trying to achieve a cheap CO2 storage system

[DuncB]

Purge a completely full cleaned keg with sodium metabisulphite and water using the ferment gas will probably get you the lowest residual Oxygen. Repeat that for each reservoir keg and then build the pressure by daisy chaining them.
You need to start the process after all of the fermenter oxygen is used up.
This thread has a lot of relevance details on this idea.
https://www.homebrewtalk.com/threads/keg-purging-with-active-fermentation.628658/

 

[beren]

Not east to do with corny kegs, but with a sankey you could pull a vacuum to help purge instead of liquid.

 

[mashdar]

Not east to do with corny kegs, but with a sankey you could pull a vacuum to help purge instead of liquid.

Is there a typical vacuum a sanke keg can take?

 

[beren]

Is there a typical vacuum a sanke keg can take?

I have not measured it but the spear ball is held in place with a pretty strong spring. Hard enough to press down with your thumb.

Hard to directly measure, the way the coupler works. Someone using a keg as a fermenter with a tri clamp on the bottom might be able to measure easiest. Put the spear back in, use the tri clamp side to pull a vacuum.