On Tuesday, May 9, 2017 at 9:29:09 AM UTC-4, Jim Wilkins wrote:
"rangerssuck" wrote in message
...
So, I got to thinking that there might be a better place than AGL,
Praxair or Aigas to get the nitrogen or Argon. A couple of minutes
with google turned up a welding supply / gas supply about a mile from
my house. They will rent a 330cf cylinder for $4.00 per month and fill
it with nitrogen for $29 or argon for $65.

This is way cheaper than I would expect to pay at any of the big
shops, and couldn't be much more convenient. A huge bonus.

==========
Once it holds a vacuum I'd make a rough estimate of the volume with
dried compressed air, then a better one by evacuating it and filling
from a nitrogen tank nearest the estimated cf capacity. The (absolute)
pressure drop in the cylinder and the rise in the piping give you the
volume ratio. The more the gauges change, the better the resolution.
https://en.wikipedia.org/wiki/Boyle%27s_Law

-jsw

That's a good one, but it requires that you know the actual cf in the tank first. I have no idea how accurately that gets measured when tanks are filled.