[Beowulf] cooling question: cfm per rack?

Jim Lux james.p.lux at jpl.nasa.gov
Mon Feb 14 01:31:26 EST 2005


----- Original Message -----
From: Maurice Hilarius

Dean Johnson mailto:<dtj at uberh4x0r.org wrote:
On Sat, 2005-02-12 at 09:49 -0500, Robert G. Brown wrote:
>
> Relative airflow can probably be measured with a kid's toy -- one of the
> little pinwheels -- and counting revolutions with a stopwatch.
> Normalizing that to absolute airflow in CFM is a bit tricky (since the
> result depends to some extent on the resistance imposed by the measuring
> apparatus) but somebody out there may have designed a version of this
> with a real fan and magnets set so that the counting is done
> electronically.  In fact, I could build something to do this out of OTC
> parts if I had any way to normalize the count.
>

Could you not use one of those cheapish wind speed devices that amateur
weather folks use? That would give you a rating, presumably in miles per
hour, and then figure backward based upon the area of the little fan
thingy. That would likely be not too expensive and a great deal easier,
and more accurate, to deal with than counting a pinwheel.  ;-)

 -DeanOne can also go to an auto wreckers, and from ,any newer models of
cars get a Mass Air Flow sensor (MAF) from teh throttle body.
Modern cars use these, in  conjunction with an O2 sensor on the exhasut, to
manage fuel injection.
The MAF returns a variable DC voltage, usually in the range of 0 to 5V
(depending on air speed).
Make a tube, mount the MAF with the probe end in the tube, attach to back of
device being measured.
Supply 12V DC,Connect to output for measurement.
Obviosly this would have to be calibrated.
It is cheap, and very accurate and very relaible..

If you want to make it more useful , a lot of modern cars also use a
barometric pressure sensor, and the calucs can be done using bioth outputs.
This helps a lot as things like current weather conditions and altitude have
a large bearing on air pressure.
Measuring flow by speed only, and ignoring pressure is a fairly inaccurate
method.

Lastly, one can measure the humidity, as this also has a pretty large
influence on the cooling capacity of the air being moved.

For around $25 one can cannibalize the parts and cabling from a modern car
wreck.
All that is left is to provide a DC 12V source, a computer with a 4 channel
A/D chip on a proto board, and some calibration.
The calibration will be the toughest challenge as you will need accurate
precalibrated instruments for a test session, but at least this is one time,
and may be borrowed..

----

The problem with automotive mass air flow sensors is sensitivity at low
flows.  Consider, for a moment, a 1.8 liter engine turning over at 1800 rpm
(call it 30 rev/sec..) That's 1.8*15 liters/sec of air (27 liters/sec),
being drawn through a tube some 5-10 cm in diameter
(call it 60 cm2).. that's 450 cm/sec or 4.5 m/sec... 885 linear ft/minute a
fairly fast airflow in HVAC terms.... And that's the bottom of the range for
the automotive sensor.



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