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.
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.
"I would have thought you could have logically solved this by just looking at extremes" Indeed Watboy, but there is the rub. Sometimes logic falls upon deaf ears.
"Knowledge and truth may be within us without judgment, and judgment without them; but the confession of ignorance is one of the finest and surest testimonies of judgment that I know" Michel de Montaigne,
I'm not entirely sure what you're saying here, sincerely hope you are finally agreeing with what we've been trying to explain to you and the reams of information that has been shared
Doubtful, Interceptor, you have just agreed that dew point temperature will decrease as you create a vacuum, this means that the realtive humidity decreases BY DEFINITION. So you CAN derease the relative humidity with a hand pump. Q.E.D.
I watch the relative humidity INCREASE in the chamber with every treatment. I have supervised 2 treatments already today. I will certainly post is the converse happens for any remaining treatments today, and I will also let you know if the sun decides to set in the east today.
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.
Any amount of negative pressure in the housing will help prevent dislodgement of a housing or port o-ring at the surface. Even 1 inch of Hg will help preload the O-rings. A simple Google search of the keywords "o-ring" and "preload" will help illustrate this. I guess the pushback we are seeing is a result of the statement that a vacuum "makes no difference" in the risk of flooding at the surface, which is incorrect. Most of this thread has been carried by repeated misguided, transparent, overly complex attempts to support that initial incorrect statement. I have found that most people will respect those who are big enough to admit when they are wrong, and will ridicule those who are not.
Wetpixel is a wonderful resource for learning and sharing knowledge about underwater photography. The only way we can trust the information that we gain from this website is for us to call out the posting of erroneous information. I hope that if someone were to post misinformation about a subject that I am trying to learn about, my fellow photographers in the Wetpixel community would call it out. I also hope that if I inadvertently post erroneous information, you will all set me straight.
The Backscatter video on their vacuum system details their non-led unit, not their electronic unit. The electronic unit is about $100 USD more. I like it because I know that the vacuum is still in effect without having to plug in the gauge. I also have a Nuaticam system with a vacuum system that I like a lot. They both preload the o-rings, which greatly decreases the risk of flooding at the surface.
Hugyfot was one of the pioneers in the vacuum check arena. I don't recall hearing any negative comments about their system. I am currently using a Backscatter electronic system on my preferred U/W rig (Aquatica AD800), and have been very pleased with it.
I do a lot of macro and super macro with the 105 and wet diopters. I find manual focus to be very helpful (bordering on essential). You can add me to the list. With respect to the "back button focus" I assume you mean the "AF ON" button, and yes it is on the D200 as well as all currently made DSLRs on the market by the major manufacturers. I have all my DSLR camera bodies (topside and U/W) set to activate autofocus only with the "AF ON" button, and not with the shutter release. I know better what I want in focus in the frame than the camera does.