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Second thoughts about vacuum systems

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Thanks for the responses and the wealth of information. I want to stress that I have no intention of saying anything negative about the V3. I just realized when it flooded, I didn’t fully understand a vacuum systems limitations. Now I think I do. I also believe that temperature compensation is a must have to allow a lower vacuum in the housing. This should allow a smaller water intrusion to set off the alarm. A moisture sensor inside would be great. I also read where some have placed their wives maxi pads in the bottom to soak up water and prevent it from splashing around in case of a small leak. Unfortunately, I am still kind of new to this and never thought about manipulating all of the controls before submerging it. You bet I will from now on. I feel I really got lucky with this one since nothing was damaged except the housing and Ikelite took great care of me there. I may look into the V4 and will research moisture sensors some more. A couple hundred dollars spent to prevent is better than four thousand later. Thanks again.

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Yeah, I doubt you'd ever see buying the V3 (or now V4) as anything other than money well spent.

 

I sleep better on a liveaboard thanks to Miso and the V4.... :lol2:

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I wouldn't be so sure. This idea intrigues me and I don't have an answer, but sucking an 'O' ring into place might well result in the 'O' ring seating differently than pressure pushing it into place will - the forces at work may well be being applied in different ways..

The forces at work are being applied in the SAME way. There is a pressure differential being applied. Loading an o-ring by increasing the pressure on side "A" vs decreasing the pressure an equal ammount on side "B" results in the same sequence of loading forces on the involved o-ring seal. The loading sequence, In the case of a "Face Seal Joint" typically seen with the main and port o-ring seals on an U/W housing, is as follows. the compressive force of the parts being assembled serves only to approximate the o-ring to the sealing surfaces, but any compressive force generated on the o-ring by assembly is not in the same axis as the sealing force at the o-ring generated by a pressure differential. This is why the o-ring pre-load generated by a vacuum system helps to prevent leaks at the surface. The vacuum system gets the o-ring started sucking in the right direction. It is similar in concept to an automobile tire being placed on a rim. When the tire is initially placed on the rim, air can seep out as the tire is inflated untill the bead of the tire pops into the seat of the rim.

Edited by diverdoug1

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I am not sure that I agree that "the compressive force of the parts being assembled serves only to approximate the o-ring to the sealing surfaces, but any compressive force generated on the o-ring by assembly is not in the same axis as the sealing force at the o-ring generated by a pressure differential." I assume that without the vacuum system my housing is sealed with minimum pressure delta applied i.e. it will not leak in half an inch of water or if I apply a small overpressure into the housing (like heating it up in the sun) it will not leak even when the Delta P is in the opposite direction.

We design medical devices that really need to stay sealed and my engineering colleagues suggest to me that sealing, especially sealing with o-rings is not simple and that modeling the whole sealing process is extremely complex.

Bill

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bvanant, The compressive force applied by pushing the parts together is not in the same axis as the force applied on the o-ring by the water trying to enter the lower pressure interior of the housing.. In fact, once the o-ring is compressed enough for the parts to meet, the o-ring is compressed on this axis no farther, regardless of increases in pressure differential, unlike the compression along the axis between the areas of part mating. The following is a force diagram for forces on a high pressure face sealing o-ring. http://en.wikipedia.org/wiki/File:O-ring.svg. The light areas demonstrate the areas of highest strain. The forces along the axis between the mating parts is lowest, while the pressure along the axis of the parts seam in the groove wall is highest. When there is no pressure differential (or it is very low) there will be no strain along the seam axis.If o-ring sealing was caused only by the compressive force of the parts, no o-ring groove would be neccessary. This is not the case. If you were to rely on the seal provided by the compessive force of the parts on the o-ring alone, it would act merely as a "simple gasket", and would be overcome in a relatively shallow depth of water (the exact reason why vacuum systems help prevent blow-by of water in the event of water impact at the seam while at the surface). Here is a good primer on o-ring function. http://www.satoriseal.com/technical/technical_articles/introduction_to_o-rings.htm It is worth a read if you are truly interested in how o-rings function.
You can see how the o-ring is displaced to the right in the diagram, perpendicular to the mating surfaces, into the groove wall at the seam of the parts. this movement is why o-rings need to be properly lubricated. Lubricating the o-ring will allow it to slide into its seated position at a lower pressure differential, there-by decreasing incidence of leaks at shallower depths and decreasing o-ring wear as it slides into position. I find it interesting how an o-ring has a dynamic (rather than inert "simple gasket-like") role in sealing.

Edited by diverdoug1

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