On Sun, 11 Apr 2010 16:03:12 -0400, Bob Engelhardt
wrote:

Maybe OT, but not as much as many.

I have a fairly large centrifugal blower (squirrel cage) & I'd like to
know its CFM. It has a well defined opening, so knowing the velocity of
the air is what I need.

I can think of a couple of approaches. The first that comes to mind is
placing a surface in the air stream & measuring the force exerted on it.
I know that there is a relationship between the air velocity, the
surface area and the force (drag), and I could find a formula on the
web, but I worry that a little bit of knowledge could be a dangerous
thing. I.e., I suspect that this is a complicated subject and an
understanding should accompany formula use. But if someone here has the
understanding & could give me the right formula, that would be great G.

Another approach is to measure the pressure in the stream & derive
velocity from that. I think that this would be subject to even more
complications than using force, but maybe not. Anyone know? If it's a
reasonable approach, what about a formula?

Or, I could get/make a _really big_ poly bag & time how long it takes to
inflate. Much too crude for my taste.

Advice anyone?

Thanks,
Bob

Some benchrest shooters will have instruments like this:
http://www.midwayusa.com/Search/#win..._1-2-4_8-16-32

For higher velocities than these meters can handle, consider an
easily-made pitot tube, an easily-made water manometer, and pitot tube
pressure vs velocity tables found online somewhere.