Second thoughts about vacuum systems

It's not 10% difference that triggers the alarm, but the 5 mbar pressure rise from the green blinking point, which is 200 mbar below the ambient pressure measured at startup.

Whether the housing is vacuumed or not doesn't really matter, except it changes the housing's depth rating for 2m (if vacuumed to -200 mbar), and that is also more or less unimportant, as nobody makes the housing which will crash at exactly that depth. The only thing that matters is a pressure difefrence, so a vacuumed housing will function exactly the same as a non-vacuumed 2 meters deeper. So, the Buddy dive's argumentation makes no sense, as the o-rings will behave exactly the same, it's just a question of depth. All housings are designed to function with higher pressure on the outside, and the vacuum only helps keep doors and domes tight in place.

How much water must enter the housing before the alarm is triggered depends on how many pump strokes are pulled after the LED starts blinking green. As V3 doesn't have the temperature compensation implemented, it is a kind of tradeoff between the amount of water and a possibility of false alarm due to temperature change, but generally, one stroke should suffice to avoid false alarm in average sized housing, and to keep the water amount at bay. 6ml of water per liter of the housing volume is not an enormous quantity of water, and I think it's hardly enough to trigger any leak detector. Any sanitary pad will handle much more, and I'd suggest using one no matter if you have LS or not.

V4 solves all the problems above, as it measures the pressure when the pumping stops, and takes that as a reference, so it doesn't really matter how many strokes you pull after the green LED starts blinking (but don't go overboard for obvious reasons).

Of course you should operate the buttons and controls during the test, I never put that in the manual as that seemed a no brainer to me, LS is just an electronic replacement for a good ol' bucket;) And that was obviously the cause for the problem tc_rain had, the housing was sealed until the lever was operated...

Cheers,

Miso

And IMHO the camera shop owner is completely wrong! It does not make any difference if you pull from inside the housing (= apply a vacuum) or push from outside (= apply water pressure). So the idea that the o-rings are "sucked in" is nonsense. And nonsense especially if you think about the relatively small degree of vacuum you apply with your pump.

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. Clearly this doesn't cause problems most of the time, but that said, in my experience the vast, vast majority of floods are usually down to very obvious reasons (and more often than not simple user error I'm afraid!). And many floods are caused by rushing which would probably mean ignoring the vacuuming step anyway. Whilst I suppose I could fit a vacuum system and have thought about doing so, I've always shied away from doing so as I've always thought that good pre-dive preparation doesn't need yet another step - its time consuming enough as it is. Depending where it enters 6-12ml of seawater could do a lot of damage so it makes sense to see a vacuum system as a pre-dive check only as far as I can see from what has been said. Interesting thread.

is the force that'll move the o rings not just the pressure difference?

is the force that'll move the o rings not just the pressure difference?

Exerting a force by sucking the 'O' ring inwards from decreased internal pressure when the exterior is subject to air pressure may not be the same as increasing liquid pressure applied from the exterior - I say might, I'm not sure what if any the difference is. Also you would need to take into account the physical 'pre-load' on the 'O' ring due to the housing fastening system. As I said it intrigues me. Sounds simple, but is it?

Although vacuum systems can detect a significant leak during a dive, I feel that a moisture sensor is a far more sensitive indicator of a leak which has occurred DURING a dive. The vacuum systems show their greatest utility (IMHO) in 1. Identifying that your system is properly sealed pre-dive, 2.Preloading the O-rings, thereby minimizing blow-by of water at the surface on entry or surface swims. I use both systems on my housings.

Doug

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.

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....

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.

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

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.