DrJ said...


You are correct ...

Under a vacuum , there is no relative humidity. A vacuum Is an empty space in which there is a substantial absence of anything, typically and relevant to this thread anything being mostly air. Air being typically and mostly comprised of nitrogen and oxygen but also a degree of water.

Taking a board as an example, applying a vacuum to the inside of the board, even a near vacuum using a low power pump, will substantially remove the air.

Not too many numbers there Dr J, but "substantially removing the air" is a dangerous thing to do to a board. As the table above shows, at 30 degrees water vapour over liquid water will only provide a pressure of 4.2 kPa. The board will collapse, so you always have to have some ordinary old O2, and N2 in there to hold it at at least 75kPa in there to stop it collapsing.

There is an old demonstration with a kerosene tin. Water in the tin is vigorously boiled so there is nothing but water vapour in it. The lid is put on and the tin allowed to cool. Well before it comes down to 30 degrees the pressure of the water vapour drops enough to crush the tin.

The interesting thing about water evaporating is that it does so based on the partial pressure of water vapour above it, not the total pressure. Conversely the vacuum pump operates on total pressure, it can't separate out the water!

So the trick is to lower the partial pressure of water vapour below saturation, (< 4.2 kPa) so evaporation occurs, but maintain the total pressure of the board at > 75 kPa so it doesn't collapse.

The best way to do this is to pass dry air through the board.

barn said...

FormulaNova said...

<snipo>


So Barn and DrJ agree! Woo hoo! (lighten up fellas, its only a discussion) I now feel that I got it completely wrong and that it really works some other way! Black magic says I!

FWIW I think the discussion went sideways when Barn said:

Your technique will work when the vacuum pump pulls a pressure of less than 4.2kpa. Until that happens, the system will chill out at equilibrium. (seemingly correct)

Do I really need to point out that you shouldn't try pull a vacuum of 4.2kpa? [/i](not correct?)

When, (again a physics-know-nothing here) you don't actually need to pull a vacuum of 4.2kpa, you just need to pull a vacuum of something less than that to create nett evaporation? No?

Well, if I learned one two things today, I think it is that you can evaporate water from a board without a hole to let air in, (and the science seems to hold up) AND pressurised Nitrogen is a great way to blow up a board (if you have a flakey regulator).

Yes, pulling a vacuum of less than 4.2kpa would boil the water, and it will fill the board up with water vapour as fast as you can pump it out.

Pulling a vacuum less than 4.2kpa (let's say 3.5kpa) would mean there is 100kpa on the OUTSIDE OF THE BOARD..

THIS MEANS THERE IS 97kpa ON THE OUTSIDE, PUSHING IN!!!..

That means there 100,000 pascals per square metre. A pascal is a 10th of a kilogram, so there is 10,000 kilograms per square metre of board.

Formula Nova, you would need to apply this pressure to a board before the water will boil. We all know a board cannot take this force.

An Air conditioning system does not have many square metres, it can take it.

Hey, come on, I was just about ready to switch off the laptop...

Well, what about 4.19Kpa? Would the board take the 0.01Kpa difference? Would this still result in nett evaporation?

Sadly, as my gauge is a vacuum/economy gauge from a car, I think in inches of mercury. (I got a 'proper' gauge with my proper HVAC vacuum pump, but they are scaled way too differently to be used for vacuum bagging.)

I think 28 inches of mercury is a close to a 'complete vacuum', which seems to marry up with your table showing that 760mm of Hg is vapor pressure of water at 100degrees C. is that 1 atmosphere?

Anyhoo, I am sure I use 7 to 14 inches of Hg to vacuum bag stuff to the board, so I am sure it will cope with those pressure differences. Given that you are providing a partial vacuum to the board, surely you would have nett evaporation?

Edit: Barn, I still think you might be arguing that you need to draw a vacuum of close to 4.5Kpa inside the board. You don't. You just need to reduce the pressure inside the board to something less than its partial pressure.

Ian K said...

So the trick is to lower the partial pressure of water vapour below saturation, (< 4.2 kPa) so evaporation occurs, but maintain the total pressure of the board at > 75 kPa so it doesn't collapse.

The best way to do this is to pass dry air through the board.

Dang scientists using these new fangled kilo something or other units.

What happened to inches of mercury!

FormulaNova said...
Hey, come on, I was just about ready to switch off the laptop...

Well, what about 4.19Kpa? Would the board take the 0.01Kpa difference? Would this still result in nett evaporation?

The board cannot even get near 50kpa, that's half an atmosphere. 25kpa is one quarter of an atmosphere. 10kpa is one tenth of an atmosphere. 5kpa is one 20th of an atmosphere.

At 5kpa there is too much atmosphere outside the board pushing in. Board goes splat.

Ah, finally a windsurfing topic worth registering for

I'd say Barn is right - once the pump has reached its nominal vacuum, the liquid and vapor inside the board will also reach equilibrium fairly quickly and will stay this way until the cows come home. Static system. This is in a perfectly "air-tight" board though.

If the board has holes drilled, then the pump will likely not be able to develop much vacuum, so the water will not evaporate quite as easily at the same temperature. However, the movement of new and dry air through the holes will have the triple effect of displacing vapour (and thus encouraging more evaporation); pushing water out mechanically (this is not out of the question, given the Styro core); and importantly - the air/nitrogen molecules will be interacting kinetically on the Knudsen layer (the thin boundary between liquid and vapour) which results in much increased evaporation. The last one is just a fancy way of describing what happens to your swimming pool water level on a windy day.

So IMO, holes and vacuum will be more effective than just vacuum, however, I'm all for spinning, because evaporation will likely leave most of the salts behind. Oh, and if you want to do "just vacuum", i.e. without drilling holes, then a cyclical pumping might be the way to do it, as long as you allow DRY air to enter the board during periods of no vacuum.

And hello everyone!

Who cares if vacuum can remove the water in theory. We all know you can't get 100% of it out so passing dry air through the board will be SAFE and as effective as any other method, maybe faster maybe slower, but safe and effective.

First the board should be weighed and compared to the specified original weight. I'll bet not much water at all has been retained and it's only in the immediate area around the plug.
I am a bit skeptical about a board collapsing from too much vacuum, I think air will find a way to leak in before that happens.
Someone should do an experiment and see if that is even possible.
I have an A/C vacuum pump and an old slalom board I don't want that is in really good shape but it doesn't have a vent plug, any ideas how to connect it?

DunkO said...





well i totally f#c!ed it up.

went to the loo, then took a phone call and on my return some how the nitogen pressure had gone up. basically it prssurised the board and blew it to pieces.
totally gutted...

Hey Dunko sorry to learn of the demise of your board.
Not wanting to sound like an ambulance chaser or road crash stalker... any chance you can post a pic of the board after the event ?

Mark _australia said...

Who cares if vacuum can remove the water in theory. We all know you can't get 100% of it out so passing dry air through the board will be SAFE and as effective as any other method, maybe faster maybe slower, but safe and effective.

Well I care because there is some serious misunderstanding of the theory going on. The board is already toast. We're just waiting for pictures.

I made a video with a solid explanation on how a partial vacuum can evaporate water. Would anybody like me to upload it?

It might jolt Drj's memory.

qldnacra said...

This is starting to sound like a conversation on "The Big Bang Theory". How about we all just remember to do up our vent screws before sailing problem solved

there in lies the problem...remembering.

I can't believe I missed the post where DunkO blew up his board... (must've skipped to page 2)

Ah well, typical seabreeze member (me) browsing, making comments without reading the 'whole' thread.

Pics (post-explosion) would be good though.

DunkO said...
all time and matter would stop, Correct me if i am wrong).

Hmmmmm! Now that conjures up possibilities [}:)]. However, I don't think time comes to a halt somehow. And as for matter; in the theoretical complete vacuum there is no matter to come to a halt, what ever that means. Might be confusing that with absolute zero .

This is very hard for me to admit, but having just read the whole thread (been away for a few days), barn is spot on the money, and his numbers fall out pretty much spot on too. There are always slight variations because we're dealing in an ever changing ambient environment, but he's sure close enough to classified as correct.

At the other end of the problem - remembering to do up the plug; The main problem arises when there might be (or possibly usually is) the odd barest hairline cracks in the outer skin of the board. Having heated up the board in the sun (whilst rigging, talking, etc) we then tighten up the plug, launch the board into cold water thereby causing the board to contract somewhat rapidly, creating a partial (relative) vacuum inside the board. If there are any crevices at all water will be sucked into the board.

The absolute ideal (though totally impractical) would to be able to have the board just a little cooler than the water before sealing up the plug and launching the board. This would create a mild (relative) +ve pressure inside the board, thereby precluding the ingress of any unwanted fluid.

2 penneth on your way out please .

Who cares who's right or wrong about pressure and stuff.
I just want to see Dunkos pics of the exploded boards.
C'mon DunkO show us the carnage!!!!!

best method is have a CHInook style two bolt deckplate. your bung when fully removed 'clips' into the plate and is just big enough not to fall out. Have used this method for years, along with other mates i have told. the good thing is you caN NOT connect your mast base/go sailing until the bung is removed from the plate!!simple

geared4knots said...

best method is have a CHInook style two bolt deckplate. your bung when fully removed 'clips' into the plate and is just big enough not to fall out. Have used this method for years, along with other mates i have told. the good thing is you caN NOT connect your mast base/go sailing until the bung is removed from the plate!!simple

where was this post three days ago, as you can see that is the base plate i have.

will post photos of the carnage soon, drowning my sorrows at the moment.