I use one on mig and tig - the one I have is accurate and matches my high quality flow meters.
Very useful for testing. When I use someone else's machine I use it to ensure the flow is correct relative to what I run in my shop.
They can show a leak by showing less flow than indicated back at the tank.
Three times I found one they were on a mig and were back at the gun connection.
On one tig setup I found the flow guage read much higher than it actually flowed - no leaks but had to be set wide open to flow 20 CFH.
It's this one, $34: http://www.netwelding.com/prod02.htm#WAT-PFM
There are cheaper ones out there as well.
Beware of false knowledge; it is more dangerous than ignorance. ~George Bernard Shaw~
Airco 300 - Syncro 350
Thermal Arc 161 and 300
Tried being normal once, didn't take....I think it was a Tuesday.