FFM?

By Jason Rogers (c) 1998

Given the inherent risk of unconsciousness that is a part of rebreather diving, many users are tempted to use full face masks in order to provide airway protection. Adapting masks intended for open circuit diving to fully closed does however present some difficulties. Most OC masks are designed either for surface respiratory protection (firefighting) applications and adapted for underwater use, or intended for use with large amounts of surface supplied gas. Common examples of these are the Interspiro AGA, and the Kirby Morgan Bandmask.

In the case of the Interspiro type of mask, the water seal comes from a tight fit, combined with a positive internal pressure, maintained through the use of spring loaded exhaust valve, and spring loaded diaphragm. Very little water enters the mask but any which does is expelled through the exhaust valve, located at the bottom of the mask. (In the case of the AGA, the exhaust valve is combined with the demand valve, but in many other surface use masks, the exhaust valve is a separate unit) Cool dry air entering the mask is directed across the faceplate, and then into the oralnasal mask built into the mask. Use of flapper valves separates the path of the inhale and exhale gases, reducing dead space, and keeping the faceplate from fogging up.

The problem with this is that the rebreather only provides for ambient pressure within the mask, and so leaks may leak in, not out of the mask. Leakage into the mask will have to follow the gas path back into the loop. In fully closed sets, or semiclosed sets that have high mounted dump valves (e.g. the Atlantis/Dolphin) this water will accumulate within the loop. Additionally, the gas coming out of the loop and being directed across the faceplate is warm and moist. When it contacts the cool faceplate, the gas will dump moisture, and the faceplate will tend to fog up. All this makes this type of mask unsuitable for use with a rebreather.

The Bandmask style of FFM is even worse. Intended for surface supply diving, little or no attempt is made to provide a gastight face seal. Rather it is intended to leak like a sieve, and the water that comes into the mask is expelled via the exhaust valve located in the bottom of the demand valve housing. Additionally a separate hand operated valve can be opened which provides a constant flow of gas into the mask, to defog the faceplate and blow water out of the mask. Gas usage can be prodigious, as can be water ingress. It's more than enough to make this type of mask useless for rebreathers.

There is however another type of mask. Here in Australia, they go by the generic term of "CressiSub", however they are produced not only by CressiSub, but also by TechniSub and ScubaPro. They are similar to a normal scuba half mask but with an extension. The lower skirt has 'grown' to cover the lower half of the face, and the skirt goes across the forehead, down the cheeks and under the chin, sealing the entire face off from the water. There is a simple oval hole, corresponding to the user's mouth, and the mouthbit of the rebreather is simply jammed through the hole. This provides the user with a normal mouthbit to grasp with the teeth and a dry space covering the rest of the face. There is normally no flow of gas within the mask, much like the situation in a normal scuba half mask, and the defogging techniques used with a half mask work as well with this type of FFM. A 'spider' or head harness retains the mask, and so most of the loads from the hoses that would normally be taken by the user's mouth, are transmitted to the whole head, greatly increasing comfort, and allowing the use of larger bore or more rugged breathing hoses. Any leakage will accumulate in the lower part of the mask, and by exhaling via the nose the user can clear the mask, just as with a half mask. In the event of loss of consciousness, the bit will usually be retained in the mouth, but in any case, the nose and mouth both remain dry, thereby preventing drowning. Users switching from a mouthbit to a CressiSub will find that the integrity of the rebreather/user interface is markedly improved and the quantity of water accumulating in the loop is quite minimal. Users of sets that have stiff loop over pressure valves (or no valves at all as is the case with some homebuilts) will find that they can dump gas from the loop nasally, just as they can with a half mask. Also it is possible to remove the mouthbit, while the mask is in place, allowing the diver to speak. This is useful during the prebreathe phase of the dive, where the diver can exchange information with others, without wasting oxygen by breathing atmospheric air and so requiring a second O2 flush.

The disadvantage of this type of FFM, inherent in all FFMs, is that in the event of a loop failure, the entire mask must be removed in order to change to bailout supplies. Apparently there is a new FFM on the market which provides some form of docking port, allowing you to swap supplies without disturbing the whole mask, but I've not had the opportunity to dive this type of mask and see if it will work in a CC application.

As with all FFMs, underwater vomiting will present major problems. I recently scrubbed a dive due to fear of seasickness causing me to vomit into my FFM. I don't have a shut off type mouthpiece, and my best guess for how to handle this would be to try to remove the mouthbit, and vomit into the mask, then replace the mouth bit, flood the mask with water, and then clear it. I have never tried this, and if anyone has met this situation with success perhaps you'd like to contribute to the next newsletter!

If using the spit 'n wash defogging technique, the user will also find that rinsing a mask that is attached to the set quite awkward. A small squeeze bottle of water will greatly facilitate this process!

Another slightly unexpected disadvantage I found when I made the switch to FFM on my rebreather was that I no longer liked to dive alone! The advantage of having a protected airway is that you won't drown, but this is of little use if you simply sink to the bottom to die. I now much prefer to dive in the company of others, who can swim over and push a few buttons on my set in the event of my passing out.

Ask the A.A.R.G.