Re: [PSUBS-MAILIST] Seals again

[s_lindblom@conknet.com (Steve Lindblom)]

Stan wrote:
>You make some very good points, so I begin to wonder why not the best of
>both... seals and pressure compensation?   Overpressure is easy enough to
>avoid, which eliminates a the blowout you describe as a drawback.  I just
>find it hard to believe that pressure compensation is a bad thing.

Not a bad thing, just quite possibly an unecessary thing. Like wearing a
parachute on an airliner. Or maybe a better analogy might be to, when you
can't decide whether to wear a raincoat or a parka, wearing both. Why is
the KISS principle so suspect on this list? Note that neither of the
pressure compensation systems mentioned here are benign - both add
significantly to task loading or complexity/failure points.

You say "overpressure is easy enough to avoid". How? By adding even more
components?


>In the debate over pressure compensation or no, I wonder what the shaft seal
>life expectancy is in each case.  It would seem to me that the more pressure
>differential on the seal, the shorter the operating life... probably by
>orders of magnitude, I would think.

We don't have to speculate on what the life might be, because these seals
are used in so many applications that their performance is well documented
and extensive engineering data is available. In my search for seals I've
focused on DPVs, because I'm lazy, and checking out what they use is a
cheap way to get someone else to do the engineering for me. Expensive DPV's
function at depths to 400', and the cheap ones down to 150-180'. Seal life
doesn't seem to be a problem. If it is, well, the seals are cheap and easy
to replace, so it can be done routinely as prophylaxis. What more do you
want?

If there was a record to demonstrate that seals would not do the job alone,
then I'd applaud the ingenuity of those who have come up with compensation
systems. As no one seems to have spent any time at all on the simple
solutions before heading merrily off to the complex, one has to wonder.

We had a guy die recently in the techdiving community who many of us knew
personally. He had, once upon a time, had a scary out-of-air emergency, and
(rather than resolving to watch his gauge more closely in the future, or
any number of other low-tech solutions) embarked in a bizarre quest to
build an absolutely safe SCUBA rig, which evolved over the years, as he
strung one mechanical fix on top of another, until he had a SCUBA rig the
size of a steamer trunk, that weighed several hundreds of pound, with eight
separate tanks and god knows how many hoses regulators and valves. He died
in it.

The reason I mention this, is that after he died, many of us who'd been
laughing at his rig behind his back realized how inevitable in hindsight
his death was, and how we all bore some responsbility for his death, in
that we just treated his rig as a joke rather than trying to talk him out
of it. Not that it probably would have made any difference, but for the
sake of a clean conscience one has to try.