One way to compare fan air pressure output, accurately

intx · Post by **intx** » Mon Jul 12, 2004 8:18 pm

I'm sure most of us have noticed at one point or another that not all manufacturers measure their specs the same way. And most enthusiasts here use fans at voltages they were not intended to be used (below 12v).

Here is a method I suggest for measuring a fan's air pressure, which will reflect the CFM it pushes.

1. Get a plastic bag, preferably clear and larger than a sandwich bag. However you probably don't want a shopping bag unless you have a powerful fan. I used a bag that's about the size of the bags you usually find in the vegetable section of a grocery store (the clear ones). The fan must not have any holes in it, obviously.

2. Wrap this bag around the fan's output area, while being careful not to get any of the bag caught in the fan's blades. You can use a rubber band, or simply carefully tape the bag around the fan.

3. Power the fan on.

I've thought of two ways of measuring the fan, depending on how powerful it is and how large your bag is.

The first method is to count how long it takes to fill up the bag.. I don't like this method because it's hard to keep track, and the bag must be in an equivalent deflated position every time for accurate results.

The second method is to place a light object on the bag. Something like an eraser, credit card, or whatever is light and nearby. If you're measuring strong fans, use a heavier object. To accurately test multiple fans, affix the object to a certain position on the bag.

Now you can measure the different voltages, and how high the object is lifted off the ground. It's actually quite fun when you're just playing around trying different objects.

This should be a fairly accurate method of comparing fans. No, you can't get a CFM rating, but you can really tell which fan has the power to push air. While this is more of a method to measure air pressure and not CFM, there will always be air pressure your fan has to fight against, whether it's from a fan grill, or your system enclosure. The amount of CFM a fan pushes is greatly related to the air pressure it pushes.

Just an idea. I noticed someone on another site used a little piece of tissue paper to test. (Or you can use a miniature flag.) I'm suggesting this as a more measurable way of testing fans.

acaurora · Post by **acaurora** » Mon Jul 12, 2004 8:27 pm

Interesting concept. However, what happens when the light/heavy object somehow bounces off and hits the blades? =[ I remember reading here in SPCR, or somewhere else, I forget, that they tested using strips of some sort. Not tissue paper, but of some other rather lightweight to medium weight material, and measured CFM by how much the strip was raised by the fan's output. The strip would be mounted to the top center of the fan's frame.

In any case, a good suggestion, nonetheless.

intx · Post by **intx** » Mon Jul 12, 2004 8:44 pm

I need some ascii art.. yes, my ascii art skills are horrible. I apologize for any blindness this may cause. I'm also not responsible

...../------------\........../---------\
---+.................\.ooo/..............\
|F|...........................................|
|A|...........................................|
|N|...........................................|
---------------------------------------
bag ^

(Ignore the dots, spaces wouldn't work.)

The 'ooo' representing the object you placed. There only way it would hit the blades, would be if it somehow magically bounced off the top, all the way to the left, down, and then 'right' into the fans. Or if the bag breaks, in which case it shouldn't even bounce because the leaking air would let it sink gradually.

Unless you put one of those bouncy-balls (in which case you're just asking for trouble), there shouldn't even be a chance. The airflow at lower voltages isn't enough to suck an object back towards the fan.

In any case, I suggested you affix the object to the bag for measurement. If it's affixed to the bag, it shouldn't come off the bag at all until you choose to remove it.

The thought the object would bounce off the bag never occured to me

Safety is always a good precaution, but I think the bag is more likely to get caught into the blades and cut up into a million pieces then bridging one the circuits is much more likely to happen. Try it and you'll see what I mean. The bag inflates gradually averagely 2-9 seconds to inflate on a fan capable of 68.9CFM (full voltage) at various voltages.

While I'm on that subject, the bag should not get caught into the fan from the right (in my diagram anyway) side, as the air pushes against it. However on the left (suction side), be sure there isn't any loose ends to be sucked in. In an event that does happen (unless your bag is overly used or cheaper than a sandwich bag and you're using a vantec tornado) the worse that should happen is you hear a lot of flapping noises from the bag, and the blades will spin slower. You won't end up with a dead fan, just inaccurate results and perhaps serious noise clicking pollution

The horror!

acaurora · Post by **acaurora** » Mon Jul 12, 2004 8:50 pm

OH... lol nevermind, I misunderstood. I misread "place a light object on the bag" as "place a light object in the bag". ~.~ Man I really should go to sleep, making me misread all this stuff.

intx · Post by **intx** » Mon Jul 12, 2004 8:53 pm

Ah! that's why.

Yeah, that would be a lot more likely to happen!

Not sure how much measuring you can do with an object in the bag though, but interesting concept. Never tried it... with the right mind (I'm far too stupid) someone may be able to come up with a better method relative to having an object inside.

Bluefront · Post by **Bluefront** » Tue Jul 13, 2004 2:24 am

Here's something I tried that works....never posted about it however. I have a highly sensitive scale for measuring small objects....goes down to one gram(digital readout). I made a little cage to hold the fan right over the scale. When you turn on the fan, you get an accurate digital readout of the pressure.

Works neat....too bad the scale costs $100.

The problem with your bag method is the accuracy of the measurement, which could never be reliable enough to make any valid comparison of two similar output fans.

Pjotor · Post by **Pjotor** » Tue Jul 13, 2004 6:28 am

Modification of intx's idea:

Get a fairly long pipe. A meter should be enough. The diameter should be the same as the fan which is to be tested.
Craft an object which has some weight, but which doesn't fit snugly in the pipe -- maybe a metal plate with holes in it. Put object in pipe.
Mount the fan to the pipe and power it on. Measure the height of the object when the fan is on. This would be a good indication of air pressure and can be used to compare different fans.

Measuring the flow volume per second/minute/whatever is slightly trickier, though. I like the idea with the bag, but it's hard to measure under consistent conditions.

hvengel · Post by **hvengel** » Thu Jul 15, 2004 10:58 am

The idea of using a sealed bag to measure fan pressure is interesting but may not relate to the real world. In the real world fans move air and operate at a modest pressure differential. In fact one of Mike's rules for building a silent or quite pc is to select (or build) a case that has a low impedence air flow. The bag test is testing fans at the highest impedence that they can handle (the fan is areodynamicaly stalled and has 0 air flow) and what the sealed bag test is measuring is the peak impedence of the fan. This may be a valid measurement for high impedence air flow applications but probably not case fans.

The test using the scale comes closer to real world use. But this only measures the relative air flow at the lowest impedence (free air). So these two tests measure the fans performance at both ends of it's operating range. But are not directly comparable because the measurements are in different units.

A better test would mount the fan to a tube like a 3 inch pvc pipe. In the tube would be mounted a butter fly or some other way to restrict the flow. In addition a clear plastic tube would be mounted in the pipe. One end of this tube would be open to the inside of the pipe between the fan the the restriction device the other end would be open to the outside air. The clear tube would have water trapped in a bend in the tube. This would be used to measure the back pressure on the fan.

This way you could use the butter fly to adjust the back pressure which could be accurately measured with the water level in the clear tube. You would use the scale method for reading the RELATIVE air flow at any given back pressure. Back pressures of 0" to .25" water are the correct range for the types of fans used to cool PCs.

Using a tube and water to meausre air pressure is a very old technique and was the standard way to do this at one time. Below is a crude text diagram of the measuring device. The pressure is measured by measuring the difference in height of the water on the high pressure side of the bend as compared to the low pressure side. As you can see it is a very simple device. It is also fairly accurate. This could also be used to measure the air flow impedence (pressure) of your case.

__________ __________
outside | | < higher
air | | pressure
-------- | | ---------
| | | |
| | | | _______
|XXX| | | measured
|XXX| | | pressure
|XXX| |xxx| ---------
|XXX| |XXX|
|XXX| |XXX|
|XXXXXXXXXXXX|
|XXXXXXXXXXXX| XXX=water
----------------------