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I'm curious to understand fully how water vapor cycles inside of a typical scuba tank. my limited understanding is that moisture is damaging to the internal tank. i also understand that in some limited very well set up stores, they have gone Thur very complex and expensive steps to remove as much water from the air stream. this is Thur various complex multi steps that each have benefits/draw backs to them. but one example is a very large ion-exchange resin bed that will ionicaly remove atomic water molecules from the inlet air stream, other methods of complex heating and cycling of the air as it passes Thur stages i don't exactly completely understand the chemistry or mechanical reasoning of how they work. Finlay many shops just have fairly dry air as a default of filling tanks indoors in a dry environment. no matter what method used some limited water vapor will eventually be inside under pressure. the various tank manufacturers have also methods of internal tempering that further resists the effects of water and oxygen on the internal surface.

my question is under high pressure what water is inside of the tank supposedly is as vapor not liquid. is that to assume that as a result of the high pressure/vapor phase of the water that it does not affect the internal of the tank as much. or what.

second part of this question is that rusting is accepted as an electrochemical process. in boats an actual voltage is measurable the instant an outboard or motor comes in contact with water. the voltage is actually molecules of metal ions moving across a gradient. in marine applications it is possible by understanding this chemical/electrical process, to reduce or almost prevent this process. example storing outboard in the down position in water is worst, storing outboard dry in the down position will have more electrical conduction to "earth" than electrically isolating it in the trim up position... taking this to the extreme storing your outboard on top of plastic sheeting and under a slight reverse electrical polarization will instead of rusting pick up random ions like electroplating. 
......is it then possible to store the tanks over plastic sheeting also in a electrically neutral environment to further optimize life of tanks.

sorry for long question. any thoughts or comments appreciated.
Letting air out of a tank FAST (as in bleeding it down to take in for a HYDRO or VIP test) is as I understand, one of the worst things for a tank.  That is because the sudden drop in pressure inside the tank causes condensation to form. 

On the other hand...What happens during a HYDRO test?  They put water in the tank under pressure to see if the tank can hold it!!!   :Smile
oh that leads in to my next question, of understanding longer term storage stress on tanks. everyone agrees that should be stored under pressure, but is there an optimal pressure, either compleatly full or half pressure that is beter for tank.

my original question was mainly to understand what happens with moisture vapor in tank under high pressure.
because i noticed that there is a verry wide range in how varrious dealers are handeling moisture in the fill stations.actualy i think most are not going to any trouble to do the extreame removal. so the current belief must be that there is no problem with some limited amount of moisture in the tank under pressure.
ion exchange beds are a pain and expensive to maintain especialy if unnessessary.
For industrial compressed air dryers are used and they work two ways:
as defined in the second law of thermodynamics......

1. A Gas under high pressure can hold less moisture vapor i.e. it condenses
2. Cold air can hold less moisture therefore  refrigerant coolers are used (to freeze the air).
     As lower the temperature the less moisture will be suspended as vapor (non linear)

The above should explain the mechanics behind compressed gas and moisture. As usual there are a few different ways to dry one is cooling, freezing another is sillica gel a third for very dry air etc. The efficiency is is always a question of flow rate in those systems

Andreas
Storage pressure came up some time ago, you should be able to find it. I posted the email I got back from Luxfer.

If I recall, they kind of dance around giving a commitment but something like 500PSI is reasonable. Luxfer never did commit to a number. I think it's a lot of reasons, most of which you can think of in a few minutes but the one I consider the most logical is if they say you shouldn't store long term its admitting Luxfer tanks suffer from sustained-load cracking, Which they don't (anymore). So they want to say leave them full ("it don't matter") but that's just asking for trouble.

Most of the science says the work hardening (AL) from repeated drain/fill cycle is more destructive than constant stain at high pressure.

I don't think it matters and I store mine full.
yes thanks for these response. this is kinda the info im searching for. is it reasonable to further extrapolate that the ideal use would be to fill to max before using, then dont cycle it so low, say less than 500psi, long term thru several months of non-use leave them at 500-1000 then fill as used.this would prevent that max cracking and also reduce the work hardening that AL is known for having problems.
Interestingly i had forgot my basic chemistry of gas. and acutaly im not sure you stated this correctly that as pressure increases solubility lowers??? if this is so, then at max pressure without absolute dry air, water would poool at the bottom of the tank
..........Found it........
I got the actual answer off another forum. very interesting info so im sharing it.

...
Here is a link that suggests that the dewpoint of the air inside the tank is a function of pressure. Even if the air is -64 deg F at 1 ATM, it could get to +28 deg F at 3100 psi:

Moisture In SCUBA & SCBA

So, if the tank got below 28 deg F, the water would condense on the surface of the tank.
...................
If you Google for 'dewpoint versus pressure' you can find a lot of information. Among the hits:


............

If i understand correctly(the dewpoint formula relationship) by increasing pressure you increase the dewpoint point pressure. If this relationship exists then that would explain the benifit of storing tanks long term under higher pressure mainly as moisture benfit . also interestingly as a buyproduct of the gas temperature law, i notice that there is a benifit to fill your tank on colder days as the air used to fill the tanks would have the lowest amount of moisture. and since the total pressure is the combination of the partial pressure of the varrious gasses, the usable air might be in some ways significantly more!WOW.....Winter Air fills may be almost a collectable comodity
What I remember is steel tanks should be stored at around 500# and Al tanks can be stored at full pressure.
the really good dive shops around the country that are filling tanks have driers working all the time. The heat takes the moisture out of the air and lines, and by the time the air is delivered off the banks of tanks it should be pretty free of moisture.

WHICH SHOPS IN MINNESOTA DO THIS? 

Nic
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