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Everything posted by diverdoug1

  1. 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.
  2. 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.
  3. 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
  4. I don't think my D800 AF performance is as good with the Sigma 15mm as compared with the Nikon 16mm, but I like the Sigma for WA closeups becaues of the closer focus ability
  5. Jcclink, you do NOT need to press the shutter release in any way to start focusing manually
  6. As far as the VR is concerned, I find it not useful underwater.
  7. I think there may be some confusion between the rules stated by the cattle boat with regard to submerging, and Florida law. Scuba diving is allowed in the areas not in the sanctuary zone, so submerging is obviously allowed. I find the best photo opportunites are provided if you rent your own boat. You can rent a pontoon boat for up to 8 people for $25 per hour at the Crystal Lodge Dive Center (352) 795-6798. Just follow the appplicable laws regarding manatee interaction. It is a much nicer and relaxiing experience than trying to go on a cattle boat.
  8. Here is an NAD-700 http://www.perthscuba.com/shop/na-d700-housing-for-nikon-d700-camera/
  9. Here you go, Hope this helps http://www.bhphotovideo.com/c/product/587081-REG/Ikelite_6812_7_6812_7_iTTL_Underwater_Housing.html http://www.bhphotovideo.com/c/product/886159-REG/equinox_underwater_housing_for_nikon.html http://www.backscatter.com/sku/su-10nd700.lasso http://www.backscatter.com/sku/ike-6812.7.lasso http://www.backscatter.com/sku/ex-hddslrd700.lasso The third link is a new Subal for $3,676.65
  10. If you are going yo use the warm cooler trick, make sure to only open your housing in a low humidity environment prior to diving. If not, you might get fogging issues when submerging the housing in cold water.
  11. I use xit404 manual focus gears for my 60 and 100mm macro lenses. I have been using them for nearly 2 years without any problems.
  12. Happy New Year everyone!
  13. The pre-loading of your O-rings afforded by a vacuum system will certainly make things safer in the surf zone.
  14. Your minimum focus distance. For that lens is 7.8". It looks like if you are not using a diopter, the apparent image of the dome will be closer than 7.8 inches when using the shorter extension rings.
  15. I used to shoot a D300 in an Ikelite housing, and a number of the buttons would not function properly if either the eyepiece or the LCD cover were left on.
  16. Scuba-s, If you are shooting macro in Manado, I hope you get to go the Lembeh Strait, (less than a 2 hour trip).
  17. If you are using a rubber eyepiece in your viewfinder, remove it.
  18. The 45 is not a pain unless you are trying to shoot downwards, then it is a huge pain. I guess I am a slow learner, I dive with mine 3 times a week, and I still prefer to shoot WA with the 180 (I even tried leaving the 45 on for 6 months and still liked it when I tried the 180 for WA). Different strokes for different folks I guess.
  19. I use a 180 for my wide angle work, and a 45 for macro. I find the 45 more difficult to see sublect that are below me and to track fast wide angle action, but it allows shooting up on macro subjects. I think if I could only have one, I think I would buy the 180.
  20. For sale: a Retra LSD PRO (light shaping device) optical snoot. This is designed to fit on Ikelite DS125 / DS160 / DS161 strobes. It is the silver model. here is a link for a review of the Retra LSD PRO is below http://wetpixel.com/articles/review-light-shaping-device/ A review of the Retra LSD PRIME (the bargain version) by Alex Mustard is here http://wetpixel.com/articles/field-review-retra-lsd-prime-snoot Condition is like new but my last name is written on the body of the snoot. This snoot is one of my favorite toys, but I switched to Sea&Sea strobes so I had to buy a new Retra to fit the D1 strobes. Retail price is 649 Euros, I am selling for $350 USD plus the cost of shipping from Sarasota Florida, payable to Paypal. Please PM me if you would like me to email you pictures of the Retra.
  21. Which Aquatica housing do you have? The current housing for the D300S (part #20064) allows for a pop -up flash.
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