitted on the crest of the shoulder. In this case, a valve is also fitted on the other shoulder and in some cases, valves are fitted in the feet too.

 

Various attempts have been made to manufacture a loose neck seal to allow suit ventilation that can be sealed rapidly prior to immersion. Thus far, quick tightening systems (Figure 25) around the neck tend to leak. The reason is simple – the engineers believe the neck to be a simple cylinder, and any form of circlip that tightens around it will provide water integrity. This is not so, the neck is a complex oval shape with a protrusion anteriorily for the larynx. Thus to date, the only simple and reliable method of providing a seal around this shape is by the use of a continuous rubber neck band (Figure 26). The softer and more pliable the rubber, the better the seal and the better user acceptance. The disadvantage is that user acceptance is not good except in groups such as the diving community who use it on a daily basis. Some companies supply the suits with the neck seal tapered with three consecutively wider concentric rings, the user cuts the seal down to the ring which fits him / her best. People generally find it hot and sweaty, irritable against beards or clean shaven faces. Nevertheless, with current technology, this is still the best way to achieve a reliable watertight neck seal.

Figure 25: An example of a ratchet type seal to close the neck.

 

Figure 26: Still the best method of making the neck watertight is the continuous rubber neck band.

 

(c) Method of entry into and closure of the suit

i) Access through the neck

Going hand in hand with the neck seal, must be the design of the suit closure. For very simple quick don suits intended for rapid abandonment with as much clothing as possible, the simple bag type suit with wide entry through the neck is best. The suit is then sealed by the drawstring (Figure 17). The disadvantages to the drawstring are as discussed above: leakage on water entry and water entry when in the water if a poor freeboard has been achieved by using a not very effective life jacket.

ii) Access through the front

The second method is one of several forms of front entry suit with either a continuous neck seal, split neck seal or hood seal. In each of these a waterproof zip is used. Modern zips, albeit expensive are very good if properly maintained. First, the zip can be run from the crotch in a vertical direction to the center of the split neck seal on the front of the larynx (Figure 18). Unless secured very tightly before immersion, it has the disadvantages of leakage and discomfort when sealed. Second, the zip runs from the crotch to the side of the neck seal (Figure 20). The problems are similar to those of the zip that ends in the front of the neck seal. Third, the zip runs from the crotch to the front or side of the hood (Figures 21 and 22). Problems with the hood seal are discussed above. Fourth, the suit incorporates a continuous rubber neck seal. The wearer must gain access through a frontal opening and pull the upper torso portion of the suit including the neck seal over the head before zipping up securely. A diagonal zip runs from the crotch to the left or right shoulder to secure it (Figure 27). It is better to end the zip at the shoulder and not at the crotch. If the ending is at the crotch and the user does not secure it correctly, then the suit will flood up very quickly. Fifth, the suit again incorporates a continuous neck seal, but the zip starts at the left hip, runs across the back, and then diagonally across the chest up to the right shoulder. Providing the longer zips are well maintained, both these diagonal zip designs make it very easy to put the feet and legs into the suits and pull the neck seals and upper portion of the suit over the head.

Figure 27: Access to the suit can be gained by a diagonal zip. This is a good design for easy donning of the suit.

 

Sixth, the torso portion of the suit is opened out in half by a W-shaped zip. This starts to one side of the umbilicus, runs diagonally up and backwards around the back of the chest and then returns down the other side diagonally downwards to the other side of the umbilicus (Figure 28). It can be operated single handed. This type of zip provides the greatest aperture for donning the suit. It has the advantage that the suit can be worn for instance in the crew room or bridge of a ship only half donned with the sleeves folded in front across the chest.

Figure 28: W-shaped zip that provides easy access to the suit for donning.

 

Seventh, entry into the suit is gained by a long, horizontal aperture running across the chest from right to left armpit (Figure 29). The disadvantage of this system is that due to the folds in the suit it is not very easy to make the final seal. Pull tabs have to be incorporated into both wrists to approxminate both edges of the zip, so that the slider can run smoothly across the chest and make the seal. This only adds complications to the suit and increases the cost (Figure 30).

Figure 29: Access to the suit can be gained by a horizontal zip

 

Figure 30: The horizontal zip needs additional ties to straighten out the two side of the zip prior to closure.

 

iii) Access through the back

All the back entry suits depend on a continuous rubber neck seal for the neck. The first type of suit is closed by a horseshoe zip (Figure 31). This starts on the front of the left side of the chest, runs around the left shoulder across the tip of the shoulder blades, around the right shoulder and down to the front of the right side of the chest. If sized to the individual correctly, it can be operated single handedly, makes a very comfortable fold in the suit, and it is easy to don the suit. Like the W type of zip suit, it gives very good access for donning and the front half can be folded down. The suit can be worn partially donned with the sleeves tied in the front.

Figure 31: The horseshoe zip. It can be secured single handed and is easy to don.

 

The second type of suit favoured by commercial divers is closed by a horizontal back zip running from armpit to armpit (Figure 32). This again is a good system, it makes a comfortable fold in the suit, and the suit is easy to don. The disadvantage is that a second person must secure the zip, the suit cannot be donned single handed.

Figure 32: The rear zip, which makes a very comfortable crease on the back, but the drawback is that it cannot be secured single handedly.

 

iv) Other methods of closure

Under development by the US Navy is a very long zip that starts on the front of the mid chest, runs through the crotch and up the back of the suit. What advantage this system has over current systems is unclear at present until a production version is trialed.

(d) Closure of the wrists (and decision about integral or separate gloves)

The best guarantee for a water tight seal at the wrist is to incorporate the glove, whether it be a five finger glove or a lobster claw type of glove into the suit (Figure 33).

Figure 33: The standard three finger lobster claw type glove incorporated into the ship abandonment suit.

 

This, in practice works very well, but any tasks that require fine tactility will not be easy. A second option is to incorporate the glove into the suit and have some form of zip that allows the hands to be free. This works better in theory than practice. If the hands are free as may be the case for helicopter passengers, then on the command ditching the hands must be placed rapidly inside the gloves and the gloves secured by the zip. The first hand gets zipped up correctly, but the second hand due to loss of tactility of the dominant hand, barely gets secured. If there has been any hand injury, the second glove will likely not be secured.

A modification to this idea is to fit a glove to the suit and protect water running up the sleeve by a continuous rubber wrist seal. However, this does not solve the tactility problem. Not only is it still difficult to secure the second glove, but also unless the glove is well designed and reinforced at the apex of the zip, the uneven pull on the zip often rips the neoprene rubber glove.

The general consensus of opinion is to secure the wrist seals with a continuous latex rubber seal and place one glove into a pocket on each sleeve (Figure 34). It is very important during servicing to powder the seals well with talcum. This prevents the thumb or fingers from puncturing the seals during donning. For more sophisticated suits such as the submarine escape immersion suits, the wearing of a cape leather glove on the suit protected by a conventional rubber wrist seal provides enough insulation to allow function to do critical tasks. Then, for the long term survival an over mitten with foam insulation is provided. Servicing the suits with continuous wrist seals is labour intense. Suits can become unserviceable because the individual pushes his/her finger or thumbnail through the rubber seal. The old seals and glue must be stripped off the suit, the fabric cleaned and the new ones glued on. This all takes time and money, and occasionally will prevent people flying offshore until a serviceable suit is obtained. A concept in use by the diving community is a quick release rubber seal that is held onto the suit by a rubber toroid seal which fits into a circular plastic groove secured around each wrist on the suit. This allows for the seals to be changed in under a minute. This concept warrants investigation by the marine industry (Figure 35).

Figure 34: Probably the best method is to secure each glove on a pocket on the sleeve and make the wrist watertight with a continuous rubber seal.

 

Figure 35: An example of a quick change rubber wrist seal.

 

(e) Fabric for the suits (with or without incorporated insulation)

The suit is basically made from an outer shell fabric that provides the water integrity and an inner liner which provides the insulation or Clo value. The two can be combined (insulated suit) or separate (uninsulated suit).

The outer shells of the original suits were made from neoprene or chloroprene coated rubber. These are impervious to sweat. When cotton ventile fabric was invented, everyone thought the problem had been solved. This was not to be the case. As previously stated, the fabric is expensive to manufacture and expensive to mass produce a suit. Furthermore, oils and grease degraded its water integrity and to achieve the water integrity it must be made of two layers of materials. The invention of Gortex and then fire-retardant Nomex Gortex fabric has certainly improved the water integrity of the suits once the system of hot taping the seals was perfected.

For those manufacturers who choose to produce insulated suits in two parts, then the options for the liners are very good. There are a number of synthetic pile liners available that can provide different Clo values for different cold water conditions (Figures 36, 37 and 38). They are all hard wearing and launder well. In addition, the new liner can be had with a wicking layer that transfers the water vapour from the skin to the surface of the suit, however, the shell fabric must be breathable for this to work. There are also other thin, flexible foam liners (Figure 38) available that can be used as well. The advantage of producing a suit with a separate liner is first that it is much easier to launder, second, the wearer can add or subtract thickness of liner to match the environmental conditions and third the suits wear well and are not as expensive to maintain.

Figures 36, 37 and 38: Three examples of liners. The one on the left and one in the center are of synthetic fibre of medium (left) and greater thickness (center). The one on the right is made of one of the modern synthetic foams.

 

    

    

The insulation of the suit can be provided by an inflatable liner. The advantage to this is that the suit can be worn as an uninsulated suit for normal purposes (i.e. with the equivalent Clo value of a business suit), and the insulation is only added when the person is in a survival situation. This is a very good idea and is the direction that research should pursue. The Royal Navy marketed the first operational inflatable immersion suit using CO2 for their submarine escape suit in the 1950s/1960s. The principle is used in their current Mk10 submarine escape suit. I.L.C. Dover (Delaware) produced an experimental CO2 inflatable aircrew flying coverall for DCIEM in the mid 1970s that worked well, but it was very labour intense to maintain its gas tightness and it was a very expensive suit to manufacture. Nevertheless, it proved the concept was good (Figure 39). In the 1980s, Shell, the Shark Group and the University of Surrey invented an advanced, inflatable helicopter passenger immersion suit including an integrated lifejacket (Figure 40). This was made from urethane coated nylon. This made it much cheaper to manufacture than the original ILC Dover model. It is R.F. sealed to compartmentalize the CO2 gas. Thus, a leak in one section will not compromise another section. A further advantage is that it keeps the insulative thickness over the back and pressure areas, preventing gas from migrating to the front.

This suit is now in service and represents the latest technology in immersion suits with an integrated lifejacket.

Figure 39: The experimental CO2 inflated ILC Dover pilots immersion suit. One boot has been removed to show the inflatable liner.

 

Figure 40: The Shell, Shark Group, University of Surrey RF heat sealed inflatable survival suit worn with an asymmetric lifejacket to ensure self-righting.

 

Other manufacturers choose to bond the outer shell fabric with the inner insulated liner. The disadvantage to this is that it is not possible to change the insulation according to operational conditions. The shell fabric is usually made from a durable nylon mixture of fabric attached to a three or five millimetre foam rubber (Figure 41).

There are now PVC or urethane coated fabrics that provide good protection from oils and greases; there are good high tensile nylon fabrics that resist ripping and tearing; and there are bondable elasticated fabrics that can be mated to any of the foams such as Ensolite to provide stretch to improve the workability of the clothing. There is also Gortex that can be bonded on a number of different fabrics. Therefore, for the first time, those who work on or over the water for a living can choose the fabric best suited to their specific operation.

Figure 41: Ship abandonment suit that relies on a durable nylon cordura mixture of fabric bonded to neoprene foam rubber.

 

(f) Closure at the feet

Several ideas have been used to close the suit at the feet. One of the better ideas is to provide a pair of Wellington type boots bonded on to the legs. These are very good for walking around the deck, going up and down ladders and scrabbling nets, but have the disadvantage that they have to be sized to the individual. When in the water they are very buoyant, bringing the survivor’s legs up to the horizontal position. People of short stature have difficulty getting themselves horizontal in the water to do essential survival routines or positioning themselves to climb into a liferaft (Figures 14 and 42).

Figure 42: A helicopter suit with rubber Wellington boots bonded to the legs.

 

An alternative is to make a sockette out of the immersion suit fabric with a lightly reinforced sole (Figure 43). Thin sockettes can then fit inside the person’s footwear. Other sockettes have a reinforced sole and do not need an additional boot. In some cases, as in the ship abandonment immersion suit, the sockette can be made expandable and the footwear can be worn inside the sockette. These sockettes generally work reasonably well. The footwear fitting inside the sockette tends to make for a clumsy gait and extra caution is needed for climbing ladders and walking along companionways. Because of the wear and tear experienced in training schools, they require additional reinforcement for training suits.

Figure 43: A typical military immersion suit with sockettes bonded to the legs.

 

Summary of Chapter 4

This chapter discusses the key issues in constructing an immersion suit. Specifically:

  • The difficulty of achieving a good neck seal. The only proven, reliable way is to use a continuous rubber collar around the neck. Split neck seals tend to leak.
  • Wrist seals are also best designed using a continuous rubber collar, but suits can be very quickly made unserviceable if the seals are not well powdered and the occupant punctures the seal with a finger or thumb.
  • Entry into the suit can be made from the front or the back. There are pros and cons to both methods, but whichever method is used, it must be possible to don the suit single-handedly and the zip closure must be of good quality, otherwise the suit will leak badly.
  • Gloves are better provided for as a separate item stowed on the sleeve rather than incorporating them into the suit itself.
  • Rubber Wellington type boots integrated into the suit are the best option for footwear, but must be sized. Necessity and cost may require the substitution of expandable sockettes.
  • There are now a large variety of outer shell fabrics for the suit and inner thermal liners. Having a separate inner liner makes it easier to launder and maintain the suit and match the required insulation with the thermal environment.
  • Overall, the quick don, once-only suit with drawstring around the neck provides a cheap, practical compromise that was well proven during the Falklands War. It is very useful for donning quickly over existing clothing prior to abandonment.

PREVIOUS PAGE | NEXT PAGE