One of my guys recently gave me a 4500 firemans SCBA tanks very similar to this one;

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I THINK its 66 cu ft (the same as the one above). Im just in the process of tracking down the appropriate fittings to reduce the scba valve down to the 1/8 npt nipple we use.

So, how long do you think something like this would take to fill to 4500 with the Shoebox ?? My guess is 34 hrs. (I pulled that number out of thin air btw)