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Member Since 20 Nov 2005
Offline Last Active Mar 23 2014 07:12 PM

Posts I've Made

In Topic: 1 Diver dead, 5 rescued and 1 missing in Bali

20 February 2014 - 11:31 AM

I have a Lifeline and the thing I like about it is that it allows you to communicate directly with vessels in the vicinity.  All satelite based systems alert an operations center that then has to coordinate a rescue. That may work great when you are in a developed country with good search and rescue infrastructre, but I am skeptical about how well that works in many of the remote locations that divers visit.  Do you really think that a satellite operations center in Nebraska will be able to communciate with a panga in Indonesia that happens to be a mile from you?  The advantage of VHF solution + GPS is that it will work everwhere there are marine radios (i.e. everywhere), even if they are ancient.  That being said, I have only used my Lifeline for "radio check.... radio check..."   :-)

In Topic: Housing care - post rinse

09 October 2013 - 10:35 PM

That is a good question. A few years back I bought the Aquatica service kit and replaced all of the o-rings in my 5D housing. It isn't particularly hard but you have to be meticulous. The buttons use x-rings and I damaged one during the service. Since the kit only gives you the exact number that you need I bought a bunch of spares from Mcmaster-Carr. X-rings only come standard in Buna-N and Viton. I am pretty sure that they use Buna-N but both materials are fine for use with silicone oil. Even so it would be prudent to check them periodically. A safer alternative would be to pack some of the Christolube into a gluing syringe and squirt the outside of the button seal with extra grease.

In Topic: Housing care - post rinse

09 October 2013 - 04:03 PM

After every rinse, I use a Rocket air bulb to quickly blow out the water from the buttons.  Next to each button is a small access hole to let water in and out during actuation.  A quick puff of air gets 90% of the water out.  The less water left in the hole, the less residual salt will be deposited when it drys.  I also blow water out of the gap between the back plate and the housing.  I then towel dry everything.  The more water you get out, the less likely you are to drip when you open the housing.   If I am not diving for a week, I usually remove the ports and the back plate.  I always remove the strobe cables and dry the connectors because they are prone to corrosion.


Diggy, thanks for the silicone oil tip.  That sounds like a great idea.

In Topic: New Housing "Airlock" Vacuum from Backscatter

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 ;-)

In Topic: New Housing "Airlock" Vacuum from Backscatter

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.