[Beowulf] cooling question: cfm per rack?

Jim Lux james.p.lux at jpl.nasa.gov
Sat Feb 12 07:47:17 PST 2005

Sure, one could build it.. but one can probably buy it cheaper/easier
Omega: http://www.omega.com/ppt/pptsc.asp?ref=HHF82&Nav=grec06  $89
They have others.
Similar devices abound: http://www.nkhome.com/ww/1000/1000.html
page_number=1  (check out the first one.. $49)

Your local sporting goods place (REI, Sport Chalet, Big 5) might have
something like this too.  So might Sharper Image or Brookstone, or one of
those gadget stores

Heck, Harbor Freight Salvage, a big retailer of inexpensive moderate quality
imported stuff might have them..next time you're down buying cheap imported
Chinese machine tools...check that bargain bin next to the register.

Other approaches..small propellor on a small DC motor run as a generator
(only works for fairly fast flows >several m/sec) run to a DVM.
Small propellor and magnet/reedswitch driving a counter (as in your
inexpensive DMM). (this is what the commercial units are)

The challenge in home fabrication of such devices is getting it to be
reasonably orientation insensitive, which implies pretty good balance, and
to work in very low flows (<1 m/sec), which implies fairly low friction.

I imagine, if you had a LOT of time on your hands, you could probably modify
the heated film/wire sensor from an automotive mass air flow sensor for this

(I spent the better part of a year trying to come up with a low budget way
to measure velocity profiles across large (decameter scale) artificial
tornadoes.. We eventually settled on a pitot tube rake with water manometers
using video to do data logging.)

----- Original Message -----
From: "Robert G. Brown" <rgb at phy.duke.edu>
To: "Mark Hahn" <hahn at physics.mcmaster.ca>
Cc: "David Mathog" <mathog at mendel.bio.caltech.edu>; <beowulf at beowulf.org>
Sent: Saturday, February 12, 2005 6:49 AM
Subject: Re: [Beowulf] cooling question: cfm per rack?

> > if your pressure is reasonably even, the same tiles should flow the
> > same CFM.  I'd LOVE to find some way to measure airflow, since I'd
> > actually consider doing things like adding patches of duct tape to
> > the underside of too-high-flow tiles.  I suppose that the empiricist
> > approach is just to sample all your system temperatures, and if some
> > are too high, reduce the airflow to racks which are "too cool".
> 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.
>    rgb

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