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Unread 11-11-2004, 02:46 PM   #1
Cathar's Avatar
Join Date: Sep 2002
Location: Melbourne, Australia
Posts: 2,538
Default Testing waterblocks vs pumping power

Been thinking on this problem a bit and attempting to find a way to convey to an everyday user what waterblock performance is like.

The problem with flow vs C/W graphs is that they do not convey any pressure based information. One can look at a graph of two blocks of two vastly different levels of restriction, and performance per LPM may be higher than the less restrictive block, but the less restrictice block will perform better per unit of pressure drop.

Then we get to the issue of a pressure drop vs C/W graph, where real world pumps do not provide a constant pressure drop as flow rate varies. So while in the example above the second block performs better per unit of pressure drop, it will also be allowing a higher flow rate due to its lower restriction, and so any centrifugal pump that is applied to it will apply less pressure to that block, than to the first more restrictive block.

Ideally we have to present both graphs for a full picture, and the reader needs to analyse and understand both graphs.

That's how it should be done, but it's requires substantial work to glean an overall picture from 4 graphs (flow vs C/W, pressure vs C/W, block pressure vs flow, pump PQ curve). In order to present a composite picture of block performance vs pumping power, then how about the possibility of displaying a graph that is just that? Basically a graph of waterblock C/W vs Pumping Wattage?

So I was thinking about how to present such in an easier to understand picture, and thought that DC pumps presented one possible answer. Using a variable voltage power supply, and tracking the current that flows through to the pump, one can measure how much pumping power is being applied to the cooling loop. On top of the other charts, plot a C/W vs Pump Wattage chart.

There are some drawbacks to this approach though, and primarily it falls down to the test loop's resistance. Pumping power gets spent overcoming all the restrictions in the test loop, and we would not test loop resistance to be terribly high. The way around this though would be to calibrate it. Plot a graph of the test loop's pump power vs flow rate curve with no waterblock in the test loop. Then when plotting data points for a tested waterblock with a certain amount of pumping power, adjust the pumping power by subtracting the test loop's required power at whatever flow rate is being measured. In this manner I believe that the pumping power vs performance curve can be easily calibrated to remove the test loop's characteristics from being an influence.

The second drawback, which is perhaps more serious, is what to exactly use as a pumping model. There are high-head/low-flow pumps (Laing DCC), and high-flow/low-head pumps (Eheim 1250), and there are pumps that fit somewhat in the middle and offer both (Laing D4). We would probably be after a middling sort of DC pump that offers up to ~20LPM of peak flow for the ultra low flow resistance blocks, and up to 10mH2O of pressure head for the ultra high head favored blocks.

That could be achieve in one of two ways. Either by putting 2 x Laing D4's in series, or by using a pump like an Iwaki RD-30 which at 24v offers 10mH2O head and 20LPM flow. Both types of pumps do support running at very low electrical powers (through decreased voltage). The D4's in particular can run as low as 1W each before failing to turn any longer, and can handle being run at up to 15v easily (for 5mH2O pressure head, or 10mH2O for two pumps in series), or at about 25W of pumping power each.

Thoughts? Would this be an appropriate way to present an easier to understand mosaic of waterblock performance that encompasses a good approximation of real world pumping characteristics with varying pumping power in a single easy to present graphing format that the layman can look at and determine which blocks are good for which pumping power scenarios, and for any given pumping power, which encompasses both flow and pressure, what to expect?

[Corrected some statements that were missing adverbs]

Last edited by Cathar; 11-11-2004 at 03:21 PM.
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