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Oh Why, Oh Why
is My Scuba Tank Dry?
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.
If still not convinced, please come in and I'll let you read the materials I have read, which are written by some of the most respected experts in the industry.
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...
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