Moisture In Tank?

[all4scuba05]

I understand that the compressors at the LDS' remove moisture prior to it going into the tank. But what about the ambient air that entered the tank during a vis? Is there too little moisture in that air to worry about?
Its been said that the air the compressor puts in our tanks is "super dry" but i'm wondering if moisture in the tank is because it doesn't get "sucked" out after screwing the valve back in after inspections. Other than that, how would moisture get in there anyway? Don't we always keep the tank with a atleast a few hundred pounds of pressure?

 

[moneysavr]

Your air should be filltered and the shop should have a in line dryer,but it shop to shop,I have seen some places I will not fill my tanks!
Air will have some moisture,not wet just a small % if you have water in your tank its due to it was run empty and should of had a vis done prior to a fill.
If at all in doubt ask to see the shops air bank and fillters, some may not let you due to Inns co. but others may,some shops even post their air test results.ask questions.
Dive safe,
Brad

 

[Lead_carrier]

The small amount of ambient air will have not detrimental effects on your tank. No where near enough to cause any problems.

 

[spectrum]

The small amount of ambient air will have not detrimental effects on your tank. No where near enough to cause any problems.

If the tank were closed with warm humid ambient air and left closed at 1ATM in a very cold location reaching the dew point would be remote posibility. With the first fill you will be diluting that "moist" air 200X so once that has cycled though a dive /fill cycle it's essentialy all dry.

Pete

 

[captain]

I believe most water gets blown into tanks by improper filling procedures. The orfice on tank valves and filler yokes can get water in them any number of ways and if both are not blown dry before filling water can be blown into the tank.

 

[all4scuba05]

I see the light...thanks guys

 

[pescador775]

Light? Heh. Firstly, a tank is not compromised by running empty unless water is allowed to enter, either by some strange backflow process while diving or during filling where a small amount of water might be present in the valve orifice and subsequently gets blown into the tank. An empty tank with valve closed is safer from moisture than if it has several hundred psi, or especially, if it is full. The empty tank myth is dive instructor, dive shop bullsheet However, an empty tank which is not sealed from the atmosphere will "breath" during temperature changes. Over a few months, this may result in a small amount of "flash" rust or small "dots" of rust inside the tank's wall. Air which is being pumped into a tank contains a small amount of moisture. This moisture is simply added to the moisture present in the air left over from a visual inspection. The air that is being pumped into the tank, since it contains some moisture causes the dewpoint inside the tank to rise as the pressure increases. It does not "dilute" the moisture already present from the visual inspection, that is nonsense. Practically speaking, this small amount of moisture inside the tank is not enough to cause condensation unless the compressor filtration is inefficient, the tank is full and is exposed to very low temperatures. Even then, the condensate which has collected on the tank walls will normally evaporate (become a gas) as soon as the tank warms up. This phenomenon is most likely to occur with tanks which are full and are rated to a high pressure such as 3500-4400 psi. Tanks which are stored with a low pressure or no pressure are not likely to see any condensation no matter what the temperature. In any case, however much pressure is in the tank, the efficiency of modern compressor filter systems is such that any moisture inside the tank is not likely to cause noticeable rust in spite of the additional effect of the small residual moisture which remains after a visual inspection.

 

[stiebs]

My curiousity more lies around how the tank is dried out completely after a hydo..

 

[all4scuba05]

wow...quite some info there

 

[pescador775]

Al4scuba, yours is an interesting question and has many variables attractive to a tech head like me. Also, in answering it gives me a chance to rant against dive shops and instructors, thanks. Picture a tank with a terminal pressure of 200 atmospheres. When the tank valve is removed for more than a few seconds atmospheric air will enter. If the relative humidity is high, the atmospheric air which enters can double the final humidity count inside the tank. That is, the compressor will inject air with a relative humidity about 1/200 (0.5%) or less of normal atmospheric air. Add one atmosphere of air at 90% and you get the picture. However, this also speaks to the fact that modern compressor filter systems inject little moisture into the tank in the first place, so a doubling is not necessarily crucial to your equipment and its integrety. As the tank is drained and refilled this humidity level should decrease since the original humid air has left the tank and is replaced by a second fill from a compressor producing dry air, and so forth. However, one caution; if you are embarking on a dive requiring extremely dry air it might be prudent to purge the tank or conduct the visual inspection on a cold day when RH is 50% or less. Purging is not hard. Immediately following the inspection, charge the tank to 100-200 psi and then drain the tank. After that, fill with hyper air.