Moisture in the tank?

[The Chairman]

The discussion was contamination DOWN STREAM fro a leak. This "leaching" of the surrounding gas is well documented in both high and low pressure systems that are flowing past a leak. While the various pressures are indeterminate, it is thought that reduced pressures around the leak due to the diverting gas will cause a cavitation/vortex effect that will suck some of the ambient gas right along with the gas moving in a tube. This is true in automotive exhaust systems where a leak any where upstream of an Oxy sensor will cause it to give an erroneous reading resulting in a fail safe operation of the feedback system in your car.

Expecting an appreciable amount of water vapor to swim upstream against the flow is a fool's errand.

 

[miketsp]

The discussion was contamination DOWN STREAM fro a leak. This "leaching" of the surrounding gas is well documented in both high and low pressure systems that are flowing past a leak. While the various pressures are indeterminate, it is thought that reduced pressures around the leak due to the diverting gas will cause a cavitation/vortex effect that will suck some of the ambient gas right along with the gas moving in a tube. This is true in automotive exhaust systems where a leak any where upstream of an Oxy sensor will cause it to give an erroneous reading resulting in a fail safe operation of the feedback system in your car.

Expecting an appreciable amount of water vapor to swim upstream against the flow is a fool's errand.

The discussion I quoted is not only about downstream.

"But, take a helium cylinder with a known impurity analysis (99.9999% pure Helium). Open the valve to atmosphere and vent it for say 10-15 seconds. Then, close the clyinder valve. The impurity analysis will show a higher level of air contaminates in the pure gas."
Here they're only talking about a pressurised cylinder and impurities getting back into the cylinder. The impurities are measurable after only one experiment. Imagine a cylinder in daily use.

"If the compressed air is very dry, even though it may be at a high pressure, its partial pressure of water vapour will be very low (essentially zero). The pressure of the outside air may be much lower than that of the air inside the pipe, but the partial pressure of water may actually be higher. In that case, the water will diffuse against the pressure gradient due to the driving force of entropy."

I agree it's small but over many hours of repeated use something will get in.
Remember that in normal use the pressure is high but the air flow speed coming out of the tank is relatively slow or even zero most of the time. If you blew any water into your 1st stage on the previous usage that will turn into water vapour when repressurised and will certainly diffuse back into the tank.

 

[rjack321]

From a practical standpoint, that amount of gas "swimming" upstream is irrelevant. There's more moisture in the 14 psi of absolute pressure which started in the tank when the valve was off and it was empty.

 

[captain]

No dought reverse diffusion can occure but I feel the issue is of no great importance in regards to water contamination. By far the greatest chance of water contamination comes from liquid water blown into the tank during filling.

 

[oxyhacker]

A few ppm in analytic-grade He does not equal enough moisture to make bit of difference in a typical scuba tank.

Also, since the water molecules are swimming upstream against the airflow like little salmon, taking advantage of the dead gas spaces adjacent to the walls, we got to assume that the longer and more convoluted the path, the longer it will take them and the less will make it through, and tanks are usually used as part of a relatively sealed system, rather than being vented directly to the atmosphere from an opened valve as in the example cited.

The auto exhaust analogy is a red herring since auto exhaust systems are subject to all sorts of interesting pulse effects that do not occur in a normal HP gas system.

 

[pescador775]

Recently, I removed a couple of steel tanks from storage and put them in service. The tanks were loosely plugged and spent about 5 years in the back of an open shed. I pulled the plugs and inspected before screwing in the valves and filling. There was little to no rust in the tanks. One of the tanks has some light rust before storage and it was the same, no change. I will probably tumble it when the weather cools. The motor heats up a tad in the summer heat. A tank will "breath" in atmospheric air when stored in this condition. Over time, this could cause rust, but it would have to be in a humid environment, like New Orleans; that would do it, I guess.