Arrrgh i shouldn't post while at work. All i had was to move that comma 3 digits on the right... but I can't even count up to 3 !!
At least i'm not alone. Here's a famous quote:
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SECOND BROTHER:
And the Lord spake, saying, 'First shalt thou take out the Holy Pin. Then, shalt thou count to three. No more. No less. Three shalt be the number thou shalt count, and the number of the counting shall be three. Four shalt thou not count, nor either count thou two, excepting that thou then proceed to three. Five is right out. Once the number three, being the third number, be reached, then, lobbest thou thy Holy Hand Grenade of Antioch towards thy foe, who, being naughty in My sight, shall snuff it.' MAYNARD: Amen.
KNIGHTS: Amen.
ARTHUR: Right! One!... Two!... Five!
GALAHAD: Three, sir!
ARTHUR: Three! [angels sing] [boom]
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hehe

(edit) besides 1.38 ml is like what, a fly's fart...

to the point: the Daden res will not have issues since it is probably has a safety factor of 1000 like everything else they make:;):

read on if needed... the most interesting thing about gases is they increase in either pressure or volume as temperature increases. we all have seen the perfect gas law from high-school chem (i hope):

P*V=n*R*T

you need to ignore the water volume & consider only the gas volume because your teacher said so a while back :). expansion "P" changes can be greater than "column pressure" (water weight) at the bottom if enough air is sealed in the system. i would also consider the resistance of the system from inlet to outlet of the pump, the inlet has reduced pressure & the outlet has elevated pressure. the pressure drops mostly when it passes through waterblocks, so the pump outlet line sees the most pressure. restrictions along with large air volumes would dictate the pressure safety factor required for a cooling system's tubing, clamps, etc...

Uh... yeah, right.:confused:

Ok, so that everybody is still clear...:shrug:

There is an overall pressure, that affects all the components simultaneously. It is similar to atmospheric pressure.

There are also individual pressure points, where for example, the pressure is high inside of a waterblock, because there is a lot of flow resistance. This pressure is due to the speed of the coolant, and not the flow rate.

Ex. If you try to cram 1'000 gph through a 1 foot pipe, there will be a lot of pressure on the walls of that pipe. On the other hand, if you try to run the same 1'000 gph through a 24 foot pipe, the pressure will be minimal, and the 24' pipe can handle the little pressure very easily. SAME flow rate, different speeds, different pressures.

These individual pressure points exist regardless of the atmospheric pressure.

The res may induce an overall pressure.

please, is my english that confusing??? if you will, think of the cooling system pressure drop graphically. the pressure is on the Y-axis, distance from the pump outlet on the X, radiator & waterblock indicated:
now does my post make sense? the small gradual drop due to tubing is not shown. the magnitude of the outlet pressure & the pressure drops due to the res & blocks increase with velocity (speed of water). the pressure may equal to ambient pressure anywhere along the water path. gravity & air-expansion create overall pressure, which shifts the graph up (BTW, gravity vs distance along the water path is somewhat random in most cases).

erm, we all forgot the most important tfactor:D
the lengthwise expansion of our tubing!!! after i turned on my airconditioner today, the water level actually rose. the joke's on us;P

Errr.... Ok... I think... (joke!)

It's there.

The pressures that you're talking about are what I refer to as individual pressure points.

These pressures are measured against atmospheric pressure, as a reference point. It's interesting to note that the pressure at the pump inlet is actually below atmospheric.

The effect of compressing the air in a sealed res, is to increase the overall pressure in the entire rig. For example, as above, if the pressure in the res was high enough, you might be able to take a positive pressure measurement at the pump inlet. Of course the pump outlet pressure would be that much higher too.

Yes. And another thread shown overall pressure has no influence on cooling abilities.

Bingo that's the formula i've been using... (not telling this time, or ppl would accuse me of posting lots of math garbage)

We were *also* counting water expansion. If we say the air is heating as well (to keep it simple, let it raise 5°C) i'm not sure it will change anything... Maybe 2 fly farts ?

Air has nothing to do there... Restrictions, pump-induced pressure, etc. come from the flow of water.
Besides a large air volume trapped in the circuit (for ex. in a big res) would not compress more (less volumetric change due to water but more due to heat - i.e. nothing).
Again the thread was about captive air creating pressure - and i hope we shown one hadnt to worry about that.
Pump induced pressure and restriction induced local pressure are what one needs to worry about (along with water weight itself), they're far greater.

So i guess the final verdict is that's it's okay to keep the cap closed and it wont cause any problems? My only input is that it's so dang expensive. $25 for a 3" tall 2.75"Diam cylinder

Correct. No problem.

$25? Well, I used a planter's peanuts container... holds about 3 pints. Cost? $3.87 at Wal-Mart. I put a barb in the side and one in the top....use lots of goop... works great.

*added bonus = you to eat peanuts. I like mine honey roasted.