Fan Testing Equipment

[GHz]

I'd like to purchase some equipment to test fans. I already purchased a B&K SLM, but I'd also like to measure CFM and possibly static/differential pressure. Here are the two candidates for anemometers:

Kanomax 6813 w/AP275... this is pretty much the same as the 6803 that SPCR uses. I'm also considering the Kanomax A031 or A041 which are thermal or hot-wire anemometers. Not quite as accurate as the vane type that SPCR uses (2% vs. 1% accuracy), but probably would be easier to use depending on the testing platform/methodology (still working on that). The A041 is a little more expensive, but offers the capability of measuring pressure. Still, it's only 3% accurate (for measuring pressure), so maybe the money would be better spent on a separate manometer?

For those, I have found the Extech HD700/750/755 series. The only difference I can tell (they all cost about the same) is the effective range (0.5/2/5 psi). Can someone explain this to me? Why wouldn't you just buy the unit with the highest range if they all have the same accuracy?

Any thoughts?

[MikeC]

Trying to accurately measure axial fan airflow with a single hand-held device is.... a nightmare. We've gone through 4 devices -- 3 with impeller vanes and one hot wire. None of them really are good enough to give you consistent results -- the hot wire devices just don't seem to have good enough resolution, and the ones with vanes are all subject to some interaction with the measured fan's impellers.

The Kanomax is defintely the best, with the best vane, but it too, varies with axle fan blade design. If you use flow straightener with a tube, the variance w/ blade design can be reduced but then the added impedance lowers the measured flow. By how much? Who knows? Amazing how complex it all gets. Which is why fan makers use such sophisticated multi-chamber devices with pressure-compensating fans etc to do their measurements.

Yet, in the end, airflow and cooling does not have any kind of linear relationship, and it varies tremendously with specifics -- the heat, the shape/size, the channel, the ambient, etc, etc.

All this is why we're no longer interested in measuring airflow. It was always to satisfy audiences' wants, not really our initiative. We've known for a lo-o-o-ong time that airflow is a dubious hinge for cooling around computers. We're ok to accept what the seller/maker claims -- with a big dollop of skepticism.

So... not sure if that helps. lol!

[GHz]

Thanks for the insights, Mike. I think I'll try anyway since--as you said--most people want/expect to see CFM.

I wonder if perhaps turbulence might have been throwing off the readings on your last fan test platform. What I've read about HVAC is that you're supposed to take the FPM readings away from turbulence. Since your last rig was in a box, maybe the flat surfaces of the walls caused a lot of air deflection (thus, turbulence). Maybe a big long tube with a bend in it (enough so that there is no direct path from inlet to exhaust) might be better for measuring FPM/CFM since there should be less turbulence.

If I find that measuring CFM is totally impossible, I'll try the thermal method. I thought about putting a lightbulb in a box with the fan at the intake side. The lightbulb would not be directly in the path of the intake nor exhaust (somewhere in the middle of the case) and then measure the intake and exhaust temperature. The delta could be used as the value for weighing fan performance. Do you think that would work?

Also, do you see any value in measuring pressure as a performance metric? Would that be any more or less difficult than measuring CFM?

[datapappan]

Measuring pressure takes some sort of tunnel, or other controllable impedance - then I would gather you're only measuring fan characteristics in that particular setup.

C.f. Mikes measurements, which are done in free air - it says nothing about how it will sound inside a case (cavity resonace etc) as well as that case might be placed differently for every user.

(Mikes work gives you great comparisons though).

/d