Tmccar1:
Does anyone have any recomendations or preferences for FFM's? Specifically
with the Prism?
An opinion on diving Rebreathers with Full Face Mask * Peter F.
Readey
In shallow water a standard Full Face Mask (FFM) with a low dead space or
oral nasal is appropriate for communications and for some cold-water
applications with communications. It has been our experience if an oral
nasal type system has a weeping seal (nicked seal, debris etc), the user
often does not recognize slow water ingress into the system, which can
result in serious flood in the case of inversion, or any significant change
in body position.
Furthermore, a standard, open-circuit type, oral nasal design has proven to
be inappropriate for deep-water heavy exertion diving leading to
considerable problems of CO2 build-up.
A ³T-bit² design alleviates some of these problems to a large extentbut does
not resolve the large air volume typically associated with a FFM. On some
systems it is not possible to completely clear the FFM with a single lungful
of gas. Kirby Morgan Super Mask with Rebreather pod and ScubaPro low volume
masks are a good combination of T-bit style FFM, with low dead space
designs.
Although the T-bit FFM assemblies with the lowest volume will function at
depth, bailout to open circuit with a switchover block are often problematic
as the regulator orientation, being far away from the users mouth has
hydrostatic issues ranging from positional free flow to extremely high
inhalation resistance at a given body position. Not to mention single point
failure at the regulator.
A retaining strap attached to a neutrally buoyant mouthpiece has proved
acceptable in many extreme environments.
This is a contentious issue as there is not a single or simple solution to
the problem. Although some of our customers use a FFM with the PRISM, Steam
Machines, Inc does not recommend the use of a full face mask with less than
100 hrs of RB specific dive time and only for specific application with
extensive, appropriate training for that purpose.
