How to Measure Airflow Rate?

D

David L. Jones

Guest
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 :)
 
"David L. Jones" <altzone@gmail.com> wrote in news:1122532438.215399.113510
@o13g2000cwo.googlegroups.com:

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 :)
Click to expand...
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.
 
Geoff C wrote:
"David L. Jones" <altzone@gmail.com> wrote in news:1122532438.215399.113510
@o13g2000cwo.googlegroups.com:

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.
Click to expand...
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.
Click to expand...
That's what I was thinking...

You could try a smoke detector at the oultet I suppose.
Click to expand...
Also thought of that one, but I thought there might be some lag in the
smoke detector response time?

Thanks
Dave :)
 
On 27 Jul 2005 23:33:58 -0700, "David L. Jones" <altzone@gmail.com> wrote:

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 :)
Click to expand...
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
 
dmm wrote:
On 27 Jul 2005 23:33:58 -0700, "David L. Jones" <altzone@gmail.com> wrote:


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

Cheers
Terry
 
On 27 Jul 2005 23:33:58 -0700, "David L. Jones" <altzone@gmail.com> wrote:

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?
Click to expand...
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" <altzone@gmail.com> wrote in
news:1122537492.608921.223730@g44g2000cwa.googlegroups.com:

Geoff C wrote:
"David L. Jones" <altzone@gmail.com> wrote in
news:1122532438.215399.113510 @o13g2000cwo.googlegroups.com:

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.

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 :)
Click to expand...
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.
 
Geoff C wrote:
"David L. Jones" <altzone@gmail.com> wrote in
news:1122537492.608921.223730@g44g2000cwa.googlegroups.com:

Geoff C wrote:
"David L. Jones" <altzone@gmail.com> wrote in
news:1122532438.215399.113510 @o13g2000cwo.googlegroups.com:

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.

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 :)



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.
Click to expand...
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.
Click to expand...
Will take a look, thanks.

Dave :)
 
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