In this instructor guide Ocean Reef informs divers using its full face masks that these masks my start to free flow in a head down under position. And that divers need to adjust the exhaust valve, adjust the reg, and finally also adjust their breathing. Quite some things to adjust.
But please see for yourself:
|Source: Ocean Reef PADI Instructor Guide|
Please notice how the diver in the picture has his left hand at the exhale valve and seems to adjust it.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 and use the flow adjuster until the leak stops. Adjust your breathing).» [my emphasis, source: PADI SPECIALTY COURSE IDM Instructor Guide]
Maybe this mask behavior results from a very special design feature of the Ocean Reef full face masks: separated(!) inhale and exhale valves are located away from each oher by quite some distance. Normally, with this special design the exhale valve is well below the inhale valve. However, in a head down under position, it's the opposite. And this then causes the free flow as described in the instructor guide, due to the negative pressure difference in this position.
What really is bugging: Ocean Reef really tells its customers to adjust their breathing? There is not explanation of what they mean by this, and honestly, I have no clue what to do in such a situation. Maybe you are expected to stop your breathing? Seriously?
But what is even worse: in case I unexpectedly get into a head down under position due to excess air in my dry suit's legs, an Ocean Reef full face mask may cause additional distraction as it may free flow.
|See! No problem head down under.|
(In the Sauerkraut, BY/D)
While I was exercising my own drill in the Sauerkraut in Aufkirchen, I had some more fun diving head down. No free flow, no reg adjustments, no breathing adjustments. Simply diving and safely relying on my equipment. That is the way I expect my equipment to work, simply for safety reasons.
Please note that the typical full face mask design avoids any such free flow issues by deploying a boring old and well-proven placement. A placement where the inhale and exhale valve are in nearly the same place: so everything is neatly integrated into your regulator.
Even as the Dräger PND full face mask features a separate purge valve, this valve on purpose has a higher threshold than the exhale valve in the regulator (Dräger uses a special spring with well-defined characteristics just for this). There's a good reason: this avoids the problem of the Ocean Reef design. As I can testify from my own tests and diving experience with the Dräger PND full face mask with the Apeks (X)TX100 stages: there is no free flow with the head down under. Neither the reg exhale nor the purge valves open unexpectedly. There is no need to adjust the regulator. I could find the same correct behaviour also with the OTS Guarding mask during my training for certification: head down under ... no issues at all.