In another article I already cited the notice which full face mask manufacturer Ocean Reefs gives to its customers: their full face masks can start to free flow while in head down under position. According to this training notice, a diver needs to adjust both the separate exhaust valve as well as the regulator, and even breathing.
At first I simpy thougth to myself «who is going to be diving with her/his head down under?» and quickly laid that special behavior of the Ocean Reef full face masks away. More or less any full face mask construction has its own «features» and odds. Until it dawned to me: this situation can happen as the unwanted and really dangerous result of excess air entering the legs of a dry suit. This is the situation when position and buoyancy is on the brink of desaster.f. down/under, with your head down and your feet up (the mask will start to free flow spontaneously from the exhaust valve. Close the valve [this refers to the separate exhaust valve] and use the flow adjuster [of the second stage] until the leak stops. Adjust your breathing).» [my emphasis, source: Ocean Reef: PADI SPECIALTY COURSE IDM Instructor Guide]
ATTENTION! In this critical situation with loss of buoyancy control due to excess air in the dry suit legs the Ocean Reef full face masks may start to free flow, according to Ocean Reefs instructor guide. And in this dangerous situation they suppose a diver to also adjust her/his Ocean Reef full face mask, while trying to avoid shoting up to the water surface.A free flowing full face mask is exactly what I'm in need of in such a dangerous situation. When I talked to several divers (instructors) about this passage from Ocean Reef's manual they immediately came to my conclusions including freezing regs albeit having never been told about them. So it seems that there is at least some common understanding.
My opinion: personally, I'm expecting of a full face mask that it neither causes any distraction nor increases the danger in any diving emergency situation. A full face mask has to work unintrusively in such situations. Period. We don't deal with toys or showcase items. There is an old and simple rule: «form follows function». A flawless function is essential in any case and situation. Form only follows second.Unfortunately, it's getting worse. Free flowing first and second stages will cool down rapidly. This is because gas expansion (going from high to lower pressure) requires energy to be taken from the surroundings. Luckily, I was at least partl awake when we got to this topic during my mechanical engineering studies.
|Classical design ...|
no problems head down under.
But your first stage is in a position that is much, much worse: going down from, say, 180 bars to 10 bars (high pressure to intermediate pressure) requires a big amount of energy! One of the first things cold water divers learn is that most of the time first stages freeze. Thus, all the luxuriousness spent on an integrated full face mask design is lost to real dangers.
You will probably not be surprised to learn that no other full face mask manufacturer so far has constructed its full face mask in a way as Ocean Reef does. Sometimes, intellectual property (read: patents) can do good, it seems. For instance, Dräger PND, OTS Guardian, Interspiro MK II, Poseidon ... all these masks avoid to separate inhale and exhale valve by a large distance as Ocean Reef does with its masks. By design, this separation makes any full face mask extremely sensitive to pressure differences, such as the ones caused in a head down under possition.
If you don't separate the valves by a large extend, then you avoid such problems. You can also see this in the photo: no adjustment necessary. No free flow, even with my head down under. Okay, mockers may note that I may just have stopped breathing ... but just watch the Kraut Dread training video.