Oxygen depletion

[WBADD]

Not to beat a dead horse, but I am sure the theory has some basis, but the practical results seem to be BS.

Here are my results of not diving for a couple of years.  Don't ask why I kept these tanks around partially filled.  Maybe it was my thought of using them any day now and not wanting to waste the expensive gas.

36.3% in a steel 72 for 3.5 years then analyzed at 35.7.   
18.3/40/42 in an aluminum 80 for same amount of time, 18.9/x/x
Probably some 02 was used, but the difference is more likely due to two different sensors or user methods done years apart.

It seems likely some or possibly  a lot of o2 was used to make the significant amount of rust dust I discovered in the steel cylinder when performing the past due VIP inspection.  However, it is apparent that it is a small percentage of the overall o2 content.  These cylinders were at aprox 2400 psi. so the total oxygen content is over 160 times what you have at 0 pressure.  Maybe the others test results were done with a tank having 50 psi, this would use a higher percentage of the avaible o2 to form the same amount of rust.

If that is not enough words, think about it this way.  Most rust formation (and o2 consumption) will take place earlier in the storage process rather than later due to the protective oxide layer.  Most would agree that tanks left for 3 months or so will not have a measurable difference in o2 content so the theory that after a year or two you would be down to less than 10% seems unlikely even with a lot of moisture to accelerate the process. 

It would have been very interesting to find out what the other gases were in the tank that had only 6%