So what's the difference between one tube size and another??
Way back in the day, when many w/c setups were 3/8", and only some were 1/2", we had the super-hot AMD T'bird CPU's, and poorly designed open-flow waterblocks with little internal furniture that demanded that as much flow be rammed through them as possible to perform. Further, most pumps that were available were like the Eheim 1250. High-flow, low-pressure pumps that dumped a fair amount of heat, and radiators taken from cars that also demanded flow rates in excess of 10lpm to get past their performance knee.
i.e. 1/2" ID made sense then. The benefit still wasn't huge over 3/8", but it was noticeable. Tests back then showed that the move from 3/8" to 1/2" meant anything from a 0.2-1.0C improvement, depending on various factors. A 1.0C improvement was enough for most people to make the jump, and so 1/2" tubing got its following. I also feel responsible in part for the jump to the 1/2" band-wagon, since I conducted a fair few tests back then to justify it.
Fast-forwards to today, and we have well designed middling restriction blocks which are more flow agnostic. We have well designed radiators that are more flow-agnostic (both in terms of fan power and liquid flow-rate). We have pumps that are near optimal for PC water-cooling that strike a good balance between pressure, peak-flow, noise, and heat. In short, everything has, quite rightfully, been pegged back from old-school high-flow excess, and tempered with a more balanced approach.
The one thing remaining is tubing size....
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