Vacuum leak detector question

The initial question was I have an housing for the em5 and I want to put a vacuum system can I now be less careful with rough water entries? Based on the housing the op has (nauticam em5) i would say no because that specific housing can be opened if you accidentally hit the lock button and rotate the latch.

I have been staying out of the argument but would like to clarify that this is not what I asked. As I sought to make clear in a second post, I decided to purchase the vacuum system for its advertised benefits and got to thinking that perhaps an ADDED benefit of such a system might be a lower risk of flooding at the surface. My thought was that the creation of negative pressure at the surface might make it less likely that the seal created by the O ring would be compromised by the water forces that can occur in particularly rough shore entries and with a giant stride (when unavoidable). I was curious about whether my thought had any validity. I was not asking if I could be less careful with the housing or asking about any of the side issues that have arisen such as damaging the housing or smashing the port during a dive or opening the housing in the water.

I have been taking photos underwater for several years without accidentally opening my housing during a dive. This is my second Nauticam housing and I have in the past used housings by a few other manufacturers. If the negative pressure prevents the housing from being accidentally opened then that is a potential added benefit, but not what I was asking about and would rather not have it appear that I was hoping to now become careless with my gear or concerned about opening the housing while underwater.

By the way, would the vacuum system make it less likely for the housing to flood if it were snatched from my hand by a great white shark, then dropped several hundred feet to the sea floor where it was picked up by an octopus and carried to a shallower depth by said cephalopod and later thrown onto the shore in a hurricane? (Just kidding...not serious here...don't answer)

I never suggested you wanted to become a careless diver and I am sympathetic with the desire of wanting to find added benefits out of a somewhat expensive investment.

Nauticam themselves define their product as an integrity check system and you should use the system within the specified design. It is true that having a high vacuum system (negative pressure > 400mbar or 10inHg) does potentially lock a housing making it much more difficult to leak however this is not what the solution you bought is designed to do, otherwise be sure they would have advertised it.

You really want to create the minimum vacuum needed to set the system on so that the temperature compensation can be extremely accurate and alert any possible leaks almost immediately also when you are in water so that you have time available to ascend safely and save your equipment. If you instead over compensated it to make lock it and make it more leak proof this alert would be delayed negating the temperature compensation.

I think the benefit of this system as pre and during dive integrity check is great and you do not need to find extra benefits to justify your investment

Seems like the discussions that go on with nitrox when people say it makes you less tired, it does not really, the benefit is only additional non decompression time but some people want to think they are less tired and there is an added benefit when there isn't really one just for the feel good factor

"My thought that the creation of negative pressure at the surface might make it less likely that the seal created by the O ring would be compromised by the water forces"

Draq, your thought is correct. Preloading the o-ring (even with a low negative pressure) will suck it into it's sealing position against the intersection of the two mating surfaces. That is how an active sealing o-ring works. A very high negative pressure which "locks" the mating surfaces in approximation is not necessary for this to occur. The normal operational vacuum for the Nauticam system is more than adequate for this sealing to occur. Regardless of the one dissenting incorrect opinion.

"My thought that the creation of negative pressure at the surface might make it less likely that the seal created by the O ring would be compromised by the water forces"

Draq, your thought is correct. Preloading the o-ring (even with a low negative pressure) will suck it into it's sealing position against the intersection of the two mating surfaces. That is how an active sealing o-ring works. A very high negative pressure which "locks" the mating surfaces in approximation is not necessary for this to occur. The normal operational vacuum for the Nauticam system is more than adequate for this sealing to occur. Regardless of the one dissenting incorrect opinion.

An opinion is an opinion and I am happy with mine like you are with yours.

My thinking is that having a vacuum system of that nature doesn't change anything in my preparation and checks and nothing on how i enter or exit the water the same care applies with or without this system. Which means that the potential additional benefits are intangible in real practice and therefore irrelevant as you don't know what more or less you can do in virtue of the small vacuum applied

If the fact that there is a vacuum helping makes you feel better dive more relaxed I am happy for you

Quod Erat Demonstrandum

Not to get too far off subject, but for additional benefits of a vacuum system, wouldn't that eliminate internal condensation issues and the need for moisture munchers?

Not to get too far off subject, but for additional benefits of a vacuum system, wouldn't that eliminate internal condensation issues and the need for moisture munchers?

The pressure tester simply reduces internal pressure. The air that's left in there will still have the same moisture content, and still be subject to the same temperature differentials.

Condensaton occurs when a partialy saturated gas is cooled or compressed to the point where the molecular density of the solute gas molecules reach the maximum threshold. Decreasing the the molecular clusters in a given volume (if you pump the housing down to about negative 10 inches Hg the decrease in molecular clusters is about 30%) will allow a greater degree of compression or cooling prior to reaching the condensation threshold. So YES, a vacuum system can decrease internal condensation issues. For illustration, if you take a volume of gas which is very humid (near saturation) and compress it (via like gas injection) in a pressure vessel, condensate will form on the lining of the vessel.

Condensaton occurs when a partialy saturated gas is cooled or compressed to the point where the molecular density of the solute gas molecules reach the maximum threshold. Decreasing the the molecular clusters in a given volume (if you pump the housing down to about negative 10 inches Hg the decrease in molecular clusters is about 30%) will allow a greater degree of compression or cooling prior to reaching the condensation threshold. So YES, a vacuum system can decrease internal condensation issues.

I stand corrected

Thanks DiverDoug (the benefits of the vacuum system keep stacking up )

And here's a little back ground reading on it, if anyone's interested

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

I think it is different if you are condensing a gas or instead condensing the vapor within air (dew point)

High pressure means drier air in fact when you breath compressed air you feel lack of moisture as the dew point is pushed further down

A change of pressure of 200 mbar is unlikely to make a huge difference though so the disbenefit is almost irrelevant

(google is dangerous)

Interceptor, I am a hyperbaric physician , so what would I possibly know about vapor pressure? Or o-rings for matter?

In your Wikipedia link explains why at the same temperature it may rain in Denver (lower pressure) and not New York

I don't think you can decide to suck out air and leave moisture behind so you are not making it less humid just by sucking

And yes Stuart is correct a compressor will have a drying filter but that is because moisture can create rust in the tank. The actual dew point in a compressed air cylinder is around -70 C so rest assured nothing is wet and therefore feels bloody dry when you breathe it at 2-4 bars when diving

I don't think you can decide to suck out air and leave moisture behind so you are not making it less humid just by sucking

That's the point. You're not sucking out air and leaving moisture behind, you sucking out the air and taking the moisture with it.

Here's some simplistic maths for you to illustrate the point.

Let's say the housing has a volume of one litre, the environment your assembling your housing in has 1000 particles of water per 1 litre of air in it, you're at perfect ambient of 1 bar.

When you close the housing, it will contain 1000 particles of water (forget the space the camera takes up for now).

If you have an amazing pump that's actually powerful enough to suck out all of the air in the housing creating a perfect vacuum inside, you'd have no particles of water left in it. Humidity would be at absolute zero (for the split fraction of a second before your housing imploded).

In the more realistic world, if you reduce the pressure inside by 5 Hg (or 0.16 bar), you'd be sucking out around 16% of the air inside, along with the water particles contained in it. So you would now only have 840 particles of air in the litre volume inside the housing, the humidity has therefore reduced.

Nope the relative composition of the gas is not changing and the lower pressure pushes the dew point down making vapour condense at relatively higher temperatures

You are just using a hand pump you can't make any assumptions of changing the composition of the gas. You are reducing the pressure and reducing the number of moles keeping the same relative composition and moisture %

Like in your Wikipedia link at the same temperature it rains in Denver and not in New York the gas stays the same

It was not my Wikipedia link, but I did read it, and the example you mention assume a constant dewpoint. That is not hte case in a housing as you evacuate its contents. You do make a chamber less humid by definiton as you evacuate its contents. Given a fixed temperature, relative humidity in a vessel will be directly proportional to the partial pressure of the solute vapor. As pressure in a vessel decreases, the partial pressure of that solute gas, and thereby the relative humidity of the solute gas will decrease.

Stuart it rains in Denver before it rains in New York at the same temperature. That is why you have snow on the mountains and not on a plain. As in a higher pressure environment you have a higher dew point it feels drier at the same temperature and lower pressure it feels wetter. It is in your same Wikipedia article paragraph 2

Compressing air increases the dew point temperature and has a drying effect

The Denver New York example is at the bottom of section 1 in the main body.