oxygen bottle question

I have a question about oxy pressure and cubic ft.
I have two oxy bottles, one is 70 cubic inches of volume the other is 100 cubic inches. Both have one outlet, with a one way check valve attached to each bottle to keep each bottle from feeding into the other. these two bottles are connected to each other with a tee after the check valves, then continue on with one line. Both bottles are serviced to 2000 psi each. I have a small leak down stream that leaks app 400 psi in 20 hours. the 400 psi leaks out of the larger bottle only and the smaller bottle stays at 2000 psi. How is this possible, I would think that both would equalize and keep the same pressure. Evidently the cubic feet of the larger has something to do with it. also, each bottle has its own calibrated gauge and this is on breathing oxygen. I have not let the bottles set long enough to find out if both bottles will equalize pressure after the larger bottle reaches the cubic feet of the smaller bottle. thanks for any thoughts

If the leak is at the large bottle- either the shutoff valve or the connection to the check valve, the small bottle would not see the pressure drop in the large bottle.

Is there a built-in regulator with the valve? There usually (always) is on portable medical O2. If the regulator on the small bottle is set even slightly higher than the large one, it will work the way you describe.

At some point, should the leak continue, the small bottle pressure would also decrease.

buster55 said:

Both have one outlet, with a one way check valve attached to each bottle to keep each bottle from feeding into the other....I have a small leak down stream that leaks app 400 psi in 20 hours. the 400 psi leaks out of the larger bottle only and the smaller bottle stays at 2000 psi. How is this possible, I would think that both would equalize and keep the same pressure.

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If the check valves are working correctly and it seems they are the leak has to be between the large bottle and the check valve. If it was past the check valve both bottles would loose pressure. Size of bottles has nothing to do with it.

As above from Jancollc. I ask how doo you know the leak is downstream ?

FWIW do you open the valves fully ?

digger doug said:

. . .do you open the valves fully ?

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If these are regular medical O2 bottles, I don't think they have the 'top seat' like an industrial bottle, but I'm not sure. They usually open with about one turn or even less. The regulator is integral with the valve.

If the leak really is downstream of the tee my first thought is that the smaller tank is not actually flowing any gas due to a defective check valve or other obstruction. Have you tried shutting off the large tank and venting a little gas downstream to test for this?

Since your gauges are calibrated I would rule out gauge tolerance as the cause. If they were really cheap gauges they could be off enough to make it appear that the small tank is not losing pressure when it actually is.

Scottl said:

. . .Since your gauges are calibrated . . .

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He didn't say, but in this case 'calibrated' probably means marked in lpm O2, etc. Not likely any more accurate than any other ga.
It would be interesting if the OP would post the latest on his problem.

Hi Buster55, I think you should mix up a little dish soap and water, find the leak and fix it. Then you will know exactly where the leak was and stop wasting gas. If these are portable O2 cylinders and regulators, the small round plastic gasket might be worn out.
Tom from Mass