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I'm hoping one of you might be able to shed a little light on a 
regulator failure I saw yesterday... so far I haven't thought of 
anything that completely explains what I saw.  This is not about
"technical" diving by anybody's definition, but I don't know of
a more appropriate list where the readers would have an adequate 
understanding of regulator internals for this question.

The guy I was diving with had a Cressi-Sub Air Max with a piston 
first stage.  I'd never seen one before, so all I know about them 
was what I saw yesterday and what I see on their web site:

    http://www.divemar.com/cressi.html

Before the dives I checked his gear to see that it was hooked up 
properly, that the tank valve was on, and that the 2nd stages 
breathed OK.  The only problem I saw was that he accidentally 
pulled the mouthpiece loose on his primary, which was easily 
fixed with a spare zip tie.

The first dive was uneventful... No problems except that the guy 
was very new to diving, with the buoyancy/trim control and air 
consumption problems that are common with inexperience.  He also 
was a little bothered by what he perceived to be cold water (about 
58F on the bottom but more like 67F in most spots), so we agreed 
to stay above the 2nd thermocline on the next dive.

9 minutes into the second dive we were at 40 feet.  I checked
his pressure and he had 1800psi remaining.  We continued, and
very shortly thereafter when I checked for him he was "gone".
I looked for a minute, surfaced, and he was there.  He said
right after we started swimming again he got two hard breaths
off his primary, then nothing at all.  He tried his octopus,
got nothing at all, so he went for the surface.  He said he 
tried both purge buttons at depth, and got nothing from that 
either.  At the surface, he was able to purge and to breathe
from either 2nd stage, but both were extremely hard breathing.
On the surface swim back to the entrance, he breathed from the 
primary a little, and it locked up on him again.

Back on land... The pressure was still 1800psi, and the tank
valve was on as it should have been.  I tried breathing from 
both 2nds, and each was extremely hard.  Purges and power 
inflator seemed to work.  I borrowed an intermediate pressure 
gauge, and when I hooked it up it read 120psi.  I pressed one 
of the purges, saw the IP drop normally, then go up to 126psi 
when I released it.  Then over about 10-15 seconds, the IP 
crept *down* to 120psi.  I repeated this several times, and 
it was consistent.  I shut everything down and took the first 
stage off the tank, and the filter looked clean.  The whole
reg looked externally to be in good shape.

Any idea what might have gone wrong?  The IP seems low, but 
not ridiculously so (I see on the web site that they recommend 
setting it to 130).  I'm puzzled by why the IP would creep down 
after I purged the 2nd stages -- I've seen regs creep up before, 
but mainly I've worked on diaphragms so perhaps there's something 
about a piston reg I just haven't considered.

I'll never see this guy or his regulator again, so there's no 
way for me to follow up and know for sure what it was.

- Todd

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