Running the test at low flow rates (0.5gpm range or below) also gives a very nice view into how well the blocks transfer the thermal energy from the CPU to the water if you monitor the inlet and outlet temps of the water. At the lower flow rates you get a measurable difference in the inlet vs outlet water temps. The greater the difference , the better a block is transfering the thermal energy to the water (given that all the blocks in the test are operated at significantly less than maximum flow rate and at the same flow rate). At higher flow rates the delta temperature is so close between blocks that it you don't get usable data (a good example is Morphling's recent tests).
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