Hi all, I would like to share some of my experiences with different types of tubing and fittings. For the record, I am co-owner of the watercooling shop www.vcore.dk - located in Denmark. We started back in 2003 with the White Water, when 1/2" ID still ruled the earth supreme (except in Germany).

First some comments regarding the tubing and fittings discussed here:
Cathar’s excellent graphs show much the same as some local tests have shown here in DK: That 8mm ID is a sweet spot with the right barbs, 3/8” ID is plenty for high performance and 1/2” is overkill for 99,99% of users.

And precisely 8mm ID hose has for years been the most common in European watercooling – but in 2 different versions:

Version 1: So called “10mm Push-IN” (push-fit) fittings with 10x8mm stiff PU hose.
(This has also been used by Asetek for their well-known, and now outdated, Waterchill systems, although they used a special hose with only 6,5mm ID)
Version 2: So called “10/8mm Push-ON” (compression) with 10x8mm soft PVC hose.
Pros: Both are easy to fit, secure, gives decent flow, and looks good and clean.
Cons: PU Hose is very stiff and does not bend well at all. It is often necessary to use 90 degree angles to get around the sharp bends. The thinwalled 10/8mm PVC hose is not stiff at all, but rather it almost collapses under its own weight after a while.
The Push-ON fittings has only about 6.85mm ID, which reduces flow some.

To get around the problems with the stiff PU hose, some people instead use a soft 10mm OD PVC hose in the Push-IN fittings. This is possible to do, but NOT recommended, as they will easily leak, especially after a few months - as others have also stated in this thread.

However using a 10x7mm ID (10mm OD) soft PVC hose with Push-ON (compression fittings), works very well, and is to me the perfect combo for many things. Only drawback is that 7mm ID is a tad small for super high performance. But that is easily fixed in a system with parallel loops, by using 3/8” ID with hi-flow barbs for the critical parts between reservoir, pump and radiator, and the 10x7mm for connections to and from the waterblocks.

Regarding Push-IN fittings for 3/8” tubing: (aka Push-Fit aka Quick-Fit)
Cathar: I can certainly see that 3/8” ID with Push-IN fittings are a very sweet spot theoretically. But unless I have misunderstood (and then I humbly apologize), you need a stiff PU or PE type tubing with 1/2” OD for this, and a PU hose with sidewalls that thin is completely unsuited for the tight bends and radia in a PC-case. It would be necessary to use a lot of 90 degree angels I would think (as with the common 10/8mm PU) – which would completely ruin the good flow.
And if you instead use a soft PVC hose, then you have problems with leaking over time and evaporation through sidewalls (plus it pulls out of the fitting very easily!).
So despite its theoretical merits, I don’t feel large diameter Push-fits are suited for practical watercooling.
- What is your experience with it? Which hose have you used?

And the positive things about the Push-IN system:
The smaller the diameter, the better it works really! Aqua Computer uses 8x6mm PU hose (they call it “Plug&Cool”), and this hose is actually quite nice. Handles bends well enough, although 90 degree angles will still be needed sometimes – but that is true of most tubing.
And when we get down to the very cute 6x4mm PU, as used by www.watercool.de for their “MICRO” concept, it works even better. Perfect in a seperate loop for DDR RAM watercooling and similar stuff.


And some of my views and thoughts on the topic:
Our webshop is basically a very time consuming hobby, so I have had ample opportunity to try a lot of different watercooling equipment and setups. We sell everything from 4mm ID (!) hose to 12,7mm (1/2") tubing, and use all types of fittings: Regular barbs, Push-In (aka push-fit) and Push-ON (aka compression) - often combined in the same system using multiple loops, and varying tubing size according to the heatsource.

Generally my personal objective with watercooling is not to obtain the lowest possible temperatures (contrary to many in this forum I realize), but instead to make the quietest possible systems, with watercooling on most all components: CPU, NB, SB, GPU, Mosfet voltage regulators, HDD and RAM.
Systems with this many waterblocks are "multi-parallel" using a distributor manifold and perhaps 3-5 seperate loops.

What you need to keep in mind, is that in Europe it is quite common to also have watercooling on HDD and perhaps RAM and even voltage regulators around CPU. So even if the tubing ID is only 7mm, the actual area for water to flow can be perhaps 2-4 greater when using multiple loops, and then the flow/backpressure situation suddenly looks quite different. It would be very interesting to make simulations or tests of flow and backpressure in that type system...

Also when using watercooling in this way, you automatically start using smaller tubing, else it is simply not possible to fit all the hose in the case. I usually use a combination of 10mm ID (or 3/8”) and hi-flow barbs with 10mm ID, mixed with 10x7mm (7mm ID) Clearflex on Push-On (compression) fittings.
The Push-ON fittings we use are actually made for 10x8mm hose, but as the thin sidewalls make this type very prone to bending/collapsing, we started using 10/7 mm PVC instead. As mentioned above, this has proven to be an excellent combination to work with, as it provides a very tight bending radius, together with acceptable flow, and very secure and nice looking fittings. The ID of these fittings (with G1/4 thread) are 6,85 mm - so it almost 7 mm straight through: http://www.vcore.dk/shop/1-4-10-470p.html
The compression nuts can be a bit hard to fit as the 10x7mm tubing is 1mm oversized for the fittings, but tools are not needed, and it sits very secure.
I am wondering if this combo would somewhere right between the two types of 8mm (yellow and green curves) on Cathar’s graph – or if it perhaps is about the same as using 8mm with 6mm barbs (yellow line) ?

We have been trying to find the same type Push-ON fittings to use with Clearflex or Tygon 3/8 x 5/8 (9,6 x 15,9mm) which has a good inner diameter for high flow, and also excellent bending radius due to the thick sidewalls (which most people here probably know all about). Ideally it should be a bit oversized on the inner diameter to keep a full 3/8” ID through, so it should be around 15/11mm or 16/11mm maybe.
But apparently that doesn’t exist – so should anyone know of such a fitting, I would love to hear about it, as I think that would be very near perfect! This would give the same flow that Cathar gets with 3/8” and “quick-fit”, but instead with a good hose with tight bending radius.

Sorry for the long post!