Emergency breathing is probably best defined as being able to breathe when your normal supply or breathing system has been interrupted or lost. The loss of your primary breathing supply can be sudden and dramatic, or it can sneak up slowly. Regardless, if you survive a loss of gas incident it can be a life altering experience that will change the way you dive from that point on.
Once the diver is in the emergency breathing mode all focus must shift to getting back on a normal supply or getting to a point of safety. In either case, the dive should be terminated unless the loss was momentary, is clearly identified, remedied, and it is certain that there is no damage to equipment that could threaten the safety of the diver and the diver still has enough EGS gas.
A loss of normal umbilical supply can be caused from a pinched or severed umbilical, umbilical obstruction, topside valve alignment error, topside component failure, or helmet component failure. Probably the two most common causes are a pinched umbilical or improper topside valve alignment. A sudden loss will usually come from one of two causes, a severed umbilical, or a pinched umbilical at / or close to the diver. All other losses will usually result in more gradual loss of umbilical pressure that will probably be noticed by the diver in the form of increased inhalation effort that gets progressively harder with each breath. The longer the umbilical, the greater the warning because the pressure drop will be slower. If the umbilical is pinched off within 10-20 feet of the diver, or the umbilical gets severed, there will probably be little or no warning until he tries to inhale at which point the diver will experience the loss during inhalation at which point there is no gas to inhale and none to vibrate the vocal cords, resulting in the inability to talk, scream, or yell to topside. This is where panic can get a foot hold fast because the diver only has 20-30 seconds at best before unconsciousness sets in and here is where a fully functional emergency gas system becomes the most important thing in the diver’s life.
Regardless of the type or brand helmet or full-face mask used, it should always be used with a fully functional EGS system that can be activated quickly and easily. There is no excuse or reason for not having an EGS system. Companies and/or organizations that condone the practice of not using a fully functional EGS system for all dives do so against the recommendation of KMDSI and Dive Lab and may face serious LIABILITY issues in the event of an accident or death. A diver with empty lungs will pass out much sooner than a person with full lungs. History has shown that in almost all cases, divers that think they are tough enough to make it to the surface without a fully functional EGS during an out of gas experience are usually “Dead Wrong”.
Kirby Morgan pioneered the side block/demand regulator system over 40 years ago and that basic design remains the most copied, and the standard by which all others are judged. All KMDSI helmets have a side block that allows two separate sources of breathing gas to be attached. Both supply sources feed a common manifold system that allows gas to be sent to the demand regulator via the bent tube and at the same time directly into the helmet via the steady flow defogger valve. Some divers keep the EGS cylinder valve shut, thinking that they won’t have to worry about EGS gas leakage if the EGS valve gets bumped and open slightly. Keeping the EGS cylinder valve shut is not recommended because the first stage regulator will suffer a squeeze and will flood with water. Flooding of the regulator will ruin it, but worse than that, the flooded regulator may very well extrude the yoke sealing o-ring when pressurized in an actual emergency resulting in a total loss of the EGS. In addition, diving with the EGS cylinder valve shut will require the diver turn on both the cylinder valve, and the side block EGS valve in order to get gas. For these reasons, it is strongly recommended that the diver leave the side block EGS valve shut and the cylinder valve open. It is also extremely important that the EGS first stage be equipped with a pressure relief valve connected to a low-pressure port so if the first stage seat develops a slight creep you don’t blow the hose. With this line up, the EGS regulator and whip stay pressurized to the EGS valve on the side block allowing gas immediately if needed. This line up practice is standard for all US military surface-supplied diving, and is recognized internationally as the safest way to configure an open circuit EGS system.