I will readily admit, I was not born working in a
dive shop and sometimes I don't have all the answers. So
when customers ask me why we don't use a water tank to do air
fills, I don't always know how to answer that, except to say
that my boss says it's not necessary. Not wanting to sound
stupid, and having enough faith in Dave's judgment to feel he
must have a pretty good reason for having this opinion, I
decided to satisfy my own curiosity. What I came up with
will probably surprise you - it sure surprised me.
The first argument I always hear concerning wet
versus dry fills is safety. It is a common belief that a
tank, pool or garbage can full of water is going to protect me -
and you, if that tank should rupture. Well my friends, I
have been studying the visual inspection manuals and judging
from those photos they publish of tanks that have ruptured, no
can full of water is stopping that missile from breaking through
it - and the concrete wall next to it, the building next door,
and maybe a few other things.
I've heard stories of tanks rupturing and never
being FOUND, so the safety argument doesn't hold water, if I may
make a pun! The only thing that will protect me - and you
- from that tank rupturing is if I follow all safety precautions
set forward by the DOT when I fill a tank. These include
checking the hydro and visual inspection dates on the tank
before filling, giving the tank a quick visual check to make
sure there is no new damage (which by the way is my right, as
the person who is most in danger, to make the decision to fill
or not fill that tank!) and not filling that tank over it's
service pressure limit. That limit is stamped right on every
tank. This is not the manufacturers OPINION as to how much
pressure the tank holds - it is the law.
Now, just because a tank is cooled in water
doesn't mean it's O.K. to overfill it. There are very good
reasons for not over filling a tank ESPECIALLY if it has been
cooled in water. Let's say the fill station operator uses the
water bath, and fills that tank to a full, cooled down 3000
psi. That tank will have to be transported to the dive
site in most cases, correct? Inevitably, that tank would
be subject to a rise in temperature, whether in the car, or the
hot sun on a boat. There is simply no way to guarantee
that tank will stay as cool as it was while immersed in the
bath. Without that margin of safety (the drop in pressure
that nobody likes!) that tank will be a prime candidate for an
over pressure incident. Of course tanks are now equipped with a
burst disc designed to relieve pressure if over pressure should
occur, but I sure would not feel I did my job in a responsible
manner if a tank I filled "blew" on the way to the dive!
Of course, this is assuming the water bath did actually work.
Experts argue that the water is never cold
enough, nor is the tank in the water long enough to do much
cooling anyway. Typically, customers just do not want to
stand around and wait, so the tank is yanked from the water
pretty quickly - before the water has had a chance to stabilize
the INTERNAL temperature of the tank. Which raises a
question in my mind: Wouldn't it then make sense that if we are
over heating the interior of a tank by a too - fast fill, but
cooling the exterior with water, that all we really are doing is
adding to the stress that metal is exposed to, and therefore
shortening the life span of that tank!?
There are other very valid reasons for not using
a water bath, which are backed up by the observations of some of
the most noted experts in the field. Water can and does
enter the tank valve during the filling procedure, either by the
wet hands of the operator, or by inadvertently being immersed
while open. The filling whips themselves get wet, and
submerged on occasion too. Then the water is forced into
the tank under high pressure during the fill! This is one
of the leading causes of water intrusion in scuba tanks! Think
about it; if you've ever had a visual inspection done and were
told there were signs of water intrusion in your tank, but you
swear you never sucked that tank dry, it could have been due to
the filling practices of the shops you have used.
I could pretend I know what I'm talking about and
try to impress you with mathematical equations, but I will just
tell you what I have learned: I learned that it is simply a fact
of life that every tank will experience an average of a ten
percent loss in pressure between the fill and the dive. (Ideal
situations are actually 6 percent, and in the worst case
scenario, only 15 percent!) That means that a tank filled to
3000 psi will actually have about 2700 psi in it when it arrives
at the dive site. It's not anybody's FAULT. Nobody ripped
anybody off. It's nature and Physics, not hocus-pocus or a
dive shop that doesn't give "good fills". And any attempt
to alter that outcome is just plain silliness, just for a few
extra gulps of air! (The funny part is that, typically, the
customers who complain the loudest are our LOWEST SAC rate
divers! They always bring in their tanks still half full!
Go figure!) That shows you guys were well trained - you dive
safe. You dive calmly. You know how to breathe! So how come this
10 percent thing is such an issue? Nobody ever tells me they
actually ran out of air. In fifteen years I have only done it
once - after a scary, insane 138 foot dive in which I had the
doodad scared out of me - and that was with a steel tank. My
point: You just don't usually need the extra air. Most of the
divers that have trained here tell me they usually have more air
than table time anyway.
You will see us fill our tanks over 3000 psi on
our gauge - OUR GAUGE being the operative phrase here. We
are aware that our gauge reads slightly higher than our other
test gauges, and we check it regularly. We don't want you
to feel cheated. But we also want you to arrive at the
water with all the air in your tank, not rushing out the burst
disc. I don't want to ever be the cause of an accident because
somebody forced me to go against my better judgment. I
know all of you would agree with that logic. If you give
us a little extra time - say twenty minutes, so the temperature
in the tank can stabilize - we'll make sure your tank leaves
here full.
As good divers we should always be learning. We
should always be willing to make small concessions for the sake
of safety. None of us knows it all. So why take
chances? Remember, this is supposed to be fun, not a proving
ground. Save that for the people who get paid to take
chances. We want you and your tank back in one piece...