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New Housing "Airlock" Vacuum from Backscatter


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#181 drsteve

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Posted 25 September 2013 - 04:18 PM

Adam, you are partially right, but you have confused the hardness of the o-ring material (durometer) with the o-ring resiliance, which is the tendency to become permanently deformed.  The durometer rating is related to the clamping pressure required to make the seal, whereas the resilience depends primarily on the material used and the percent compression. If you have enough clamping pressure (ie bolts) you don't need to use a soft material.  For example Buna-N o-rings are inexpensive and have good resilience, which is why they are so commonly used.  A face seal using a 70 durometer Buna-N o-ring with a 15% compression is good for over 3000psi without even trying.  If you use bolts, you get plenty of clamping force.  You don't need a torque wrench, you can simply tighten the screws until you hit the hard stop of the two surfaces touching.  The problem is that the suitcase clamps can't do this, so the housing vendors rely on an incomplete face seal and let the water pressure complete the seal.  To me this is a poor trade-off and explains the tendency for housings to flood in the rinse tank.  Frankly, the pressures experienced in recreational diving are trivial for a good o-ring design, so unless the o-ring is physically damaged, issues like compression set should be almost irrelevant.  If you are interested, there is more than you ever wanted to know about designing seals at  http://www.logwell.c...er_Handbook.pdf

 

I too need to give this thread a rest.


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#182 John Bantin

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Posted 26 September 2013 - 11:38 AM

I, like a few others, can't seem to leave this thread alone!

 

As I understand it, if an O ring seal is to be tightened until metal to metal contact is ensured, a soft O ring of 50 or 60 shore is required. Unfortunately, this softness of O ring would deform and need replacing after almost every use. The reason this features in machinists handbooks and the like is that most engineering applications for O rings are for permanent installations, rather than situations in which the seals are regularly disturbed.

 

Even with a bolt type closure, the only way to guarantee metal to metal contact would be to specify a torque to do so. 

 

Firmer, harder O rings, located in a suitably deep groove, have sufficient material hardness to form a seal that will not extrude despite the metal not meeting. There are quite literally millions of examples of this in action-not least of which would be the O ring sealing your regulator onto your tank!

 

Just to emphasise again, I would imagine that few underwater photographers rely on "gingerly dipping" their housings in a rinse tank. Cleaning, inspection and careful preventative maintenance are the key. Vacuum detectors simply prove that you have followed these procedures correctly.

 

I think I need to stop following this thread:)

 

Adam

 

 

Adam, The Nauticam version will arrive any day and then you can sleep soundly in the knowledge!


Edited by John Bantin, 26 September 2013 - 11:39 AM.

I buy my own photographic kit. Diving equipment manufacturers and diving services suppliers get even-handed treatment from me whether they choose to advertise in the publications I write for or not. All the equipment I get on loan is returned as soon as it is finished with. Did you know you can now get Diver Mag as an iPad/Android app?

 

#183 Deep6

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Posted 26 September 2013 - 11:48 AM

Adam, The Nauticam version will arrive any day and then you can sleep soundly in the knowledge!

Oh thank you John for that reassurance!  :dancing:


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#184 adamhanlon

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Posted 27 September 2013 - 10:20 AM

Hi John,

 

I know it is a big ask, but if you read my reviews, you would know that I have been using the Backscatter one for some time! I don't find that it affects my sleep in any way at all. 

 

http://wetpixel.com/...zen-dp-100-port

 

http://wetpixel.com/...ty-check-system

 

I should also point out that it was kindly supplied by Backscatter free of charge for review purposes. There are now other similar units available from many other manufacturers and suppliers, including the one that John espouses in his Undercurrent article.

 

For what it is worth, rinse bucket O ring leaks are normally due to O rings not being located properly,being damaged or being dirty. A correctly installed, undamaged and clean O ring will not leak in a rinse bucket ( or anywhere else!) If it was common, most underwater photographers (including John before he saw the "light") would have flooded their housings many times over. Housing leaks are not caused by poor design or engineering, but by user error.

 

Even so, I can't find any reference to face to face contact in static seals being necessary in the text above....Perhaps I missed it.

 

Adam


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#185 diverdoug1

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Posted 27 September 2013 - 11:46 AM

Here is a rinse bucket tale of woe that disputes Adam's above posting.  Last year on a Palau liveaboard I had just returned from a dive.  The crew had unloaded all the cameras from the skiff onto the deck of the liveaboard.  I went to rinse my camera and..........BLUB!  Major camera destroying leak.  The crew had inadvertently bumped my macro port and slightly dislodged it.  I would have seen the blinking light of a lost vacuum had I had my vacuum airlock back then, and disaster would have been avoided. I also feel that had the housing had been under vacuum, the port would not have dislodged in the first place.


Edited by diverdoug1, 27 September 2013 - 11:46 AM.


#186 adamhanlon

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Posted 27 September 2013 - 02:07 PM

Doug,

 

Sorry to hear about your flood-I lost a camera and lens under similar circumstances a few years ago too. Since then a port lock and locking extensions are a major feature on any housing that I buy. Vacuum systems do help hold ports in place, but there can be a fair amount of leverage/force involved in moving gear between boats etc.

 

Since you expect the housing to be watertight post dive, most people would not check the lights on their housings in such situations. I know of one flood that was caused by a curious third party actually opening the vacuum bulkhead in order to see what it was in-between dives and not closing it again. The diver failed to check the lights before leaping off the boat with it, as it had sealed properly on the first dive! 

 

Human error is nearly always the cause of floods!

 

My comments on rinse tanks were to do with the idea of testing a housings watertightness prior to the dive. To be honest, I don't rinse my housing post dive with the camera in it, as I can see little reason in further testing my housing's waterproofing if it is not strictly necessary. 

 

I do rinse it thoroughly once the camera has been removed at the end of the day's shooting.

 

Adam


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#187 diverdoug1

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Posted 27 September 2013 - 04:08 PM

Not to take this thread too far off topic, but I see that Adam stated he just rinses his housing at the end of the day.  I personally rinse after every dive, working all the buttons.  I wonder if my rinsing with fresh water between dives is necessary.  I have been under the perception that this would help prevent salt crystal formation.  Any other opinions out there?



#188 Deep6

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Posted 27 September 2013 - 07:54 PM

Not to take this thread too far off topic, but I see that Adam stated he just rinses his housing at the end of the day.  I personally rinse after every dive, working all the buttons.  I wonder if my rinsing with fresh water between dives is necessary.  I have been under the perception that this would help prevent salt crystal formation.  Any other opinions out there?

If available, I rinse the housing (working buttons) after each dive.  If not, then I cover with a wet towel or rag and keep the housing in shade.   I recently had a rinse tank flood.  No damage to camera or lens.  I think one of the crew accidently release the housing back lock.  The wet alarm went off.  Now I am the only one to rinse my housing. 

Bob


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#189 adamhanlon

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Posted 28 September 2013 - 01:18 AM

Hi folks,

 

I've started a rinsing thread!

 

http://wetpixel.com/...showtopic=51431

 

 

Adam


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#190 Viz'art

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Posted 03 October 2013 - 09:20 AM

The problem is that the suitcase clamps can't do this, so the housing vendors rely on an incomplete face seal and let the water pressure complete the seal.  To me this is a poor trade-off and explains the tendency for housings to flood in the rinse tank.  

 

I just want to set the record straight, the so called “suitcase” latch are actually designed to preload the O-ring to a set amount of pressure, we use 45 lbs,  but it could be anywhere from 30 to 70lbs, depending on a manufacturer preferences. This preloading will be compressing the o-ring to a depth equivalent of about 100ft of ambient pressure, and that is before it even touches any water. Sorry but, the claim that “suitcase” clamps do not offer a complete face seal or are just holding the front and back together in wait of ambient pressure is not accurate.

 

If you ask me, so far as I can tell, I have yet to see a “faulty or inadequate” design of housing closure system among the group of housings manufacturers that tends to your underwater photography needs. That group would include us as well as our competitors past and present. No matter what the method, be it side sealing or compression sealing, a housing manufacturer decides to use for sealing and securing their housings, they will be staking their reputation on it.  So c’mon, I mean, this industry is pretty Darwinian, if you are ill adapted for the task, you will simply become extinct!

 

O-rings and their roles in protecting our equipment from ambient pressure is basic elementary knowledge for anyone involved in manufacturing underwater equipment, No one would seriously think, that if a closing system was incompletely, or poorly, doing its job, especially with all the method of sealing a housing available, that a manufacturer would keep using this type of closure for decades. There is a substantial amount of photographic and video manufacturers using these “suitcase” latches successfully; they are also commonly used as a mean of closing up and sealing many of the military and scientific equipment that requires ambient pressure protection.

 

Adam’s got a point when he says that floods occurrence is normally due to O-rings not being located properly, being damaged or dirty, they do not spontaneously fail unless there is some interference of some sorts involved.

 

Before even pumping up the vacuum in a housing, A simple and very efficient way of testing to see if the main O-ring is properly sealing is to gloss up the O-ring, make sure the mating surface is clean (both of which you should do all the time anyway) and simply close the back, clamp down the latches and open the housing again, you should see a clear foot print signature of the O-ring on the mating surface (see picture). At this point, what can I say, it is the user’s responsibility to check and that there is no obstruction, nicks or scratches on the path of the foot print signature.

Attached Images

  • Signature-.jpg
  • O-ring-illustration.jpg

Edited by Viz'art, 03 October 2013 - 09:24 AM.

Jean Bruneau / Aquatica Technical Advisor

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#191 Deep6

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Posted 03 October 2013 - 10:00 AM

Thank you Jean.  Very informative.

Bob


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#192 Viz'art

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Posted 03 October 2013 - 05:15 PM

Bob, I forgot to mention that part of a pressure test cycles involves letting the housing sit in the tank of water without pressure for a while, then after a bit, it's gradually down to 90m/300ft (or more) for a period of time before making its way back up and sitting in the tub for another moment. with that method of testing if something does not seal as it should, the manufacturer just won't let it out of the shop.

 

Oh, and I like the Carpe carp,  :-) having learned some Latin as a kid, it really cracked me up!


Edited by Viz'art, 03 October 2013 - 05:18 PM.

Jean Bruneau / Aquatica Technical Advisor

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#193 diverdoug1

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Posted 04 October 2013 - 06:47 AM

Jean thanks for the diagrams, I never properly understood how a side sealing o-ring worked!

#194 sharky1961

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Posted 08 October 2013 - 06:22 AM

Hi,

 

I wanted to contact  uwcamerastuff by there homepage. It seems that there domain (http://uwcamerastuff.com) has expired.

is there another way to contact them??


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#195 johnspierce

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Posted 08 October 2013 - 06:35 AM

Hi,

 

I wanted to contact  uwcamerastuff by there homepage. It seems that there domain (http://uwcamerastuff.com) has expired.

is there another way to contact them??

 

Wow, that must have just happened -- I was there 2 days ago.  It's run by Bill Libecap (member= blibecap), I'm betting it was just an oversight.  You could send him a PM through Wetpixel.


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#196 blibecap

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Posted 08 October 2013 - 06:54 AM

Hi,

 

I wanted to contact  uwcamerastuff by there homepage. It seems that there domain (http://uwcamerastuff.com) has expired.

is there another way to contact them??

We are aware that we are having problems with our web site. Tech support is working to fix the problem. Please feel free to contact me directly, BLibecap@gmail.com. 

 

Sorry for the temporary outage. 

 

Bill


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#197 blibecap

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Posted 08 October 2013 - 02:36 PM

We are aware that we are having problems with our web site. Tech support is working to fix the problem. Please feel free to contact me directly, BLibecap@gmail.com. 

 

Sorry for the temporary outage. 

 

Bill

Our web site is back up now


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#198 drsteve

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Posted 08 October 2013 - 09:40 PM

Jean, you inspired me to measure my 5D housing to see how well it conforms to the standard mechanical engineering face seal guidelines.    Attached is the face seal design from the Parker o-ring catalog.  The first thing to notice is that it assumes sufficient clamping pressure that the surfaces are pressed together so that the two metal surfaces touch and the o-ring is completely trapped within the groove.   My 5D housing uses a standard 1/8" ring.  The terminology is unfortunate because a 1/8" o-ring has an actual thickness of 0.139 +-0.004.  The guidelines specify that the groove should be between 0.101 and 0.107 deep, which would give a compression ratio of 20% to 30%.  The width for a standard o-ring should be 0.177 and 0.187 to give room for the o-ring to expand laterally when compressed.

 

The groove in the housing measures 0.100 deep and 0.136 wide.  The depth conforms to the standard  depth assuming enough clamping force to compress the two halves until they touch.  However the cam latches do not provide enough force to do this.  When closed, there is a gap of 0.032 so that the o-ring compression is only 0.008 or 5.7%  This is seriously under compressed, which is the point I made in the original message.

 

I believe that you test housings to insure that they seal reliably, however since the initial seal is seriously under compressed it will work only if everything is perfectly clean and greased and nothing gets bumped in the rinse tank.  Of course everyone everyone should keep their o-rings perfectly clean and greased, but there are plenty of stories of someone getting a stand of hair or a piece of lint under their o-ring causing it to leak. 

 

The if o-ring were designed per the guidelines, it would be much more robust.  A hair or a spec of lint wouldn't cause it to fail.  The good news is that the vacuum system provides enough clamping force to make the seal much more reliable.

Attached Files


Edited by drsteve, 08 October 2013 - 09:42 PM.

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#199 diverdoug1

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Posted 09 October 2013 - 09:05 AM

drsteve, I have a couple of questions if you would be kind enough to answer.
1. Is there a fixed amount of o-ring compression required at zero pressure differential before it is considered adequately compressed, or does it vary between different types of 0-rings?
2. For the O-rings used in your Aquatica housing, what percentage of compression is considered adequate?
3. How many inches Hg of vacuum are required to bring your o-ring compression into this adequate range?
Thanks, Doug

#200 drsteve

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Posted 09 October 2013 - 02:10 PM

The rule of thumb for static face seals is to design around a nominal 20-30% compression with a worst case including tolerance stackup of  >10%.  Reciprocating seals on shafts (i.e. buttons) are typically designed around 15% compression with a minimum compression  >10%.  It varies somewhat for different materials.

 

I measured my 5D housing since it has a traditional rectangular o-ring groove.  The vacuum system is on my 5D3 housing which uses a dovetail groove.  The calculations are a bit tricker for the dovetail, but I pump my housing down by 10inHg = 4.9psi.  The cross sectional area of the rear door is roughly 50 in^2, so the vacuum is applying an additional 250lbs of force.  Jean said that they set the latches to apply 45lbs, so an additional 250lb should be safe ;-)


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