Vacuum leak detector question

When you said "well that is somewhat scary indeed" was that an insult?

Regardless of that, to answer your previous post, when you say "if this was a closed system being compressed and decompressed I would not have any issue agreeing pressure increase changes humidity levels"

WHAT IS THE DIFFERENCE????

If you take a fixed volume vessel at 1 atm, and apply a vacuum to .5 atm it will have the exact same internal composition as a closed variable volume vessel which starts at 1 atm and doubles its volume. Those are the only 2 ways that you can decompress a temperature constant system. 1.increase volume, 2.remove contents. When you posted the above quote regarding the closed system, how did you proposes the system be "compressed and decompressd"?

I did not mean it like you took it but in the context of dealing with the bends and having a scary job.

Anyway i have taken it out and would expect this to go back to civil terms as you and your buddy are going a bit too far now.

If this was something really easy to understand and to demonstrate I would have worked it out by now just by doing experiments instead it seems a much more complex matter as in fact you don't find any literature where vacuum is using on its own to remove moisture there is always a condenser and vacuum is just used to clean

My mum used to do vacuum jam jars thinking it was the vaccum to reduce the moisture but instead it was just taking the oxidant away which is what formed the mold itself the jam stayed well wet

I agree, we should keep this discourse civil, so that we may continue the discussion unfettered by moderation.

Now, please answer the question, how do you propose the system be compressed and decompressed in the instance where you state "If this was a closed system being compressed and decompressed I would not have any issue agreeing that pressure increase changes humidity levels"?

Increasing the volume of a sealed vessel by X% will yield the exact same internal composition as decreasing the internal fixed volume vessel's pressure by X%.

I guess the difference is that dalton law refers to a close system (I have no issues understanding that!)

So if you have your 1 ATM and the gas is inside your container increasing the pressure or reducing the volume increases the partial pressure of gas no problems with this

In this specific case although the relative composition is the same in terms or weight (I do not have reasons to believe any different) the gas are different as I am not keeping the same mass of gas but am actually changing although not changing the proportions. As the mass of gas is changing the relative pressure of the moisture compared to the total pressure of the environment in % is not changing so it does not feel more or less humid. I need to work out how my weather sensor works inside to have a more plausible explanation

I still think pumping compressed air in the housing before closing and drying it off plus operating in an environment that is dry to start with is the way forward to avoid condensation and myself I have never had condensation in a housing so am not sure why am even bothering here!

Anyway am off to bed now have a good one

If you have no issues understanding Dalton's law, you should be OK with understanding Boyle's law. In the non-fantasy world your above post is simply nonsensical. Barring pixie dust, leprechaun's tears, and mythical weather sensors, decreasing the pressure in a vessel will decrease the relative humidity and inhibit condensation formation. The manner in which the decreased pressure occurs is irrelevant, all that matters is moles of gas molecules per unit volume. This is not conjecture but fact delineated by the laws of physics.

If you don't like that your position is incorrect, then shake your fist at God and the heavens above, or write a nasty letter to Robert Boyle, John Dalton, and perhaps Isaac Newton.

Q.E.D.

There exists a pressure, below which water cannot exist as a liquid at any temperature. Its called the triple point and is 611 pascals or 0.006 ATA. You draw a vacuum down to this level, and there cannot be condensation, even if the air was fully saturated with water. At this point, above 0 C, the water will only remain as a gas. Below 0 C, all the water will freeze. Lets just look at the above freezing temperature zone. If you are above the triple point, as the pressure lowers,and you approach the state where liquid water cannot exist, logically it would become harder and harder for liquid water to exist. Water doesn't become more inclined to be in the liquid state, then pop back to being a gas once you hit the triple point. And it doesn't matter how the pressure is dropping be it by pumping out mass in a fixed volume, by expanding the volume on a fixed mass, lowering the temperature or any combination of. Add to that, you are pumping mass out of a fixed volume, you are removing water from the system as well. You are making the system harder for liquid water to exist that has less water in it.

If this was a closed system being compressed and decompressed I would not have any issue in agreeing that pressure increase changes humidity levels.

However this is the other way around you are decreasing pressure by pumping matter out you are not applying pressure on the same amount of air.

 

Reducing the pressure using a pump is, by definition, decompression.

The only difference is that we are taking the housing pressure below ambient pressure instead of naturally allowing it to equalise with ambient pressure. However Dalton's Law still applies.

If this was something really easy to understand and to demonstrate I would have worked it out by now just by doing experiments instead it seems a much more complex matter

As I said earlier, I can see where you made the initial mistake, I initially made exactly the same mistake and also got it wrong.

as in fact you don't find any literature where vacuum is using on its own to remove moisture there is always a condenser and vacuum is just used to clean

 

Here's a dew point calculator, pop in the dew point at ambient and the line pressure, it will then tell you the dew point at the new pressure

http://www.dew-point.com/equivalents.asp

Here's another calculator, with a plain as day statement at the very start

Dew point changes as pressure changes (and vice-versa). Therefore, if you know an air streams dew point at a given pressure, then change the pressure, the dew point will change.

 

http://www.howelllabs.com/resources/dew-point-conversion-calculator/

Even the link you shared earlier on has a calculator on it that clearly shows a change in air pressure will have an effect on the dew point ie the temperature at which condensation will occur.

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

This is quite interesting, although a bit more complex. Read it carefully, if you start to misinterpret it to mean that removing air from a housing won't reduce dew point, then you've taken a wrong turn. Go back, and start again.

http://www.smc.eu/portal/WebContent/local/UK/Pneu_Book/Dew_Point_.jsp?tree_options=tree_products.js&box=box_basket_v2.jsp

With all of this stuff, all that matters is that pressure changes. It doesn't matter whether the pressure change is effected by increasing or reducing volume on a sealed object, or by introducing or removing gas from a rigid sealed object, the net effect is exactly the same, you are merely changing different parameters within the equation.

Your educational principles are a bit dated if you think putting someone in a corner to think matters is the way forward. I have non problems admitting when am wrong. Like you I said that the dew point that is a temperature would go increase with pressure. The matters geot confused when your speak about relative humidity as actually what changes is not the % of moisture but the total number of particles of dry air and vapour. Which means now air can hold more moisture before it condenses. I could not work this out myself as the weather station measures the % and therefore reported no changes. The dew point is the absolute humidity not the relative and therefore this now makes sense to me. So I agree that of you pump air out the number of particles reduces and so does the vapour which means that the temperature to which those particles would condense is lower than if I had not created vacuum. Now the fact that if I set the housing outdoor I get 84% humidity and if I do it indoor I get 55% does seem far more important in terms of avoiding condensation but it would seem that if you create a low vacuum you may go in water a tad colder (don't know how to possibly quantify this) before your housing fogs. Now back to work

(Metaphorically) going into a quiet corner, without distraction and taking time to think things through is a very effective way of gaining understanding. I suspect you didn't finally get to this conclusion whilst sat on your sofa watching a psychedelic thrash metal performance on the TV

Odd, that you still haven't done so then.

No, you initially said quiet the opposite.

But no matter, big hooray, we got there in the end (shame you didn't read post 76 properly, it said exactly this is nice simple language)

Good calculator I see that it gives around 3 C at normal conditions

What is more interesting is that it looks like if I am diving in cold water I am better off preparing the housing outdoor than indoor?

Wraysbury today water 8 air 11 humidity 89% dew point 9.11C some condensation could occur

Prepare the same housing outdoor at -200mbar dew point 5.85C (% humidity is unchanged)

Prepare the housing at home 19.5C (humidity 60%) dew point 11.47C would fog with vaccum -200mbar 8.47 would also fog

This of course assumes that the equipment inside the housing stays at the same temperature, in reality certain cameras reach easily 30C temperature and end up acting like a heat generator. This moves the dew point up again and the condensations will occur no matter if at 1 or 0.8 bar.

Which makes me think two things, the first is that I need temperature compensation in the leak detection (which I had already concluded as in the works) the second is that some form of moisture packets are required regardless as whatever happens condensation will most likely occur in practical circumstances nonethless

 

What is more interesting is that it looks like if I am diving in cold water I am better off preparing the housing outdoor than indoor?

This really depends on the conditions indoors and outdoors. Prepping the camera in the coolest, and driest conditions possible will be best. So inside with the A/C running would be prefferable to outside on a hot, muggy day. A cold crisp winter day outside would be preferable to prepping inside a warm house with a humidifier.

Also, if you choose to use dessicant packets, you need to put them in your sealed housing well in advance of the dive, because it tikes time for them to absorb the water vapor in your housing.

This of course assumes that the equipment inside the housing stays at the same temperature, in reality certain cameras reach easily 30C temperature and end up acting like a heat generator. This moves the dew point up again and the condensations will occur no matter if at 1 or 0.8 bar.

 

Once the housing is sealed, you have created a contained atmosphere, the dew point will stay the same. A temperature increase inside the housing will decrease the relative humidity, a temperature decrease will increase relative humidity. Once temperature is reduced to generate a relative humidity of just over the 100% mark, condensation starts to form (if things worked the way you just explained, i.e with temperature rises also rising the dew point, and vice versa, then dew point would never be reached)

Think of humidity (in simple speak) as being the amount of water that has not been evaporated by heat.

Am not sure about this. The cameras in a housing never fog right away only when they get hot. The water temperature outside never changed the only thing that changed was the temperature inside?

This really depends on the conditions indoors and outdoors. Prepping the camera in the coolest, and driest conditions possible will be best. So inside with the A/C running would be prefferable to outside on a hot, muggy day. A cold crisp winter day outside would be preferable to prepping inside a warm house with a humidifier.

Also, if you choose to use dessicant packets, you need to put them in your sealed housing well in advance of the dive, because it tikes time for them to absorb the water vapor in your housing.

Yes this is exactly what I do. On top of that I make sure the camera itself has a packet in the pouch when outside the housing. It takes around 12 hours for those things to do their job so usually is something I do the night before for diving the day after

Once the housing is sealed, you have created a contained atmosphere, the dew point will stay the same. A temperature increase inside the housing will decrease the relative humidity, a temperature decrease will increase relative humidity. Once temperature is reduced to generate a relative humidity of just over the 100% mark, condensation starts to form (if things worked the way you just explained, i.e with temperature rises also rising the dew point, and vice versa, then dew point would never be reached)

 

Think of humidity (in simple speak) as being the amount of water that has not been evaporated by heat.

Condensation in a housing does not (just) happen because the air itself condensate is because there is a temperature difference between the housing wall and the air inside. (The just is the case where you prepare in a warmer environment and the whole thing fogs right away as the cold water example above)

So in effect the dew point temperature discussion is setting things slightly off track as are not always condensing a gas.

What happens when the temperature rises is that the vapor inside starts gaining kinetic energy and hitting the cold walls of the housing which is at much lower temperature. The water itself transitions from vapor to liquid and condenses. The total amount of liquid and relative humidity never changed as the gas never changed but the status changed as result of a physical reaction at the interface between air and housing wall. A similar phenomena can be observed on the outside of the housing if you prepare it in a cold and dry environment and then move it to a warmer one, the vapour in the air condenses on the external colder walls of the housing

Generally speaking this seems to depend from the amount of energy of the particles to move around and hit the wall and the number of particles themselves. Even if you evacuate some of those particles the heat at some point will be enough to condense vapour at the interface

When using a compressor (I must admit I find difficult to think in gas law for something so far from ideal laws) we pump in hot filtered air. Most of the moisture goes off with the hit and some other is taken out by the filters that are in place to avoid the oils and other contaminant which are carried by vapour. So compressed air in a compressor is not the same as compressing a volume of air.

All along I did say the right thing about the dew point but missed the point about what the dew point is if you read correctly. Relative humidity is not relevant to this discussion as we are not compressing a defined volume of gas. Still there are some marginal benefit of vacuum as you said in terms of reducing the total amount of moisture (absolute humidity and not relative)

Relative humidity is not relevant to this discussion as we are not compressing a defined volume of gas. Still there are some marginal benefit of vacuum as you said in terms of reducing the total amount of moisture (absolute humidity and not relative)

Your wrong again sorry to say. Relative humidity has nothing to do with the gas being a defined volume. In fact when you use the term "dew point" this is just when the relative humidity reaches 100%. Condensation occurs on a cold surface because the surface cools the air that is against it to a temperature where the relative humidity increases to 100% and the dew point is reached, then a condensate forms.

Uhm I don't find the relative humidity useful to explain anything. Hot vapor hits a cold surface and condenses that makes sense to me. Either way that's what happens if the camera heats up inside even if you prepared it in a cooler environment. Hence warming the camera up can lead to condensation despite preparing it in an air conditioned room. Seems it was more the low moisture that was important than the temperature at the end as things get to equilibrium except you go in cold water. In that case you may actually fog the whole thing right away

Uhm I don't find the relative humidity useful to explain anything. Hot vapor hits a cold surface and condenses that makes sense to me. Either way that's what happens if the camera heats up inside even if you prepared it in a cooler environment. Hence warming the camera up can lead to condensation despite preparing it in an air conditioned room. Seems it was more the low moisture that was important than the temperature at the end as things get to equilibrium except you go in cold water. In that case you may actually fog the whole thing right away

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 "dew point numerous times, this term just means 100% relative humidity.

I thought condensation on the housing is a result of state change of hot vapour hitting the cold wall. Not the condensation of the gas inside by cooling or heating. At least this is the conclusion of another wetpixel article which was pretty much the reference until now

I found it

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