nos...@all.please.net wrote:

I generally consider 'cotton mouth' to be a symptom of being
dehydrated, but there are some mouthpiece designs that cause some
people a problem, so if simpler better pre-dive hydration doesn't
work...or if its obvious that it is a very specific spot in the
mouth...then try changing brands/designs of mouthpieces.

Years ago, I can recall doing the calculation for how much water vapor
it was possible for 80 cubic feet to contain.  My recollection is that
the number came out to like ~2 ounces at its theoretical maximum and
100% efficiency.  As such, its maximum possible contribution is on the
order of "half of a spit" total for the entire dive.

Reconstructing...

http://en.wikipedia.org/wiki/Humidity#Absolute_Humidity

The chart shows a value of approx 28 grams of water per kg of air for
100% Relative Humidty at 30C (86F water temperature), and 15g at 20C
(68F).

http://en.wikipedia.org/wiki/Actual_Cubic_Feet_per_Minute

One (1) cubic foot of air is approx  0.075 lbs mass.

0.075lbm is ~34grams.

At 34g per cubic foot, 1kg of air is ~29.4 cubic feet.  For sake of
convenience, let's round this up by 2% to a nice even 1kg = 30ft^3.

A standard AL80 is actually ~77ft^3 and since we always should be
ending our dive with a reserve (500psi), the actual amount of air that
we should at most be using is going to be 77*(2500psi/3000psi) = ~64
cubic feet.  However, for sake of convenience again, let's assume we
only drain the tank down to around 660psi so that
we ...conveniently... use 60ft^3.

With 2kg of air consumed across the entire dive, that means that at
20C, the total amount of water needed to go from 0% to 100% relative
humidity is 30g.  Similarly, for 30C temperature, you need 56g.

Water is roughly 1g/cc and a liter is 1000cc's, or 33.81 ounces (US).
As such, at 30C, its (56/1000)*33.81 = 1.89 oz.

And for more temperate conditions (20C), its (30g/1000)*33.81 = 1.01
oz.

These "2oz" and "1oz" water mass values are the maximum theoretical
possible amount of water that any sort of "humidifier" system put on
your air supply could possibly deliver across the total duration of a
typical dive.

fin' cannot be all that large, since it can't be 100% efficient.  As
such, the above theoretical values have to be reduced by real-world
system efficiencies and so forth.

As such, the benefit that people who claim that these hydrators work
are reporting an effect that is most likely: (a) Pychological, (b)
Extremely Small, (c) From some other source/combination of design
features, or (d) Some combination of the above.

FWIW, there is a 'wick' type product that's sold, the Apollo Bio-
Filter.  This sort of system is probably a bit better at delivering
the theoretical maximum (of 1-2oz), but personally, I see it as a
breeding ground for various bacteria and the like, since it will
require a high degree of vigilant maintenance to keep it
sanitized...its not something that I would relish just hosing it off
and then let it sit in a tropical equipment shed overnight for a
couple of days in a row, particularly since there's probably some
section of hose that gets left nice and moist because its probably
difficult to access to sanitize and dry.

The question I would have to ask is how much additional daily
equipment maintenance are you willing to add, just for 1-2 oz of
water?

Thanks, but I'll simply hydrate before the dive.

-hh

We don't swish . . . ever.

Lee

*****

Looks like and interesting idea.

 For some value of "get used to it".  On an average dive, I
don't notice it.  On a long, shallow dive, it becomes a real
irritant.
 You can cure the problem by moving to a rebreather.  That will
provide warm, moist air.  There are other drawbacks, but they're
mostly a different set of drawbacks from open circuit.