Buoyancy Question

[ccx2]

Got in the pool to prepare for my upcoming trip to Mexico, I have a new,first BC,Sherwood Avid,and wanted to dial in my buoyancy with a 3mil shorty wetsuit and other gear. I told the LDS guy I would like to start with about 22lbs of lead because that was about what I've been using on similar dives with rental equip. He said that would be way to much and told me to start out with 16 tops,I did,he was right,and I got down to 3lbs and at that I would have to wave my arms a little to get started and then I would Desend. With 3lb at 15' I would rise and fall on in/exhale and adding 1lb seemed to keep this to a minimum. So 4 pounds it is. I pulled the Al80 down to 2000 so i shouldn't have seen much diff in that and 3000, right. This was a freshwater pool.

My question is how should I add weight for saltwater and a low tanks buoyancy ? Is there a tried and working formula ?
Also ,my BC has trim weight pockets on the back shoulders that I can't understand what they would be for,they are non ditch able (without yoga monkey arms) and the back seems like an odd place for weights anyhow,maybe with weight there it would allow one to swim in a spiral manner more easily ?
Thanks for all opinions.

 

[ChrisV]

I wear 4 pounds in fresh water with a full 3mm, I just got back from mexico and needed 12 pounds. So I went up 8 pounds for salt water in a full 3mm.

You can do a quick calc if you know your total weight including you and all your gear and weightbelt.
What you want to do is basically calculate your volume you take up in fresh water (since your neutral) and then take that volume and calculte the equivalent weight in salt water (the difference is your needed addtional weight).

For example on me...
Fresh water I wear a 3mm with 4 pounds of lead.
Total weight = 155(me) + 20(rig) + 40(tank) + 4(lead) = 219 lbs
That volume for me in fresh water would be 219/62.4 = 3.51 ft3

now take that volume and calulate the weight of salt water for the same volume
3.51*(1.025 *62.4) = 225 pounds

take that total weight and subtract out you and your rig and tank
225 - 155(me) - 20(rig) - 40(tank) = 10(lead)

I tried 12 to add a couple pounds your air consumption and it worked fine for me.

 

[TSandM]

2.5% of total weight of you and gear should be added for salt water, as already stated.

For gas, you can easily calculate it. 13 cubic feet of air weighs approximately one pound. If you have 77 cubic feet of gas (the capacity of an Al80 at rated pressure) in 3000 psi, you have close to 2.5 cubic feet per hundred psi. If you still had 2000 psi in your tank when you were doing these weight checks, you had about 50 cubic feet of gas. Since you don't intend to use the last 500 psi (or you shouldn't, anyway), you are going to keep a 12.5 cubic foot reserve, leaving 37.5 cubic feet of gas you were still going to breathe out into the water. That's close enough to two pounds as no matter; if you wanted to be very conservative, you could add three.

BTW, this method will work with ANY tank. As long as you know the capacity at rated pressure, you can calculate the cf/100 psi (known in tech parlance as the "tank factor" for that particular tank). That will tell you how many cf you are going to exhaust into the water, and you can easily calculate how much weight to add. This is handy for doing weight checks with full rental tanks you haven't seen before.

 

[t-mac]

The formulas will only give you a general idea since they dont' account for body composition. Best thing is to keep track of how much weight you need with what equipment under what conditions. Big factors are amount of exposure suit, salt or fresh and aluminum or steel. Once you dial in on those variables you can pretty well guess. Also, new exposure suits tend to be more bouyant than "experienced" ones, as do dry ones. You may find that you can drop weight on the second dive of the day, especially with a new suit. Just count your blessings that you are in a 3 mm shorty in warm water rather than a 7 on 7 farmer john in a cold quarry. It makes the experimentation more fun.

 

[ColoDale]

I use the weight pockets on my tank band in the back for trim when at the end of a dive. I have a back inflate BCD and at the surface and at the end of a dive I tend to go face forward a bit. So to balance that I use 4 lbs (2 lbs each "back" pocket). They are non-ditchable by me. I use a weight belt as well. I did not need to add back weight with a traditional jacket BCD. (All of this refers to salt water diving)
I am not sure what you mean by "...allow one to swim in a spiral manner more easily ?" I only use weight in back for trim.
Also, on entry from boats, I have trapped air in booties, 3mm jacket if used, BCD, SMB, etc. Once I get rid of that, the weighting is good and the descent is easy.

 

[Wayne at DiveSeekers]

Buoyancy questions are hard without seeing you in the water. If you are not having proper lung control or are slightly kicking that can effect what's going on regarding the weight you are using. With out having the technique divers think that they can't descend, add more weight (when they don't need it) and then are diving overweight. Rule of thumb (providing you are dialed in fresh water) about 4/5 lbs. Trim weight pockets help you divide up the weight so it is more evenly displaced.

 

[bullshark]

22 lbs. to 4? Holy crap. I digress.

The air in your AL 80 has a weight of approximately 7 lbs.

At 500 psi it will be about 2.5 lbs lighter than it was at 2000. Why you considered that measurement[2000 psig] is a mystery. You could have emptied the tank and gotten a feel at 2-300 psi. I digress.

Right away you're up to 7 lbs. for an empty tank.

The most important things to know would be your height and weight and BMI. If I could see you, I could tell you the correct amount to within two lbs. You didn't give us that information.

As it is, you have given us rather strange observations as observed by an inexperienced observer. It is further complicated by the fact that you have insufficient experience with your BC to know if it was empty or not.

Best guess Answer: start with 12-14 lbs and work down. I think you'll wind up with 10, but that would be insufficient for first day after flight-dried gear. You might even want to go with 16. You are inexperienced and will be puffing up like all new divers do for the first few dives. You were nice and relaxed in the pool, like all new divers are. Open water is way different than a pool.

The back weight pockets are trim pockets. You carry some of your weight high on your back so that you're not swimming in a vertical position. Without finning, or arm-waving your body should naturally assume a horizontal position underwater. Mastery of buoyancy skills is impossible without this attitude.

The trim-weights being non-ditchable is immaterial. Correctly weighted warm water divers never carry more weight than they can swim up in the first place, and less than half of that will be in the trim pockets. Notwithstanding, I can't think of any likely warm-water scenarios where ditching your weight could provide an advantage. This isn't the 1960's with canvas straps on a plastic backplate, or even the 70's with an orally inflated horse-collar. You aren't a cold-water diver with 7mm whole-body wetsuit that loses 15 lbs of buoyancy on bottom-crush. Those are the conditions that need to consider weight ditching.

You are a warm water diver with negligible wetsuit buoyancy (a shorty) that has even more negligible buoyancy loss at depth. Your BC is a 21st century device made of Space-Age materials and will always be capable of holding air (in some attitude). In the extraordinarily, super-improbable event that I had to do an ESA (I haven't seen one in 33,000+ hours of observed diving), I wouldn't ditch my weights, I would ditch the whole rig.

Ditching your weight is an extremely dangerous action that will likely result in lung trauma and/or embolis from the uncontollable ascent that follows. IMO the issue is discussed in scuba instruction only to cover all the bases (the books don't know the water temp the diver will use), and warm water divers would be well-advised to forget it. Then again, you were diving with 22 lbs...so it's understandable that you were concerned.

 

[freewillie]

If you want the absolute most accurate weighting, see above for weighting before you dive.

However, try coming back to the boat with about 500 psi at the surface. Then, before you get back on the boat try doing another weight/buoyancy test. That way you will know exactly how to weight for an empty AL tank with your specific gear and wetsuit.

 

[TSandM]

Oh, yes, I forgot the part of your question about the trim pockets.

Trim pockets are to help you balance in a horizontal position. For some people, putting all the weight they need in integrated pockets or on a belt will mean that, every time they stop moving, they will rotate to a vertical position. This means the swimming diver is both pushing himself forward AND keeping himself horizontal, which is wasted energy.

When you only need to carry a very small amount of weight, you may not have a lot of choices of where you put it, if you want to keep it ditchable. I think ditchable weight is probably important for new divers -- not because you will ever need to ditch it at depth, but because having ditchable weight makes it extremely easy for you or anyone trying to help you to make you positively buoyant, even if you are out of gas. But once you have enough experience to be fairly sure you are watching your gas consumption carefully, and have a sense for how long you can dive at a given depth on your tank, you may want to move the small amount of weight you require to those trim pockets -- or at least some of it. Being able to be effortlessly horizontal is the first, simple step toward reducing what, in new divers, is always a high gas consumption rate.

 

[Charlie99]

CCX2, start with your 4# at 15', with 2000psi in an AL80 in freshwater.

My recommended adjustments are:

The 2000psi weigh 4 pounds. +4 adjustment so you can surface under control with near empty tank.

You were at 15'. You need to be able to slowly ascend to the surface. +1 pound to make this easier as your 3mm suit expands between 15' and the surface.

The correction for fresh to saltwater is about 2.5-3% of total dive weight ( you plus gear plus tank plus lead)
For a 180 pound diver + 60 pounds of gear this is about 6 pounds. It is better to be a pound or two heavy than to be underweighted, so I suggest starting with +7 for this adj.

4 + 1 + 7 added to your 4 pounds in the pool would be a total of 16 pounds.

It takes a few minutes for your wetsuit and other gear to fully saturate and lose excess buoyancy, so it is best to do a weight check at the end of your dive. The air in an AL80 weighs close to 1 pound for every 500psi,so it is easy to account for the weight of air in the tank, even if you choose to do a weight check with a full tank during the surface interval between the dives of a two tank boat trip.

Charlie