Emptying a tank ...

[jviehe]

In regards to frost on the outside of a tank, I dont think its a big deal. As long as there is positive pressure in the tank, water from outside cant get in. The air inside should be dry. Heck, tanks are often surrounded by water.

 

[jwalko]

Help me understand why. It seems to me that more pressure means higher ppO2 and therefore faster oxidation. The only thing I can think of is that lower pressure allows water to condense easier. But, if you are getting dry fills, that shouldn't be a real concern unless at some point you've drained the tank and allow water to back-fill.

I'm probably all wet , so let me know!

Tank fill pressure has nothing to do with ppO2. At sea level, if the tank is filled w/ air, the ppO2 is always 21% (.21). It does not matter if the tank pressure is 500 PSI or 3000 PSI. The only way the ppO2 increases is if it is EAN or if the ambient pressure increases to cause the gas to be more dense.

Also, I don't think that low or high pressure in a tank relates to condensation unless the tank is completely emptied and the valve left open to allow humidity or outside water to enter the tank. Maybe others could offer better evidence about this. This is only my opinion based on working w/ compressed air.

John

 

[Sharky1948]

Tank fill pressure has nothing to do with ppO2. At sea level, if the tank is filled w/ air, the ppO2 is always 21% (.21). It does not matter if the tank pressure is 500 PSI or 3000 PSI. The only way the ppO2 increases is if it is EAN or if the ambient pressure increases to cause the gas to be more dense.

Actually pressure has everything to do with ppO2. ppO2 = fO2 X ATA At one ATA, ppO2 = the fraction of O2 or .21. At 3000 psi, roughly 204 ATA, the ppO2 is 42.8 Assuming you've taken an EAN class, remember the ppO2 calcs and realize that (in theory) the regulator would deliver 3000psi at about 6699 fsw.

I.e., as you said, ppO2 goes up when gas is more dense, which is exactly what pressure is all about.

 

[Sharky1948]

Also, I don't think that low or high pressure in a tank relates to condensation unless the tank is completely emptied and the valve left open to allow humidity or outside water to enter the tank. Maybe others could offer better evidence about this. This is only my opinion based on working w/ compressed air.

John

To pick on you again , the dew point (point at which condensation forms) is, in fact, related to pressure. Dew point is <edit> higher at greater pressure, assuming constant temperature.

 

[TwoBitTxn]

Air in a scuba cylinder has a partial pressure of 0.21 on the surface no matter if there is 500 or 3000 psi in the cylinder.

If you bleed off a cylinder quickly you run a risk of condensation forming inside the cylinder as the pressure drops rapidly

 

[Sharky1948]

Air in a scuba cylinder has a partial pressure of 0.21 on the surface no matter if there is 500 or 3000 psi in the cylinder.

If you bleed off a cylinder quickly you run a risk of condensation forming inside the cylinder as the pressure drops rapidly

So why does air breathed had higher pressures at depth have higher ppO2, but air at high pressures on the surface remain at .21. Air released from the tank is at 1 ATA and therefore ppO2 is .21; but, under pressure the ppO2 goes up. That's the definition of partial pressure. You may be confusing fO2 with ppO2. (See my post below)

The sum of the partial pressures = total pressure.

 

[Sharky1948]

Another way to look at it... At the surface (1 ATA) the pressure is 14.7 psi. The ppO2 of air is .21 at 1 ATA or 3.1psi. The ppN2 of air is .79 at 1 ATA or 11.6psi. The pressure in the tank is 500, 1000, 3000, whatever. What accounts for the increase in pressure, given sum of partial pressures = total pressure?

 

[Digger54]

In regards to frost on the outside of a tank, I dont think its a big deal. As long as there is positive pressure in the tank, water from outside cant get in. The air inside should be dry. Heck, tanks are often surrounded by water.

I think you are exactly right. Frost forming on the outside of a valve and tank should not matter one whit so long as positive pressure ultra dry air exists in the tank.

 

[pescador775]

Ouch, some of the foregoing commentators are off the reservation. Yes, higher PPO2 which occurs with nitrox and/or higher pressures could cause more rust. However, this effect is negligible in a tank which contains dry air. As mentioned, the fraction of O2 remains constant regardless of pressure. It is also true that the partial pressure (fraction X gauge pressure) of oxygen is directly related to tank pressure. Thus, a tank with higher pressure has higher PP02 all else being the same. The dew point inside a tank which has been subsequently drained and thus has little pressure is about -60F. The dewpoint inside the same tank which has been filled to 3000 psi from the same air source (the same good compressor) is about +30F Thus, dewpoint RISES as the pressure increases. One might think, "aha, condensation". Well, yes and no. At very low ambient temperature, the humidity inside the 3000 psi tank may release some tiny droplets or frost crystals. On the other hand, practical experience suggests that the minute amount of frost has little or no effect on the tank walls, eg, little to no visible rust. This temperature drop also occurs when draining a tank. Yet, the cooler, near empty, tank is not subject to rust inside because the dewpoint is steadily dropping as the tank releases its air and the metal cools. In fact, if drained completely, the tank's temp would have to drop to -60F to form any moisture at all. That is cold enough to form ice on the exterior wall and freeze finger tips solid. Bottom line, draining a tank, however quickly, will not form rust inside. It is a myth that gained bar room status back in the days when compressors could only achieve dewpoints around -40F. Divers who removed the valve and inspected the tank observed water and concluded that draining caused the water not knowing that the water was there all along due to the poor performance of the compressor filters. Today's compressors consistently produce air with -60F dewpoint and that is really dry. Condensation resulting in rust is very unlikely.

 

[Tanker299]

I have to say that I have learnt a great deal on this thread.
It seems opinions are still divided on the issue; both camps seem to make sense to me, unfortunately (or fortunately) I lack the necessary experience to judge the more complex opinions.
I will take the tank to the LDS, it will give me an excuse to go there off-season anyways.
The folks there can VIP the tank and let me know whether they found any moisture or not, in fact, I can actually be present and view for myself.
I will in fact take these responses and get my LDS's opinion on them. I've come to trust those folks a great deal.

Thanks indeed!

Alex