The begining of a Fan cfm/db comparison project

mdearth · Post by **mdearth** » Fri Jul 22, 2005 11:40 am

I have an idea for a fan comparison test. Please let me know your thoughts as I am seriously considering taking on my own fan comparison project.

The test would include many 80, 92, and 120mm 12v pc fans. The test would be based on the measured cfm output of each fan. I would adjust the voltage down from 12v stopping at each 5 cfm measurement (30cfm, 25cfm, 20cfm, etc). At each 5cfm level I would record the rpm, voltage, and noise volume at that cfm. This way the sound produced by a low speed fan at say 30cfm @ 9v could be compared to a high speed fan at 30cfm and 5v irregardless of the rpm. The benefit to a test like this would be not only comparing sound, but how much sound at what performance.

Example:
Fan “A” has a max cfm of 25 @ 12v @ 1000rpm @ 30db
Fan “B” has a max cfm of 50 @ 12v @ 1500rpm @ 40db
Fan “C” has a max cfm of 75 @ 12v @ 2000rpm @ 50db

Most all tests I’ve seen would compare these fans at various voltages and the sound level at that voltage, but never touching on air moving performance. What if one fan can move 25% more air at the same db but different voltages?

Since I buy fans to move air, and I don’t care what voltage I have to set them at or what rpm they spin at to achieve the desired cfm/db. In the above example of “made-up” test results you can see that fan “B” produces the lease noise at 30cfm. Or it could be read that at 25db fan “B” produces the most cfm.

Now the big question: Test tools
First I have to find a cfm meter and a way to channel or funnel all of the air from an 80, 92, 120 fan through it. Adjusting the voltages and reading the rmp if applicable would be simple. I would want to test the db and cfm while the fan was mounted in a pc case blowing out. The sound would be measured with the cfm funnel removed, in a normal home office environment. I’m not sure if the microphone distance should be close, or 3 ft away? I’ll probably have to try both. The important thing would be that every fan would be tested in the exact same way with the voltage and sound recorded at the same measured cfm levels. 120mm fans should be tested a 2nd time while pulling air through a radiator for us watercooling fools.

Testing every possible fan would be expensive, and a bit ridicules since I would not get paid for this. A hand pick of the popular fans should suffice. What are you thoughts on this and do you have any suggestions where to find a tool to measure air volume out of a tube with low resistance? I do not want to measure air pressure. The closed pc case, and radiator (for the 2nd 120mm test only) will provide the real world resistance.

Bluefront · Post by **Bluefront** » Fri Jul 22, 2005 12:18 pm

Unless you are attempting such a test for your own amusement/use, any results you achieve are probably going to be suspect/ignored/innacurate/of no use to anyone but yourself.

You'll not be able to find or even rig up a CFM meter accurate enough to mean anything...and the same goes for a db meter. If you attempt to use mfg specs for anything, the results will be useless.

I recently did a mini-test of six 92mm fans, used a CPU temp test to determine which fan had the greatest airflow at 5V. I'm convinced my results were accurate for me. But whether they mean anything to anyone other than myself is doubtful.

I'd suggest waiting for the SPCR Greatest Fan Test, coming soon (maybe).

jamesm · Post by **jamesm** » Fri Jul 22, 2005 1:46 pm

mdearth:

I like your idea. Keeping the CFM as the control variable is a good idea. Let me know what you come up with.

mdearth · Post by **mdearth** » Sat Jul 23, 2005 9:57 am

Bluefront, Yes this would be mainly for my own amusement as I love to tinker with stuff. I knew the cfm meter would be difficult to design, thats why I'm picking the brains of the people here. with regards to db measurment, the db or bad noise produced by fans should be amplafied by hard mounting the fans in a case. I wasnt going to attempt measurments uner 20db. I read your test info, was a very good idea to use actul cooling temps as your performance measurement. After reading your post, if i cant get the cfm meter to work, I may do something similar.
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Again, anybody know of a way to measure air flow? It does not have to be accurate to any standards, but must produce results enough to compair slight changes in order to compair fans.

Ive even thought about using a 40-60mm easy spinning fan with a tach wire. Mount it to the end of my tube/funnel. The test fans will spin it. I should be able to read its rpm on a motherboard by only connecting the ground and tach wires. Then base the air flow of the tested fan by the rpm output of the 40-60mm fan. I just afraid this will not measure low airflow.

jamesm · Post by **jamesm** » Sat Jul 23, 2005 10:05 am

I think your fan idea would work as it would provide a relative scale for cfms

Bluefront · Post by **Bluefront** » Sat Jul 23, 2005 12:00 pm

Ever seen an air flow meter for a car? I don't think anyone uses them anymore (they use air mass meters now). Anyway I always thought an airflow meter could be modified to test relative fan CFM.

It's a small boxy looking thing with about a 92mm opening, and a 92mm exit hole. (for nissans anyway). Inside is a big flap which opens wider with increased airflow. The shaft of this flap operates a potentiometer of sorts.....this gives a variable voltage reading.

Perhaps you could rig up something similar? This sort of reading could not be duplicated by anyone other than yourself. But it might be reasonably accurate. Using such a device could compare CFM of any size fan at any voltage.....plus you wouldn't need a computer running for any testing....easy fan swapping.

frankgehry · Post by **frankgehry** » Sat Jul 23, 2005 12:33 pm

You could build one of these things.

You could place a fan in the airflow of the test fan and measure the voltage that it generates. The relationship between voltage and rpm is supposed to be linear.

You could use a heating element like a resistor and place a temperature proble or thermistor on or near it and measure the voltage changes to determine cooling power. Relationship probably not linear, but real world.

Google anemometer.

cpemma · Post by **cpemma** » Thu Jul 28, 2005 1:01 pm

frankgehry wrote:You could use a heating element like a resistor and place a temperature proble or thermistor on or near it and measure the voltage changes to determine cooling power. Relationship probably not linear, but real world.

Google anemometer.

That type of method would be best IMO, most other methods restrict the fan's airflow far more than a case would. Possibly just a thermistor in the air path, fed from a constant-current-source, it will reach a certain equilibrium temperature depending on the cooling airflow and the resistance, hence temperature, can be measured from the voltage drop.

How you calibrate the flow in absolute units is harder.