Scythe Ultra Kaze 120mm Fans

Static Pressure Theory

Static Pressure Theory

I thought it would be pertinent to run over some basic theory in relation to static pressure and its benefits, as all fans are not created equal. Furthermore; I hope that by covering the basics it will help the newer members grasp what we are trying to achieve a little easier. So what is Static Pressure?

Static Pressure is usually stated either in inches of water (inH2O) or in millimeters of water (mmH2O). It is essentially a measure of the differential air pressure between the air pressures inside an application vs ambient air pressure outside of an application, which for airflow calculation purposes is usually 0 (zero). There is an inverse relationship between airflow and static pressure. As the pressure differential rises, airflow drops.

Ok so that makes sense so far doesn't it? Essentially static pressure, or more accurately the measure of it, is the difference in pressure. Because we are looking at utilising higher static pressure fans for possible heat sink or radiator cooling work, the static pressure differential then becomes the difference in air pressure on the inlet and exhaust of the chosen application. We can think of the differing pressures as positive (absolute pressure of the fan) and negative pressure (pressure drop experienced after heat sink/exchanger). I have included a few diagrams to help get the point across:

Equal Static Pressure Static pressure diagram

The first image (above left) is that of equal pressure, where the pressure on the outside of a case is equal to that on the inside...and not an optimal solution for cooling. The second image illustrates negative pressure where the pressure inside the case is less than the outside pressure. Negative pressure allows air to enter the case at the right direction and speed and is needed for efficient cooling.

Now the principal is similar when it comes to fan static pressure, which is illustrated by the image below:

Static Pressure Diagram

So by now you may be asking "What the hell does this have to do with fans?". Ok, as air flows through fin configurations (either on a heat sink or heat exchanger), the small openings constrict the flow, creating a pressure drop. Because air flow velocity is directly proportional to air flow volume and proportional to the static pressure drop, heat sinks and heat exchangers require a higher flow velocity pressure to move a given volume of air through a heat sink. To increase the velocity, an increase in fan power is usually necessary to maintain the same mass flow of air through the heat sink.

Ok so now that we understand a little more about static pressure theory, let's head over the page to see how we're going to test it...
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Most Recent Comments

08-12-2007, 11:22:40

Awesome detailed review Peevs. Looks like they're a winnerQuote

08-12-2007, 13:09:05

Thanks Kemp I'm glad you liked it.Quote

08-12-2007, 14:08:50


nice review, well done.

if i get a much bigger case for me next build, i'll consider these fans instead of the thinner S-Flex.Quote

08-12-2007, 15:30:31

Very sweet fans, great review Matt as usual!!! Looks like I know which fans I'll be using for my next build. I bet those Ultra 3000's in push/pull around a triple 120mm rad would be amazing.Quote

08-12-2007, 15:57:18

Thanks weihk and Fraggles.

@ FragTek: Yeah those Ultra 3000 would work perfectly in a push/pull config. Just make sure you run them at 7v though aight Quote

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