Also those that say a vacuum pump won't work because the system will reach equilibrium are wrong. Equilibrium will only be reached in a closed system, attaching a pump that constantly draws a vacuum will remove any water vapor such that equilibrium will not be reached until all the water is gone. How long this take will depend on temperature and pressure, the higher the temp the greater the vacuum the quicker..... However the risk you take is further damage to the board.

Using a vacuum in a board which has extra holes drilled in will be fairly pointless and the rate of evaporation will be purely temp related as the pressure will be constant. Although a powerful pump may increase airflow enough to effect more efficient evaporation, but due to construction and small holes flow will be poor and this method will not be as effective as a vacuum.

geez Dunk, un-salvagable?

you coming to gerroa for the comp on the weekend? if so, do you need to borrow a big lads wave board (just do up the screw)?

This is a great thread, its keeping barn out of general

DrJ said...

Sorry barn, even if your pump is controlled electronically such that when it reaches a certain pressure it switches off, what will happen then, is that water will evaporate filling the vacuum with water vapor, this will change the pressure such that the pump will come back on removing the water vapor and starting the process all over again and so on until the water is gone.

Ohhh I'm gad it's not me digging holes for once!!


I'll refer you back to the vapour pressure chart. At 30 degrees, evaporation will only exceed condensation when the partial pressure of water vapour is less than 4.2kpa..

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.

Do I really need to point out that you shouldn't try pull a vacuum of 4.2kpa?

Scientists, chemist, and engineers have been using this method to remove water and solvents from system for decades if not centuries. And the efficacy of the process is directly related to temp and pressure, greater temp requires less vacuum, and vice versa.

You're right it's related to temp and pressure. And I agree, before scientists were corrupt, they did use this technique, it does work..

Just look at the chart, all the info is there. Let's heat our enclosed system to 60 degrees, the partial pressure is now 19.9kpa. So to get the water to exceed equilibrium inside this partial vacuum, the vacuum pump needs to pull 19.9kpa.




When the pressure drops below 19.9kpa, the equilibrium cannot be met, and the water will boil filling the chamber with water vapour, which the vac pump can suck until all the water is gone.

The water needs to boil. We can get the water to boil by heating it up, or by dropping the pressure. If you don't do either of these enough, you'll get an equilibrium.

Unfortunately you still cannot pull 19.9kpa on a board otherwise you'll flatten it. I haven't pointed this out because I assumed everybody knew this minor detail. That's a 5th of an atmosphere.

The scientists who use this method are evacuating closed systems that are able to take the pressure.

You could safely pull a vacuum to 70kpa, but then you need to heat the board up to 90degrees. Good luck with that, might as well scratch the pump and heat it to 100degrees.

Am I right yet?

DrJ said...

Barn please explain the following; my garage reaches a max temperature of around 30 degrees there is negligble airflow, yet the puddle of water from my car airconditioner evaporates in less than about 45 mins. By your reasoning my garage must be under a huge vacuum or when I'm not looking reach temperatures of around 100 degrees.

No problem! But I need to know the following.

1. Dimensions of your garage.

2. Relative humidity when you shut the garage door.

3. How many millilitres in the puddle.

I don't think the issue is whether it will or won't. Isn't it more about how quickly the water will evaporate?

Assuming the air isn't completely saturated and the water molecules are in liquid state evaporation will still occur. By adding a vacuum pump and/or air flow you are just speeding up the process. So yes, as long as the air within the garage isn't saturated the air-conditioning puddle will evaporate, as will the cup of water. If you were to seal off the cup completely, the air would eventually become completely saturated and thus the water level wouldn't continue to drop...

Of course there is also the difference between boiling and evaporation...

DrJ said...

You need none of that information to explain it, and if you truly understood the physics rather than having just googled it you would have known that.

LOL.


No, I need to know the size of the room to know how much evaporation potential the air in the closed system has.

I need to know how much water is in the puddle because we want to know where it all went.

And I need to know the relative humidity, because I need to know how much extra water the air can hold before it's in equilibrium with the puddle.

SailCoothara said...

I don't think the issue is whether it will or won't. Isn't it more about how quickly the water will evaporate?

Assuming the air isn't completely saturated and the water molecules are in liquid state evaporation will still occur. By adding a vacuum pump and/or air flow you are just speeding up the process. So yes, as long as the air within the garage isn't saturated the air-conditioning puddle will evaporate, as will the cup of water. If you were to seal off the cup completely, the air would eventually become completely saturated and thus the water level wouldn't continue to drop...

Of course there is also the difference between boiling and evaporation...

I think DunkO has found an alternate technique, although I am not sure I would recommend exploding the board and drying it in the sun as a good way to remove moisture from a board...

It would be good to see pictures to see how bad the explosion was. Surely only a bit of it exploded?

(Let the physics wars begin! Although a simple explanation for me would be good)

barn said...


I'll refer you back to the vapour pressure chart. At 30 degrees, evaporation will only exceed condensation when the partial pressure of water vapour is less than 4.2kpa..

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.

Do I really need to point out that you shouldn't try pull a vacuum of 4.2kpa?

I am definitely a physics-know-nothing, but doesn't this chart show the positive pressure at a certain temperature, when water is in equilibrium (in a fixed volume)? I.e. if the pressure is lower than that shown, the water will be evaporating more than it will be condensing? (Hey, I spent a minute googling it )

(If I am completely off base, I reserve the right to remove any or all parts of this posting!)

barn said...

DrJ said...

You need none of that information to explain it, and if you truly understood the physics rather than having just googled it you would have known that.

LOL.


No, I need to know the size of the room to know how much evaporation potential the air in the closed system has.

I need to know how much water is in the puddle because we want to know where it all went.

And I need to know the relative humidity, because I need to know how much extra water the air can hold before it's in equilibrium with the puddle.

Lmao you are an idiot you need that information to calculate a rate of reaction, not to explain a principle. You are proof positive that a little bit of information and no real knowledge can be hazardous to your reputation.

Mark _australia said...

The garage analogy is not good as it is say 100ml of water in 100,000L of air space.

In a board we are talking up to maybe 1 litre of water in a 100L board which probably only has a couple of litres of air space in the styro. The air in the board becomes saturated very quickly and needs dry airflow through it to remove the water - and even better as the pump sucks a bit fater than the air can get in, the reduced pressure helps it vaporise.

Your method is sound, but as I indicated this method relies on air flow, as I suggested earlier I doubt that a small drill hole would allow for a significant airflow given the internal structure of most boards, certainly I doubt it would be a great deal more efficacious than simply applying a constant vacuum .... But for sure I have never tried it so don't know.

^^ is that your idea of an explanation ??? You explain nothing, you still have not explained how a puddle evaporates without a vacuum at low temperatures.

All you have proved is that you can use google !

EDIT * Also I never suggested the garage was a closed system indeed if it was it is highly unlikely that the puddle would completely evaporate, this is why bottles of water in supermarket shelves don't become empty over time

^^^^

Lmfao .... You have no idea whatsoever, you crack me up.

^^^^^^^

Ha ha ha ha .... Seriously you should publish your theories you might win a prize .... "When humidity is high nothing evaporates" that's a gem, it's a **** attempt at an explanation, but it is in itself gold.

barn said...

FormulaNova said...


I am definitely a physics-know-nothing, but doesn't this chart show the positive pressure at a certain temperature, when water is in equilibrium (in a fixed volume)? I.e. if the pressure is lower than that shown, the water will be evaporating more than it will be condensing? (Hey, I spent a minute googling it )

(If I am completely off base, I reserve the right to remove any or all parts of this posting!)

You're correct.

The 'positive pressure' is called teh partial pressure. All the water can see is the water molecules in the air. Under the conditions listed in the chart, molecules evaporating is equal to the molecules condensing. Lot's happening, but nothings changing.

If we drop the partial pressure below the listed vapour pressure. You will get evaporation until it returns to equilibrium.

The pump can only see total pressure, the water sees partial pressure. The pump doesn't care what the water is doing.

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? (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).