How to Measure Airflow Rate?

"David L. Jones" wrote in news:1122532438.215399.113510 @o13g2000cwo.googlegroups.com:

I would use a hot-wire anemometer cos I have one, or a vane type. Losses in

100m of duct is large. Timing smoke I would imagine is tricky because it will smear due to differences in flow rate between walls and centre as the flow will likely be laminar after some distance. You could try a smoke detector at the oultet I suppose.

Sorry, typo, I meant 100mm ducting. My total length is maybe 10-20m.

I'll look into an anemometer, thanks. Presumably you would simply use it on the output and work out the flow rate based on the diameter of the ducting?

That's what I was thinking...

Also thought of that one, but I thought there might be some lag in the smoke detector response time?

Thanks Dave :)

Apart from heated anemometers as Geoff mentioned, there's rotating vane, ultrasonic, laser-doppler-anemometers (LDA), and pitot tube anemometers.

I'm sure Tech Rentals would have a selection.

Regards David

ultrasonic,

doesnt dick (head) smith sell flow rate measuring devices...

Cheers Terry

You can use differential pressure and a decent graph (if one is available). Otherwise I'd look to use a vane anemometer. Note that any anemometer gives a "local" flow rate (really a velocity sensor) and you need to have a decent technique for moving the anemometer aperture across the duct aperture to obtain a sensible average reading due to non-uniform flow across the duct.

"David L. Jones"

** Sure is.

Try " alt.ducted.air.con.tragics " .

......... Phil

"David L. Jones" wrote in news: snipped-for-privacy@g44g2000cwa.googlegroups.com:

Yeah, 100mm is a bit different... The hot wire anemometer is good cos it allows spot air velocity measurements. However the airspeed profile across the outlet duct is usually very irregular, most flow is on an outer annulus. most of the time I prefer a vane anemometer (cheaper too) and try to get a consistent placement at the outlet so I get repeatable results when trying different experiments. Total flows are very hard to measure without affecting the actual conditions. Presuming the objective is for cooling, I usually regard temperature measurements as the premium information. If you are still very interested in flow metrics, have a look at data books or websites for companies like Papst who state their measurement conditions in the front of their apps books and this is the best sort of practical info I have seen about duct system design. Its a few years since I have looked though.

That's what I want, repeatability across various experiements. Plus I also want an absolute airflow figure to allow calculation of total system efficiency etc. So Budgies idea of taking an average across the duct sounds good too.

Will take a look, thanks.

Dave :)