[pippin88]

Originally posted by Cathar
The technical answer is that a one waterblock may offer a lower thermal resistance than another, through whatever means available (turbulence, jet impingement, fine-channels, whatever). The whole point being that the water is manipulated in such a way that for the time that the water is in contact with the hot sections of the block (which has been heated up by the CPU) that the water achieves a higher coefficient of convection, also referred to as h. The units of h are Watts per (Area * dT), where dT is the temperature difference between the surface (of the waterblock) and the liquid flowing over that surface.

Now each water-block offers a fixes amount of area for cooling, so if we multiple h by the area of the waterblock, we are left with units of W/dT, or how many watts of heat energy will move into the water based on the temperature difference between the waterblock surface temperature and the water flowing over that surface. T is often measured in Celcius, and so the term is commonly summarised at W/C, or even some-times inverted to C/W.

Now some waterblocks will have a higher W/C value than others. Higher is better. For example, one block might have a W/C value of 20, while another might have just 10.

For the one with a W/C value of 20, and a heat source of 100W, the surface of the block coming into contact with the water must be 100/20=5C warmer than the water before 100W of heat energy is transferred into the water.

For the block with a W/C value of 10 and a heat source of 100W, the surface of the block coming into contact with the water must be 100/10=10C warmer than the water before 100W of heat energy is transferred into the water.

Now since the waterblocks are attached to the CPU (and ignoring the thermal paste junction for now) this means that the CPU will be 5C cooler for the block with the W/C of 20, because it is more efficient at transferring the heat into the water. The block with a W/C of 20 is more efficient because it doesn't need to get as hot before it's able to transfer 100W of heat into the water, unlike the block with the W/C of 10, which has to get much hotter. Since the CPU is attached to the blocks and is the source of heat, the CPU is correspondingly hotter as well for the second block.

This is what's referred to as thermal resistance. The watts (heat energy) transferred into the water is the same, it's just a question of how hot does the block need to get before it transfers the heat energy that the CPU is emitting.

Please also note that the above is am extremely simplistic explanation and does not attempt to take into account concepts like varying thermal gradients through the metal itself.

Excellent explanation.

I knew that the water wouldn't get hotter on a different block, but had not come to grips with the reasons for this.

This put it all pretty clear.