Doug,
Here's the link to the tester
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=94190
It was $39.99 at my local Habor Freight store.
Here's the link to the operating instructions:
http://www.harborfreight.com/manuals/94000-94999/94190.pdf
The inlet gauge is 0-100psi, the downstream gauge has a percentage scale and pegs out at about 20psig i.e. if i block the outlet then at 20psig inlet the 2nd gauge is maxed out.
Note: I took the tester apart and found that it had a .060 orifice between the gauges.
My test procedure is as follow:
1) Set the regulator pressure so the 2nd gauge read 0% (about 16psig), the tester is not flowing any gas at this time.
2) Connected the tester to the cylinder and read the % drop in pressure directly from the 2nd gauge.
So I guess what the test is telling me is that with 16 psig on the inlet I am able to maintain 12psig in the cylinder after flowing through a .060 orifce.
I hope this helps
Stu