Vacuum leak detector question

They are one and the same, the vapor is the gas. The air contains water molecures which come out of the gasseous state and condense on the cool surface as the air is cooled, causing the relative humidity of the cooled air to reach 100% (dew point) and the condensation forms. This is the function of a common dehumidifier. Humid air is drawn over cooling elements, which causes the air to cool and deposit some of its water molcules on the cooling surfaces (condensate), which is collected in a drainage pan, The expelled air will have less water molecules per unit volume.

I think you are getting hung up on the term "relative humidity"..It just means at a given temperature and pressure, how much water the air is holding compared to how much it CAN hold before reaching dew point.. As temperature increases and pressure decreases, the air can hold more water. As temperature decreases, and pressure increases, the air can hold less air.

If the temperature in a room is 75 and the air is half way to dew point in the amount of water it can hold, the relative humidity is 50%. If you warm up the room to 90 degrees, the air will be able to hold more water, so the relative humidity will decrease, even though the amount of water in the air has not changed.

This is why a hair dryer works. The heated air now has a low relative humidity, and can hold more water (think thirsty air) and can evaporate the water more readily (the oposite of a dehumidifier, you are actively putting water vapor in the air.

Actually no, the camera heating up inside the housing will DECREASE condensation in the camera. The hot camera heating up the inside of the camera will decrease the realtive humidity (meaning that any condensate that has formed on the walls of the housing will find it easier to evaporate). You have used the term "jdew point numerous times, this term just means 100% relative humidity.

There is a few where things will react differently

- Interior of the camera heats up and generates a temperature differential between the inside of the lens element and the air inside the housing. In this instance, you'll see condensation appearing on the inside element of the lens instead of the inside of the dome port

- If there has been water ingress into the housing during assembly, e.g if some particles of rain dropped in, as the inside of the housing heats up, the pools of water evaporate and then increase the relative humidity inside the housing, dew point therefore increases in response.

However in general:

- When you assemble the housing the inside and outside temperatures are the same.

- When you enter the water, the outside temperature will be (generally) colder

- The dome port will slowly reduce temperature to match the water, the interior surface of the dome port therefore becomes cold.

If the interior temperature of the dome port is at the dew point of the relative humidity of the air inside, condensation will appear. If you're lucky, the heat generated by the camera may be sufficient enough to heat up the air inside the housing, and the interior surface of the dome port, making the condensation disappear (in much the same way as you would blow hot air on a windscreen to demist it)

However, whatever is at play, it's all about the dew point.

That's the way I understand it, anyway

OK so you are agreeing with the principle the heat generated by the camera makes the particles move the go on the cold wall and condense?

Another thing that seems interesting is that here we are assuming that the air is holding all the humidity this may not be the case some moisture may well be trapped in the camera equipment

In general the old wetpixel forum articles goes well with my experience for warm climates. I use silica packets and to be honest assemble outdoor not air conditioning never had an issue. This is after an housing fogged after being prepared in the air conditioned cabin

http://www.sealife-cameras.com/accessories/moisture-muncher-capsules

Anyway I think I got the gist of this. I would try and prepare the housing at similar temperature of the water I go and dive so that it does not fog right away into as a starter and then use dessiccant to fight the increased temperature inside and possible condensation on the wall from whatever phenomena is at play

OK so you are agreeing with the principle the heat generated by the camera makes the particles move the go on the cold wall and condense?

 

Another thing that seems interesting is that here we are assuming that the air is holding all the humidity this may not be the case some moisture may well be trapped in the camera equipment

 

In general the old wetpixel forum articles goes well with my experience for warm climates. I use silica packets and to be honest assemble outdoor not air conditioning never had an issue. This is after an housing fogged after being prepared in the air conditioned cabin

Preparing in an aircondition cabin is actually better, but ONLY if you seal the housing before going into the hot humid air and KEEP it closed. If you go outside before you close the housing, or open it breifly once you are outside, you are screwed. The hot humid air comes in contact with the cool surface of the camera lens and inside of the port, and condensation occurs.. If you have the need to open the housing at any time other than when you are in airconditioned, dehumidified conditions, it is better to leave everything outside, so the surfaces of the camera and housing are warmer.

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

 

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.

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)

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)

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?

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.

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?

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).

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.

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

Sent from my iPhone using Tapatalk

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.

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

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

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?

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