2 hours ago, Wira Wijoga said:
Did the rig become slightly more buoyant again as you're finishing the dives in the shallow (5m) ? my nikon rig does the same ... always more buoyant in the shallow less than 6m !
Thanks for the info on your setup. Was asking because my 16-35mm Nikon setup with the 80mm extension ring+Nauticam 180mm was so buoyant underwater, and even worse that it was soo hard to bring it down underwater when i was swimming with whale in Tahiti. I'm just thinking if i should sell my 80mm extension ring, now that i found out my nikon 16-35 fits into my housing+ N120 extension ring 30+ Nauticam 180mm dome set which is much less buoyant (intended for 8-15mm + 1.4 tc setup) even though the lens positioned so close to the dome's glass.
The lens may fit in the setup with shorter extension, but won't be correctly placed optically and this introduces distortion and chromatic aberration to the image. The 80mm ring places the lens at the correct spot.
When considering buoyancy the only thing that matters is the displaced volume> The difference in volume between the 30 and 80mm extensions in 0.00056 m3, so the buoyancy change going to a 30mm ring from an 80mm is approximately 0.56 kg, so strapping on a 1lb dive weight will have about the same impact and is what I would do in that situation.
Regarding the buoyancy change again only a change in the volume displaced will impact buoyancy. For example the float arms may flex a little at depth or the the o-ring on the housing back may compress to the point that metal to metal contact occurs, preventing additional volume change. Likewise the dome may be pushed further into the o-ring until it can't compress further. Vacuum won't have any impact apart from causing the compression to occur shallower than it would otherwise. The reported vacuum achieved on such system is equivalent to the housing being submerged 2m. Pulling a vacuum will reduce the weight of gas in the housing, but this would be a fraction of a gram change.