Air Flow meter - heatsink testing

[JH]

I would like to do some thermal characterization on a heatsink assy and
it would be helpful to get a "somewhat" accurate air flow measurement.
I've thought of building some ductwork and variable speed fans to
control the air across the fins but need a way to measure the air
speed, etc. I know I could spend some $$ for a lab flow meter but it
occurs to me that cars have a air flow meter in the intake system.
Could I get one of those cheaply from a salvage yard and would it work
for the sort of low air speeds (0-8mph) I want to test at? I would
need to get the hookup connection and what the signal vs flow cal
information is if its available..... The design I am working on has a
heatsink about 9" wide with 1" fins and typically we would be running
150W or so with it. I need to collect the temp rise vs air speed to
verify the design.

Thoughts welcome.

Thanks,
Jeff

 

[TuT]

I would like to do some thermal characterization on a heatsink assy and
it would be helpful to get a "somewhat" accurate air flow measurement.
I've thought of building some ductwork and variable speed fans to
control the air across the fins but need a way to measure the air
speed, etc. I know I could spend some $$ for a lab flow meter but it
occurs to me that cars have a air flow meter in the intake system.
Could I get one of those cheaply from a salvage yard and would it work
for the sort of low air speeds (0-8mph) I want to test at? I would
need to get the hookup connection and what the signal vs flow cal
information is if its available..... The design I am working on has a
heatsink about 9" wide with 1" fins and typically we would be running
150W or so with it. I need to collect the temp rise vs air speed to
verify the design.

Thoughts welcome.

Thanks,
Jeff

You could use a hot-wire anemometer based on a temperature sensitive
filament run at constant temperature. I've not personally used one of
these, but they're reckoned to work well at low flow speeds. For some
reason I can't select a fixed font for this reply, so I'll have to describe
the basic circuit:

Op-amp output to base of an NPN transistor.
Transistor collector to +ve supply voltage.
Transistor emitter to two resistors, R1 and R3.
Other end R1 to amplifier inverting input.
PTC resistive element, Rf, from inverting input to 0V
Other end R3 to amplifier non-inverting input.
Adjustable resistor, R2 from non-inverting input to 0V.
Appropriate + and - supplies to the op-amp.

The sensor output is the voltage on the transistor emitter and is non-linear
with air speed, sensitivity being greatest at the lower speeds. R2 is
adjusted for optimum sensitivity at the flow rates of interest. You may
also need to add additional component(s) to ensure that the circuit
initialises at switch on, depending on the initial status of the op-amp
output.

Can also be reconfigured to use an NTC element.

 

[John - KD5YI]

I would like to do some thermal characterization on a heatsink assy and
it would be helpful to get a "somewhat" accurate air flow measurement.
I've thought of building some ductwork and variable speed fans to
control the air across the fins but need a way to measure the air
speed, etc. I know I could spend some $$ for a lab flow meter but it
occurs to me that cars have a air flow meter in the intake system.
Could I get one of those cheaply from a salvage yard and would it work
for the sort of low air speeds (0-8mph) I want to test at? I would
need to get the hookup connection and what the signal vs flow cal
information is if its available..... The design I am working on has a
heatsink about 9" wide with 1" fins and typically we would be running
150W or so with it. I need to collect the temp rise vs air speed to
verify the design.

Thoughts welcome.

Thanks,
Jeff

0-2000 ft/min hot wire anemometer. $159.

http://www.metermall.com/Product Pages/Air Flow/405-V2 Stick Anemometer.htm

I am not connected with these people in any way.

Cheers,
John

 

[GregS]

0-2000 ft/min hot wire anemometer. $159.

http://www.metermall.com/Product Pages/Air Flow/405-V2 Stick Anemometer
.htm

I am not connected with these people in any way.

The air flow meter on my car is fairly stiff. You need a lot of CFM to get these going.
There must be a cheap meter around. You could make it with a resistive sensor
attached to a vane. Make it for $10. Cal it yourself. Air speed is one thing, but
CFM in a tunnel is another thing.
http://www.omega.com/ppt/pptsc.asp?ref=HHF12&Nav=grec06
greg

 

[John Larkin]

I would like to do some thermal characterization on a heatsink assy and
it would be helpful to get a "somewhat" accurate air flow measurement.
I've thought of building some ductwork and variable speed fans to
control the air across the fins but need a way to measure the air
speed, etc. I know I could spend some $$ for a lab flow meter but it
occurs to me that cars have a air flow meter in the intake system.
Could I get one of those cheaply from a salvage yard and would it work
for the sort of low air speeds (0-8mph) I want to test at? I would
need to get the hookup connection and what the signal vs flow cal
information is if its available..... The design I am working on has a
heatsink about 9" wide with 1" fins and typically we would be running
150W or so with it. I need to collect the temp rise vs air speed to
verify the design.

Thoughts welcome.

Thanks,
Jeff

Extech has a couple of propeller-type flow and temperature meters,
fairly cheap.

John

 

[Don Foreman]

I would like to do some thermal characterization on a heatsink assy and
it would be helpful to get a "somewhat" accurate air flow measurement.
I've thought of building some ductwork and variable speed fans to
control the air across the fins but need a way to measure the air
speed, etc. I know I could spend some $$ for a lab flow meter but it
occurs to me that cars have a air flow meter in the intake system.
Could I get one of those cheaply from a salvage yard and would it work
for the sort of low air speeds (0-8mph) I want to test at? I would
need to get the hookup connection and what the signal vs flow cal
information is if its available..... The design I am working on has a
heatsink about 9" wide with 1" fins and typically we would be running
150W or so with it. I need to collect the temp rise vs air speed to
verify the design.

Thoughts welcome.

Thanks,
Jeff

I've thought about this too. A hot-wire aenomometer might work
nicely, if you could find a way to calibrate it. Could be done in a
car if your probe is on a long enough pole to get away from the car's
slipstream -- not compatible with traffic! Might be able to
calibrate it "close enough" with a shop-made pitot tube, a simple
slant manometer made of a bit of glass tubing and some colored water,
a leaf blower with variac or lamp dimmer and some duct.

http://www.flowmeterdirectory.com/flowmeter_artc/flowmeter_artc_02111201.html

It would probably be good to pack the duct upstream with some bits of
tube, maybe a foot or so of length, to reduce turbulence and get
closer to laminar flow. Bits of 1/2" EMT (conduit) might work nicely
for that and EMT is very cheap -- about a buck for 10 feet of it.

A simple cheap way to make a pretty good hotwire aenomometer is to
break the bulbs on a couple of automotive marker light bulbs to expose
the filaments. (I'm not making this up, I've done it.) Shield one
from the flow, excite both with same current, measure the
differential voltage. They're a bit fragile, but they work quite well
at low flow rates like you mention. Might also be able to do it by
maintaining constant resistance with suitable elex (low-level AC
current excitation, AC control loop to keep sensed AC voltage at
setpoint) and then measure the DC current or voltage necessary to
maintain that temp and resistance in the presence of airflow.

Volumetric air flow in ducts as in HVAC systems is often measured
with a distributed pitot e.g. a ring of tubing with a bunch of holes
in it, or a distributed hot wire as in some mass air flow sensors in
throttle bodies. Thing is, the velocity profile across a cross
section of duct is far from uniform.

Or, just go with the pitot tube and manometer and fuggedaboud the
electronics!

 

[Smaug Ichorfang]

I would like to do some thermal characterization on a heatsink assy and
it would be helpful to get a "somewhat" accurate air flow measurement.
I've thought of building some ductwork and variable speed fans to
control the air across the fins but need a way to measure the air
speed, etc. I know I could spend some $$ for a lab flow meter but it
occurs to me that cars have a air flow meter in the intake system.
Could I get one of those cheaply from a salvage yard and would it work
for the sort of low air speeds (0-8mph) I want to test at? I would
need to get the hookup connection and what the signal vs flow cal
information is if its available..... The design I am working on has a
heatsink about 9" wide with 1" fins and typically we would be running
150W or so with it. I need to collect the temp rise vs air speed to
verify the design.

Thoughts welcome.

Thanks,
Jeff

The problem is calibration. Get a large container of known volume - say a
3 gallon water bottle or a small trash can you can measure the volume of.
Cut several large holes in the sides and bottom of container. Fit a
collapsed trash bag over the fan. Insert trash bag into mouth of
container. Turn on fan and measure time it takes trash bag to fill
container. Turn fan off, collapse trash bag, repeat and take average of
several measurements. This gives you the aproximate average flow rate.
Modify to suit needs, season to taste.

 

[Homer J Simpson]

I've thought of building some ductwork and variable speed fans to
control the air across the fins but need a way to measure the air
speed, etc.

Try using a flashlight bulb with the glass broken and running it at a low
voltage. See how sensitive it is to low speed air flow.

 

[[email protected]]

I would like to do some thermal characterization on a heatsink assy and
it would be helpful to get a "somewhat" accurate air flow measurement.
I've thought of building some ductwork and variable speed fans to
control the air across the fins but need a way to measure the air
speed, etc. I know I could spend some $$ for a lab flow meter but it
occurs to me that cars have a air flow meter in the intake system.
Could I get one of those cheaply from a salvage yard and would it work
for the sort of low air speeds (0-8mph) I want to test at? I would
need to get the hookup connection and what the signal vs flow cal
information is if its available..... The design I am working on has a
heatsink about 9" wide with 1" fins and typically we would be running
150W or so with it. I need to collect the temp rise vs air speed to
verify the design.

Thoughts welcome.

Thanks,
Jeff

Jeff,

There are lots of ways to make a device to measure the speed of
airflow. But if you don't have a calibrated "airflow meter" (which is
what you are trying to avoid buying), you will have to also devise some
way to calibrate whatever you build.

I have read some of the other posts, which mention some possible
calibration schemes that sound like they should work.

But another possible calibration scheme just occurred to me (I've never
tried this, and am just speculating): Assuming you can get your hands
on a heatsink for which you can also get a good/detailed manufacturer's
datasheet, i.e. which has some data for the
temperature-rise/watts/airflow-speed relationships, you could then try
this: Dissipate a known power level with/into the heatsink, measure the
temperature (or temperature change) of the heatsink (per heatsink's
datasheet's measurement-location/methods/specs), and adjust the airflow
speed (and/or power and temp) until your "knowns" measurements/values
match datapoints that are available from the heatsink's datasheet,
which should then give you the airflow speed. Then probably repeat
that over ranges of power levels and airflow speeds.

I don't know how accurate that method might be. You might also want to
talk to the heatsink manufacturer's engineers, first. But, on the plus
side, it sounds like it might be pretty easy to try, using hardware and
information that you might already have, or could easily get.

Good luck!

- Tom Gootee

http://www.fullnet.com/u/tomg

"He who lives in a glass house" should not invite "he who is without
sin".

 

[Glen Walpert]

I would like to do some thermal characterization on a heatsink assy and
it would be helpful to get a "somewhat" accurate air flow measurement.
I've thought of building some ductwork and variable speed fans to
control the air across the fins but need a way to measure the air
speed, etc. I know I could spend some $$ for a lab flow meter but it
occurs to me that cars have a air flow meter in the intake system.
Could I get one of those cheaply from a salvage yard and would it work
for the sort of low air speeds (0-8mph) I want to test at? I would
need to get the hookup connection and what the signal vs flow cal
information is if its available..... The design I am working on has a
heatsink about 9" wide with 1" fins and typically we would be running
150W or so with it. I need to collect the temp rise vs air speed to
verify the design.

Thoughts welcome.

Thanks,
Jeff

There are quite a few inexpensive anemometers on ebay, some of which
would probably meet your needs.

 

[[email protected]]

I would like to do some thermal characterization on a heatsink assy and
it would be helpful to get a "somewhat" accurate air flow measurement.
I've thought of building some ductwork and variable speed fans to
control the air across the fins but need a way to measure the air
speed, etc. I know I could spend some $$ for a lab flow meter but it
occurs to me that cars have a air flow meter in the intake system.
Could I get one of those cheaply from a salvage yard and would it work
for the sort of low air speeds (0-8mph) I want to test at? I would
need to get the hookup connection and what the signal vs flow cal
information is if its available..... The design I am working on has a
heatsink about 9" wide with 1" fins and typically we would be running
150W or so with it. I need to collect the temp rise vs air speed to
verify the design.

Thoughts welcome.

Thanks,
Jeff

Hot wire anemometer is the best way. You can use a small thermistor in
the feedback loop of an op amp, real easy and cheap.

 

[GregS]

There are quite a few inexpensive anemometers on ebay, some of which
would probably meet your needs.

The flow across a heatsink is much more complicated than CFM. How the air is drawn or
pushed through, direction, and things like that.

greg