Vacuum leak detector question

[Interceptor121 766]

Yes I would never do that.

 

Am looking at those dew point charts just for fun. There is one where with a given humidity % at increasing temperature the dew point goes up

 

Example I prepare the housing at 25 C 65% humidity in my air conditioned room Dew Point is 17C

I then go outside and my housing reaches the ambient temperature of 32C whilst am doing the boat ride to the dive site that is at 22C

I would say that the humidity does not change as the % number of particles is still the same

Now that the housing is 32C the new dew point is now raising to 24C in virtue of temperature increase

I go inside the water at 22C and the housing fogs as the port is now colder than the new dew point of 24C

 

If I had prepared the housing in a room at 55% dew point would have been 15C but then once at 32C dew point would have been 21.3 no condensation would have occurred

 

This is consistent with people flushing the housing with compressed dry air before locking it in to push the moisture out

 

That would explain the practice with your dew point theory but would require the dew point to change with temperature even if the gas is closed in. However if dew point is a measure of the absolute moisture that should not change which leads me to believe that fogging inside a housing depends on the total amount of moisture inside and the temperature. The theory of the vapor hitting the cold wall and changing state seems easier to reconcile otherwise if it is like you say warmer would mean dryer and there is no way this would create condensation.

 

So a dry room or some system to absorb the moisture like a dessiccant would seem the most effective way of dealing with all of those issues which is also what in layman terms most people would do

Edited April 1, 2014 by Interceptor121

[diverdoug1 56]

 

So a dry room or some system to absorb the moisture like a dessiccant would seem the most effective way of dealing with all of those issues which is also what in layman terms most people would do

Now youve got it!

Yes, The tempaerature of the room you prep in is not an issue once the sealed system reaches temperature equilibrium at the dive site. All that matters is how much moisture that is trapped inside your housing. The less moisture in the housing atmosphere, the less it will tend to form a condensate if the housing is plunged into cool water. This is why the decreased pressure are and decreased trapped water molecues are helped by the vacuum system,, and why starting with drier air and dessicants help.

Edited April 1, 2014 by diverdoug1

[Interceptor121 766]

That is what I thought in the first place to be honest. And I do not think the dew point theory as in my previous post can explain fogging inside a housing except you start with an environment so wet that the whole things fog up right away

If all you say is true warmer means dryer and easier to carry vapor around. On the other hand the moisture is trapped inside so it can only move around

 

What really seems to matter is the temperature gradient between the hot air inside and the water outside.

In general the water is never warmer than the housing. My housing reaches 32C shooting video and the water is always somewhat colder. This is compensated to an extent by the fact that the metal housing is also cooled down and as a consequence the camera. This explains why housings fog in hot climates and not just in cold water.

 

A desiccant seems the only way forward as it will intercept and trap the vapor molecule and manage their condensation in a controlled fashion as well as choosing the drier environment of those available for housing preparation (may or not be the air conditioned cabin and I like very much using compressed air inside the housing as way to remove moisture)

[diverdoug1 56]

And I do not think the dew point theory as in my previous post can explain fogging inside a housing except you start with an environment so wet that the whole things fog up right away

 

Wrong again

Fogging ONLY occurs when the dew point (100% realtive humidity) is reached at the air / surface interface. The "dew point theory" describes ALL cases of fogging/condensation formation. This is decreased by

1. decreasing the water in the housing atmospere via vauum / dessicant/ only opening in a dry environment

2. decreasing the pressure on the housing (thereby decreasing relative humidity / increasing dew point)

Edited April 1, 2014 by diverdoug1

[Interceptor121 766]

The dew point of the gas being linked to the absolute humidity is not changing according to your own link and the idea that inside a gas you have multiple dew points seems overcomplicated Or are you now saying that the dew point does change inside the housing with increased temperature? By definition it should not (absolute humidity not relative)

In my mind I see two possible scenarios prepare the housing for cold water and end up with a dew point higher than the water temperature--> the whole housing fogs immediately camera heating up does not change the moisture there is no ventilation game over

Second scenario warm water with camera far away from the dew point still fogging because of the hot vapor hitting the cold wall and changing state according to water phase diagram. I have seen this happening in Egypt and it is not that wet I can assure you

Once temperature goes up the pressure inside the housing goes up how does that go with your case two?

Edited April 1, 2014 by Interceptor121

[diverdoug1 56]

are you now saying that the dew point does change inside the housing with increased temperature? By definition it should not (absolute humidity not relative)

YES!!!! The Dew point = the point where relative humidity is 100%. (THAT is by definition). As temperature increases in the housing, the dew point (which is the point where the housing air is saturated with water, which is also 100% relative humidity) INCREASES.

If you take a housing at near dew point (say relative humidity of 99%), and cool it, the dew point decreases (and the relative humidity reaches 100%)............CONDENSATION FORMS! Then as the camera warms the interior, the dew point increases/relative humidity falls, and the fogging dissipates.

Edited April 1, 2014 by diverdoug1

[Interceptor121 766]

OK so you are disagreeing with your buddy Stuart on this. A gas in an enclosed space can change the dew point.

 

Going back to the warm water scenario I prepare my housing at 25C 65% humidity dew point is 17.54 water is 24C if I jump in right away no condensation happy days

If the housing reaches ambient temperature of 34C and I can assume the humidity to stay at 65% the dew point is now 25.85C I jump in and it fogs (this is a real scenario)

 

So either the humidity is fixed or the dew point is fixed otherwise you can't calculate the other. Can you confirm the humidity can be fixed and the dew point moves?

Edited April 1, 2014 by Interceptor121

[diverdoug1 56]

OK so you are disagreeing with your buddy Stuart on this. A gas in an enclosed space can change the dew point.

 

Going back to the warm water scenario I prepare my housing at 25C 65% humidity dew point is 17.54 water is 24C if I jump in right away no condensation happy days

If the housing reaches ambient temperature of 34C and I can assume the humidity to stay at 65% the dew point is now 25.85C I jump in and it fogs (this is a real scenario)

 

So either the humidity is fixed or the dew point is fixed otherwise you can't calculate the other. Can you confirm the humidity can be fixed and the dew point moves?

Wrong, your assumption that the relative humidity of the housing will be at 65% once it reaches 34 degrees is incorrect. If the relative humidity inside the housing was 65% at 24 degrees, the relative humidity will be lower at 34 degrees (remember, warm air is thirsty air).

Edited April 1, 2014 by diverdoug1

[Interceptor121 766]

Then your theory doesn't explain why the housing did fog and I have to revert to the temperature and kinetic energy of vapour moving the particles to the edges and those changing state touching the cold walls.

[nortoda1 13]

The topic that just keeps giving, loving it :-)

 

 

Sent from my iPhone using Tapatalk

Edited April 1, 2014 by nortoda1

[diverdoug1 56]

Can you confirm the humidity can be fixed and the dew point moves?

In a sealed vessel, so the ammount of water molecules contained remains constat), increasing the pressure will decrease the dew point and increase the relative humidity.

[diverdoug1 56]

Then your theory doesn't explain why ............................

Edited April 1, 2014 by diverdoug1

[diverdoug1 56]

I am faced with one of 2 possibilites

1. If he's trolling then

2. If he has a comprehesion deficit issue and really belives what he is posting, he has my condolences.

 

Either way, he does not care to understand or is unable, and will continue to argue in esoteric circles. I have, to no avail, done my best to help him grasp what most find to be a fairly easy concept. I apologize for helping to so horribly hijack this thread.

 

If anyone ELSE is still interested in this material, follow the links provided to see the lab set-up we use for our undergraduates, which demonstrates the physical properties described in the preceding discussion.

 

http://www.phywe.com/461/pid/23579/Condensation-of-gases-through-an-increase-of-pressure-andthrough-cooling.htm

 

and a Full discussion of dew point, relative humidity, and how it changes with temperature AND pressure.

 

http://www.youtube.com/watch?v=D_UpZwLFmVY

Edited April 2, 2014 by diverdoug1

[Interceptor121 766]

In a sealed vessel, so the ammount of water molecules contained remains constat), increasing the pressure will decrease the dew point and increase the relative humidity.

Uhm not according to this diagram? We can say with the increasing pressure and reduced volume, the dew point temperature also increases.

http://www.oxywise.com/en/news/What-is-the-dew-point-and-how-do-you-calculate-it as posted earlier today

 

When you say that the relative humidity increases that is true is the pressure was constant (isothermal heating) but in this case is the volume to be constant and temperature rising makes the pressure rise (http://en.wikipedia.org/wiki/Isochoric_process) like in one of the phases of a petrol engine.

So I do not think the relative humidity rises it may as well stay the same?

[Interceptor121 766]

Found some other documentation as volume stays the same no mechanical work is produced and the only effect of heating is to increase internal energy which makes the particles move more. As those hit the colder housing walls they produce condensation this maps the housing in hot climates scenarios. Reducing the % of moisture is key.

Cold water case the housing is prepared at home in normal conditions 21C 55% humidity the particles inside are quite warm and buzzy if the go in the water right away they may fog the housing immediately. In this case is better to prepare the housing in the fridge door like I have heard some people saying, the particles are nice and steady and do not go anywhere and are pretty cold. They don't get more excited going outdoor and when in water they get even more relaxed. Eventually the camera starts warming up the environment and getting the particles busy but your moisture packets keep things at bay. Alternatively prepare the housing inside and cool it down gently to a temperature similar to the water (better as there are not mechanical shocks) then same as before.

Summary: inside an underwater housing there is a process at constant volume and the dominant driver is temperature and moisture. To avoid fogging prepare the housing in an environment that is as dry as possible and as close as possible to the temperature of the water you will be diving into because if it was not fogging before it won't fog after thanks to the desiccant packets

On the vacuum front as the energy added by the increased temperature has to go somewhere the reduced mass will result in increased speed of the particles. Maybe creating more smaller water drops??

 

Found the equation

 

 

Where N is the number of atoms m is the atomic mass of the particle. It follows speed does not depend on the number N so vacuum does not make any difference to the speed of the particles. So the only thing that is relevant is the number of particle of vapor inside that can move. Vacuum seems to help just in regards of having less matter inside the housing and nothing other effect.

 

Additional Considerations

Boyle Law: does not apply as the system changes temperature

Gay Lussac Law: applies P1/T1=P2/T2 you can calculate how much the internal pressure changes as result of increased temperature (this is what gives false alarm to the leak detector)

Dew point (is the temperature at which the water vapor in air at constant barometric pressure condenses into liquid water at the same rate at which it evaporates) does not apply as pressure is not constant in the housing

Edited April 1, 2014 by Interceptor121

[Stuart Keasley 42]

Perhaps you could share this paper you allude to

 

In terms of the other stuff:

 

-your ground breaking equation that proves everything you say is nothing more than a measure of internal energy.

- of course Boyle's Law does not apply once the housing is sealed, it refers to change of pressure with volume. Once sealed, the volume remains the same.

- if dew point doesn't apply because of changing pressure, then the dew point wouldn't apply in the atmosphere, pressure changes there all the time.

 

Any, I can't be bothered any more. You win.

[Interceptor121 766]

It is in the Wikipedia link isochroic process. First principle of thermodynamics. The variation of pressure in the atmosphere are so slow to be considered not happening. The housing with an active device inside heating up is not the same. A process that is similar is somewhat similar is heating a gas in a sealed container the difference is that the device heating our container is inside the housing though. Anyway as you correctly said you loose and it's time to go into that quiet corner

[Interceptor121 766]

Just finalized a little experiment to confirm to you lot of experts what really happens in a housing so that you can read it through in your own time.

I set my housing at 21.3C 61% humidity and 997 pascal pressure the dew point for this is 13.16C

I put inside a container with water at 12C. According to your theory of relative humidity this should have increased as the air inside the housing was dropping in temperature.

But it did not. The housing did not fog after a good half hour and when I opened it there were no signs of any condensation. The camera that had been off reported a temperature of 15C in the battery compartment.

 

Afterwards I did the same but set the camera to record video for 30 minutes, at the end of the 30 minutes the camera internal temperature was 26C and the camera port started fogging so the housing is not getting drier (probably because the moisture content has not changed!) and it is indeed starting to fog inside.

 

Another thing to consider is the different behavior of aluminum housing to plastic housing and in general of metal versus glass or plastic. Metal conducts heat so in whatever conditions you are there is a much better chance that the systems is leaning towards some equilibrium. Plastic does not so when it gets really hot inside the heat has nowhere to go hence plastic housings fog most.

What matters to fogging is the temperature gradient between the inside of the housing, mostly driven by the camera, and the water not the absolute diving conditions (housing could fog in cold and warm water). This relates to the phenomena happening at the various interfaces between the mediums that are not strictly insulated and the moisture inside the housing which is what condenses as the particles kinetic energy makes them move more hit the walls and change status and condense when faced with a colder material (surface condensation).

So what is important is a moisture free environment and as little temperature gradient as possible. No matter what when the inside of the housing is really hot either because left in the sun or because the camera is really hot condensation can occur so desiccant is required.

 

The thermodynamic process is explained here

http://en.wikipedia.org/wiki/Isochoric_process

And the explanation of the thermal energy here

http://en.wikipedia.org/wiki/Thermal_energy

Condensation here

http://en.wikipedia.org/wiki/Condensation

 

Dew point is just an explanation of a process and not the cause of it. In order to understand what really happens you need to look at thermodynamics.

 

Prepping the camera in an warmer or cooler room has little impact as the housing will be immersed in a heat sink with infinite thermal capacity what matters is the moisture so a dehumidifier more than an air conditioning unit is what is required.

Vacuum does reduce the number of particles inside the housing and helps to some extent but as the pressure difference is rather low it has a very mild effect and should not replace desiccant packets.

 

It is always good to learn things better!

Edited April 2, 2014 by Interceptor121

[Stuart Keasley 42]

Congratulations, you've proved that water requires heat before it will evaporate.

 

You could get a Nobel prize for that.

 

As for your other comment, you misunderstand me, it's your tenacity fir being wrong that has beaten me.

[Interceptor121 766]

Back to the quiet corner please i see you need more reading . If you have a personal issue with reading mine this is another piece for you

 

http://www.wetpixel.com/articles/housing-fog-condensation-fix

 

http://wetpixel.com/forums/index.php?showtopic=17361

 

What you and your buddy seems to fail to understand is that you are not compressing a gas. The analogy is correct in the old wetpixel post you are heating a container on one end and condensation forms on the other colder end

 

The condensation by compression at ambient temperature of a gas reported earlier does not apply here as conditions are different. The theory is fine but the situation is different

Have fun

[chris_l 6]

It's a good thing we got a separate mirrorless forum for such engaging topics.

 

 

Sent from my iPad using Tapatalk

[Draq 118]

It's a good thing we got a separate mirrorless forum for such engaging topics.

 

 

Sent from my iPad using Tapatalk

In my defense, it started out as a discussion about The Nauti vacuum system on a mirrorless housing, but it has kind of taken on a life of its own

[diverdoug1 56]

Sorry about that Draq! I will try to clearly answer the 2 questions posted in this thread using what little expertise I have in the area as a hyperbaric physician. I will not get drawn into a pettifoggery once again.

 

Your question if the negative pressure generated by the Nauticam system will make it less likely to flood -the answer is yes, because the o-ring is sucked into the mating surfaces of the housing, preventing blow-by.

 

watboy, your question if an additional benefit of a vacuum system would be that it would eliminate condensation issues , the answer is no. It will greatly decrease them though. If you close your housing at 30°C and 75% relative humidity, condensation will form when the housing wall reaches 25°C. Under the same conditions, If you pump the housing down negative 10 inches Hg, condensation will form when the housing wall reaches 18°C. Starting conditions will certainly modify the results, but not the manner in which you can calculate the benefit. The explanation is below.

 

Condensation forms at the dew point which is when relative humidity (RH)= 100%

 

RH= partial pressure of water vapor (PW) / vapor pressure of water (VP) at a given temp.

 

Vapor pressure of water at a given temp. (VP) is independent of pressure

 

When you decrease the pressure by 10 inches Hg, you are decreasing the pressure inside the housing by 30%

 

Following Dalton's law, decreasing the housing pressure by 30% will decrease the partial pressure of water (PW) by 30% once the temperature equilibrates. Therefore, relative humidity will decrease by 30%

 

So starting at 30°C and 1atm and a RH of 75%, pumping the housing down to .7atm will yield a RH of 67.5% once the housing equilibrates with the 30°C ambient temp.

 

Using the dew point curve we know that at 30°C and 75% RH the dew point = 25°C

Using the dew point curve we know that at 30°C and 67.5% RH the dew point = 18°C

 

So after vacuum there will be a 7°C degree difference in how cold you have to get the housing wall before condensate will form.

 

I will be happy to answer any questions not originating from Interceptor.

Edited April 2, 2014 by diverdoug1

[Interceptor121 766]

Are you still going in with this non sense? This is not an isothermal compression of a gas as per your experiment. Go and read my post and the older ones on wetpixel. The condensation will diminish if you suck vacuum only because there is less matter to condensate. Humidity doesn't change inside the housing. If that is so hard for you try to look up at why you have condensation on windows on a cold day maybe that's easier to understand. Have been attacked by two supposed experts with non sense that I could not understand and guess what? It was non sense so please stop plaguing this post with additional misinformation and try to make sense of the real process happening inside the housing.

Edited April 2, 2014 by Interceptor121

[Interceptor121 766]

And to give some example figures pumping out 200mbar reduces the matter of 20% so you have 20% less condensation as the same conditions.

For what concerns the benefit of the vacuum you need to consider that a sealing surface is actually small just a few mm. So if you have an oring of 2mm for example over a dome of 4" you need around 1.2 kg force to pull that out. But if you hit it laterally with much less than that it would drop because of of momentum.

A larger 8" dome will need much less force to drop. Those values at the surface. Also consider that if your camera has heated up the pressure has increased and most likely the effect will be reduced.

Conclusion there are some benefits of vacuum but your housing preparation practices in terms of carefully putting the parts together and using desiccant do not change because of it.

End of story

Edited April 2, 2014 by Interceptor121