How to Measure Airflow Rate?

[David L. Jones]

Maybe a little off-topic...
I want to measure the airflow rate (from a small axial fan) through a
ducted system. I am thinking of using a smoke test and time how long it
takes to get through the system (which comprises 100m ducting and a
similar overall area channelled box).
Any traps?
What type of "smoke" works best?
What errors are typically in this sort of measurement?
How is this sort of thing is usually done?
Are there any better ways to measure air flow rate? Maybe measure
pressure somehow and use the pressure vs airflow graph from a
datasheet?
Any other traps?

Any help appreciated.

Thanks
Dave

 

[Geoff C]

Maybe a little off-topic...
I want to measure the airflow rate (from a small axial fan) through a
ducted system. I am thinking of using a smoke test and time how long it
takes to get through the system (which comprises 100m ducting and a
similar overall area channelled box).
Any traps?
What type of "smoke" works best?
What errors are typically in this sort of measurement?
How is this sort of thing is usually done?
Are there any better ways to measure air flow rate? Maybe measure
pressure somehow and use the pressure vs airflow graph from a
datasheet?
Any other traps?

Any help appreciated.

Thanks
Dave

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.

 

[David L. Jones]

I would use a hot-wire anemometer cos I have one, or a vane type. Losses in
100m of duct is large.

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?

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.

That's what I was thinking...

You could try a smoke detector at the oultet I suppose.

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

Thanks
Dave

 

[dmm]

Maybe a little off-topic...
I want to measure the airflow rate (from a small axial fan) through a
ducted system. I am thinking of using a smoke test and time how long it
takes to get through the system (which comprises 100m ducting and a
similar overall area channelled box).
Any traps?
What type of "smoke" works best?
What errors are typically in this sort of measurement?
How is this sort of thing is usually done?
Are there any better ways to measure air flow rate? Maybe measure
pressure somehow and use the pressure vs airflow graph from a
datasheet?
Any other traps?

Any help appreciated.

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

 

[Terry Given]

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

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

Cheers
Terry

 

[budgie]

Maybe a little off-topic...
I want to measure the airflow rate (from a small axial fan) through a
ducted system. I am thinking of using a smoke test and time how long it
takes to get through the system (which comprises 100m ducting and a
similar overall area channelled box).
Any traps?
What type of "smoke" works best?
What errors are typically in this sort of measurement?
How is this sort of thing is usually done?
Are there any better ways to measure air flow rate? Maybe measure
pressure somehow and use the pressure vs airflow graph from a
datasheet?
Any other traps?

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.

 

[Phil Allison]

"David L. Jones"

Maybe a little off-topic...

** Sure is.

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






.......... Phil

 

[Geoff C]

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

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.

 

[David L. Jones]

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.

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.

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.

Will take a look, thanks.

Dave