On Tuesday, May 9, 2017 at 8:37:03 AM UTC-7, Neon John wrote:
On Mon, 08 May 2017 19:45:11 -0500, dpb wrote:

On 05/08/2017 6:57 PM, Neon John wrote:

You absolutely can NOT use the ideal gas law because none of the
gasses are ideal. The Van der Walls equation must be used. ...

For the purposes here and at PT talking of, the correction will be of
only a few per cent, certainly within 10% or so.

Actually it will be much more significant than 10%.

Depends on which recommendations we're talking about. If the
test is to pump down, then fill from a lecture cylinder and measure
achieved pressure, and use the weight-before and weight-after of
the lecture cylinder to determine the gas quantity, then
the only pressure/volume relationships are on low-pressure
gas. The cylinder before/after pressure DOES need van der Waals
or other corrections, but if you go by cylinder WEIGHT, not
so much.

If you want to empty a 4.5 foot cylinder for the test, and don't have
a suitable balance to weigh that, and use the pressure gage, that's
a whole different scenario.