Housing Sentry Vacuum Seal

O ring aging due to vacuum .............

I tend to think ozone and sunlight will do in O rings faster.

Jim Church once commented to me that he goes "years and years" with the same Nikonos lens rings!

But that was back when rubber was rubber and men were men!

Okuma I am thinking along the lines the more stress you put on the o-rings by putting a positive or negative pressure on them (by either diving or adding a vacuum) the quicker the o-ring stretchs. Bit like If someone dives everyday for a year their o-rings would be more stressed than say someone who dives once a month.

Its just an idea anyway.

But as I am only new to Vaccum seals in my housing this might be a question asked to those that have had a vacuum seal in theirs for some years.

Do they vacuum their housing for several hours before diving? If so have they noticed that main body o-rings required to be replaced more regulary?

Maybe its like a first stage regulator. Doesnt matter so much on the amount of dives you do when you service your first stage but manufacturers recommend that you do every X amount of years or like some people, who only service it when something is wrong.....?

Regards Mark

Another Idea is that housing manufacturers look at using different Durometer rubber for their O-rings. Using a higher durometer rubber to have a stronger, longer lasting o-ring when used in suitable applications. Just like using 90 Duro O-rings on your Scuba cylinders. They cost about the same and last much longer than the standard 70/75 Duro o-rings.......? Might not work on those housing who main body o-rings has alot of bends but I cant see a problem with those round o-rings like for ports/extensions and domes...........?????

 

Regards Mark

The problem with harder o-rings is they will not seal as well under low pressure differences. Softer o-rings (lower Durometer) them to give more and seal better. I would not recommend moving from what the manufacture recommends for o-rings.

Bill what about those service kits you see for those doing tech dives? Are they using harder o-rings and stronger control springs due to the pressure and stres they are taking their housing too?

Regards Mark

To the best of my knowledge there just replacement o-rings with stronger springs. As you go deeper the pressure outside the housing has a tendency to push the buttons. Installing stronger springs solves this problem.

Udo,

 

What was amount of the vacuum you were using for your camera? I always wondered if too much vacuum was not good for the seal. On the other hand would it be better to have the vacuum at a level that was just above the point where the green light will change to red so that a very small leak would be detected as the vacuum decayed slowly away? The dial on these vacuum gages are broken up by one half inch increments of vacuum and therefore the resolution is not all that good for detecting very small changes in vacuum readings. I don't know if a vacuum gage with more precision would be a better way to help detect a very small leak.

 

The other feedback I have on this topic: is anyone recording the vacuum when first setting up the system and recording the vacuum just before a dive to see if there is a slow leak? I usually draw a vacuum overnight and check it again in the morning but from Udo's experience I am not sure this would catch such a small leak. Has anyone caught such a small leak before by performing a vacuum drop test (i.e. check/record the vacuum over a period of time)?

 

By the way thanks Udo for the feedback on your experience. I think most people have gotten the impression that these systems will detect leaks in all circumstances. This obviously is not the case in your circumstance. I will certainly be more sensitive to the indications of moisture buildup inside the housing now that you have you have shared your experience.

 

Sincerely,

 

The Meltdownman

Hello Meltdownman,

I use an altimeter inside my housing, with the camera installed, to monitor vacuum. If you have insufficient space for that, I think placing one inside an empty housing could be useful in diagnosing small leaks. My housing has never leaked so I can't say for sure, but if a control button leaks a small amount of water, then wouldn't you expect that it would leak a larger volume of air; And if you manipulated that button isn't it likely that the altimeter would display a detectable loss of vacuum?

When I was looking for my altimeter, I considered a more expensive didital one that claims to register 2 ft inrements in altitude (the reading changes If you raise it from your feet to eye level). This altimeter is called the MicroTim, and it costs about $200.

A small aneroid barometer/altimeter costs much less. Here is a cheap one from China that was too big for my purpose but was otherwise surprisingly well put together - I know because I destroyed it while trying to make it fit. It cost less than $20.00 but took over 3 weeks to arrive:

http://www.ahappydeal.com/product-77032.html

John

I maybe way off base (and somewhat off topic) on this but maybe Bill or someone else with an engineering background could answer:

With any of the currently discussed devices (Sentry, Airlock, Hugy) would there be any utility of using a low positive internal pressure to test seal integrity? I am thinking bike pump, 2-3 psi differential and dunk tank looking for bubbles. This might be a useful way localizing leaking seals.

Or would this be completely facticious, based on how the o rings are designed to work. It would simulate taking the housing to altitude, and most housings warn against flying with a sealed housing...

You can use low pressure to help you find a leak in a housing depending on the configuration of the clips that hold the housing together however, do not try to dive with a pressurized housing. If you pressurize a housing and dive with it at some depth the pressure outside the housing will equal the pressure inside the housing and you will not have a pressure differential. Pressure differential is what makes the o-rings seal. Finding a pressure leak may not be the same as finding a vacuum leak as they may be at different locations on the o-ring.

You might also look here

http://wetpixel.com/forums/index.php?showtopic=50446&p=331999