jplacson once bubbled...
Start of the dive, empty bladder, full lungs... float at eye level.
Close.
I believe it's "end of the dive," empty bladder, full lungs... Float at eye level. Of course, that doesn't help you at the beginning of the dive... So conversely, you could say:
"Start of the dive, empty bladder, full lungs... Float at eye level, then add the amount of weight that your tank swings from full to empty." With an AL80, that's about 6 pounds. It will be more if you're using a larger tank, and less if you're using a smaller tank.
I was told that the Luxfer tanks are only -1/-1.5 when empty, as opposed to the Catalina tanks which hit -3 when empty... the resort I was in uses Luxfer (or is it Luxfor?) so that's probably why I didn't need much weight.
Well, the truth is that Luxfer actually makes three different AL80's... They're all listed on my site. They all have different buoyancy characteristics, although they all swing about 6 pounds.
...So it really depends on which Luxfer AL80's the resort was using. However, I've found that the most common is the S080, which has been rebadged under different brand names... US Divers and Aqualung being one, if I recall.
Additionally, the actual buoyancy characteristics are going to depend on water density. That is, everything, tanks included, are going to be more buoyant in salt water than in fresh. That's why people diving freshwater commonly talk about AL80's being -3 full and +3 empty. To a very small degree, actual water temperature can change them as well.
That's why we wear buoyancy compensators... To adjust for the buoyancy changes in tanks while diving, and in different diving conditions. Ultimately, that's really what you're shooting for... To only have to compensate for the tank change - not anything else. This will keep the amount of air in your BC to a minimum, which increases your streamlining and minimizes the effect of depth changes to the gas in your BC.
If I dive a Catalina tank, I was think of just using the Halcyon STA to offset the weight. The standard STA is about 2lbs right? Plus the 1# of the AL plate... that's enough to offset an empty AL tank.
I've never seen a Halcyon STA that wasn't 6 pounds... Although they may make it. I've just never seen one. A FredT "light" STA is 2 pounds, and Scott Koplin makes one that's less than a pound in weight.
Although I know trim is something that is usually born out of trial and error...I'd like to get as much calculations done on the surface so that I can fine tune everything as soon as possible.
Agreed. I'm not sure if I posted it here in this thread or not (I'm losing track) but I touch on this subject at
http://www.bftwave.net/lcscuba/homebc.html
The idea is to bring your center of gravity and your center of buoyancy together as closely as possible. In my humble experience, diving a 3 mil wetsuit (buoyant), a steel plate (-6 pounds) a light STA (-1 pound) and a heavy tank (-1.5 pounds empty) brings my center of gravity too far back, and I'm prone to "turtling." You might not have this problem with a fleece wetsuit, since your center of buoyancy is not brought forward with a buoyant wetsuit. However, it's something to think about.
Anyway, I'm trying to see if my math is right ... with a Luxfer tank, the AL plate is enough to offset it at 1#. Catalinas are about -2# more than the Luxfer so the 2# STA should be enough to offset the tank. My body should be enough to balance off the empty wing... so with this setup...I should have, at the MOST, is a 2# trim weight on the tank strap or non at all.
I don't see that.
I see that a Luxfer S080 (what's most common) will put you, worse case scenario, at +4 pounds empty in salt water. The human body is naturally buoyant as well (especially with a full breath) and even if your suit really is neutral (I don't believe it is... If you threw it in the water, would it not float?) then you're going to need more weight to offset this.
Once you've decided exactly how much weight you need (most of us do well with a 3 mil, 6-8 pounds and an AL80 in freshwater, or 3 mil, 12-14 pounds and an AL80 in salt) then you're probably going to want to do some "fine tuning" and figure out where that weight should be placed so that your rig isn't "opinionated." For most, it involves about six pounds up and on the back... With the rest on the hips. Of course, that's an individual thing, and your mileage may vary.
