BP/W Selection & Weighting

[GoBlue!]

Hello-

I know that this topic comes up regularly, and yes I've done a lot of searching & reading other threads.

I'm currently shopping for gear, and recently had the opportunity to try a friend's Halcyon SS BP/W (Pioneer). Really loved the way it felt. I was, however, overweighted in the pool w/ my new 3mm neoprene wet suit & the 6# plate w/ an AL80 (at 2000psi). I didn't have weights with me, but went back to a pool another day (without gear....just the suit) & found I need about 4# of weight to achieve neutral surface buoyancy with the exposure suit only.

My diving is generally warm & is both fresh & saltwater. I don't own a dry suit, and if I do in the future I'd get a new setup for that type of diving.

So, given that I only need about 4# to neutralize my exposure suit, it seems that an aluminum plate & lightweight STA is what I should get with some trim weights if necessary. Seems that a stainless plate would just turn me into a rock. Am I missing something, or does this sound reasonable?

Jim

 

[Doc Intrepid]

Jim,

Check this site out here:

http://divesales.com/tanks/index.html#navigate

Sites like these provide the "swing weight" for their tanks, or the variance in bouyancy that results from the weight of the compressed gas itself. As you consume the gas, the tank becomes more bouyant.

Notice that under their aluminum 80 the swing is 5.8 lbs, from full to nearly empty. All else being equal, then, if you're perfectly weighted for the end of your dive (which is precisely where you need to be perfectly weighted), obviously you'll be nearly 6 lbs heavy - overweighted by 6 lbs - at the start of your dive.

One purpose of you weighting yourself properly, among others, is so that you can hold a 3 minute safety stop at the end of your dive without finning or exerting yourself to keep yourself down.

Along the same lines, if you've done a deep dive, or multiple dives daily from, say, a liveaboard, you may over time be pressing up against the no-decompression limits. When you're diving like that, it pays to be prudent and hold those safety stops a bit longer, then very slowly ascend that last 15-10' to the surface. Perhaps as slowly as 2 feet per minute, or 5 minutes to the surface from your safety stop. Divers have been bent while technically still within no-decompression limits. Two elements of this are pushing the NDL and how hard the diver is working, both while ascending and directly after getting out of the water.

Proper weighting helps you do proper slow, relaxed ascents. Technically speaking, you may indeed wind up "neutral" at the surface. You require some negative bouyancy to allow yourself to control that slow drift to the surface rather than rising faster than you'd prefer as your suit expands during that last 10'.

To properly weight yourself, then, do it with a nearly empty tank. Say 400 psi max. [I am aware that others advocate weighting with full tanks. I disagree. Under multi-dive circumstances the difference may determine whether one takes a hit or not.]

Put your gear on and ensure that, under pool conditions with essentially no air in your BC, you are neutrally bouyant at 10'. And hold it for at least 3-5 minutes. Add or subtract weight until, under those end-dive conditions, you're neutral. Ensure you have adequate weight so that you do not rise uncontrollably during the last several feet of your ascent. It takes a bit of time, but it is worth your effort. (Remember there will be a variance between fresh (pool) and salt water.)

As others have noted, with your wetsuit losing bouyancy on the descent and a full tank (e.g. ~ 6 lbs overweighted) at the start of the dive, you will initially be overweighted. As your wetsuit loses bouyancy, you'll be even heavier than 6 lbs over. That's okay.

At the end of the dive, where it really counts, you'll be perfectly weighted. And thats where you need it most.

Regards,

Doc

 

[Albion]

thats what im planning on doing, with similar exposure suit. also a NEW suit is more bouyant han an old one so you are only going to get more negative over time and also at depth

 

[jonnythan]

GoBlue! once bubbled...
Hello-

I know that this topic comes up regularly, and yes I've done a lot of searching & reading other threads.

I'm currently shopping for gear, and recently had the opportunity to try a friend's Halcyon SS BP/W (Pioneer). Really loved the way it felt. I was, however, overweighted in the pool w/ my new 3mm neoprene wet suit & the 6# plate w/ an AL80 (at 2000psi). I didn't have weights with me, but went back to a pool another day (without gear....just the suit) & found I need about 4# of weight to achieve neutral surface buoyancy with the exposure suit only.

So an empty Al 80 gives you about another 4 pounds of buoyancy, so you'd need 8 pounds total with an empty tank. With a 6 pound backplate, you'll need *more* weight.

Get into the pool with an almost empty tank and then see if you're overweighted with just the plate.

If you are, head to Leisurepro and plunk down the $35 or whatever for the Al plate.

 

[Don Burke]

GoBlue! once bubbled... Hello-

I know that this topic comes up regularly, and yes I've done a lot of searching & reading other threads.

I'm currently shopping for gear, and recently had the opportunity to try a friend's Halcyon SS BP/W (Pioneer). Really loved the way it felt. I was, however, overweighted in the pool w/ my new 3mm neoprene wet suit & the 6# plate w/ an AL80 (at 2000psi). I didn't have weights with me, but went back to a pool another day (without gear....just the suit) & found I need about 4# of weight to achieve neutral surface buoyancy with the exposure suit only.

My diving is generally warm & is both fresh & saltwater. I don't own a dry suit, and if I do in the future I'd get a new setup for that type of diving.

So, given that I only need about 4# to neutralize my exposure suit, it seems that an aluminum plate & lightweight STA is what I should get with some trim weights if necessary. Seems that a stainless plate would just turn me into a rock. Am I missing something, or does this sound reasonable?

Jim

If you breathe that AL80 down to around 400psig, you'll be a bit closer to neutral. That much air (the difference between 2000 and 400) weighs about three pounds.

In your case, I think you're going to end up with an aluminum backplate. While you might be neutral with the steel plate, 400psig, and no weight, not having enough ditchable weight to keep yourself on the surface with a full tank and failed wing doesn't give me a warm fuzzy feeling about your safety.

The newer Halcyon Pioneer doesn't require an STA, although some have modifed the wings to require one.

 

[SeaJay]

With everyone's comments here.

Weighting should be established with an empty tank... And a full breath, by the way. That makes a difference in some people of up to 9 pounds!

If you need 4 pounds to get neutral in your suit alone, you'll need to have that... Plus whatever your tank is in terms of buoyancy when it's empty. AL80's are +4 empty in SALT water... They're +3 in fresh.

...So the way I see it, assuming you did the "full breath" thing... You'll need -7 pounds to offset an empty tank and your suit in the pool. A SS plate weighs 6 pounds, and you can find another pound or two in a light STA or a weight or two threaded on your harness.

In salt water, you'll be MORE buoyant... Meaning to get the same effect, you'll need a little more weight. Empty AL80 tanks are now +4 empty rather than +3, and your body and suit are 3% more buoyant. I have to add six pounds when I go from freshwater to salt, without changing anything else.

Here's how I did mine, and I'm diving the same way that you are, most of the time: Wet, 3mil, single AL80, bp/wing: I use a 6lb SS plate and a Koplin 1 pound "light" STA in freshwater. I add a three pound weight to each side of my harness when I dive salt in the same configuration. Conversely, you could wear a weight belt with a three-pounder on each side in salt water. It's up to you on how you do it.

 

[FredT]

I wrote this a while back to explain how to deal with this byouancy and gear selection issue.
Hope it helps.
---------------------------

The answer to buoyancy issues is simply to be neutral in the water at all times. In other words you want to displace as much mass of water as you mass. Regrettably this is much simpler to say than to do.

Your buoyancy has several components. Some of the buoyancy components change as the dive progresses and you descend and ascend in the water column. The BC has to offset only these CHANGES in the buoyancy of you and your gear from the start of the dive at the surface throughout the dive and back to the surface. The best way to get a feel for what these are is to list them by type.

The first thing to do is determine your "residual" buoyancy. Fat along with lung and sinus cavities float, muscle and bone skink. The mix of those components in YOUR body will determine your residual buoyancy. Almost all women and a large percentage of men float to some extent. Women who don't have at least _some_ fat on them simply don't look like women, and the ladies tend to have smaller bone and muscle structure. As a result most ladies will float to some extent. Men are all over the spectrum. Take you and swimming suit to a pool and determine how much lead or air it takes to make you neutral with lungs inflated to the middle to middle top range of your normal tidal volume. This is your "base load" that everything else you take into the water either adds to or detracts from. Also please be aware that this base load changes slowly over time. As an example, 30+ years ago when I was in engineering school I was diving one or more tanks every day to catch lunch, measured 6' tall, weighed 135# soaking wet, with a base load of about 5 pounds negative in seawater. Now 30+ years and about 100 pounds later I'm still 6' tall, but the base load has changed to about 7 pounds positive in seawater. I lost much of the big "jet fin" leg muscles, and added fat everywhere, so my base load reflects that change

The next biggest non-reversing item in your buoyancy calculation increases in buoyancy throughout the dive. This is your decreasing "gas load", or the mass of the air or other breathing mix in your tank(s) that is consumed at a relatively constant rate throughout the dive, with due consideration to the increased use rate due to the compression of the gas volume inhaled as depth increases. If you are talking just "sport" diving with an 80 cubic foot tank this buoyancy change is in the range of 5 to 6 pounds, assuming you plan on coming up with around 500psi. Note that this change is independent of the tank construction. Tech and deep penetration cave divers with larger gas loads may have up to 50 pound of "expendable" gas in several back gas bottles and decompression stage bottles. You need to determine the maximum amount of gas you intend to use with a particular setup to determine this gas load swing buoyancy for your type of diving. Add that amount of positive buoyancy to your "base load". You'll need to add lead or other negative components to your rig to offset this buoyancy change to remain neutral at the deco end of your dive.

The next biggie effecting BC size for most folks is the changes to the displacement of your exposure suit. This is not a major item for those diving in t-shirts and shorts, or for most of those diving in dry suits. For the rest of us diving in wetsuits this can be pretty drastic. When planning for BC size you need to assume your suit looses ALL buoyancy to suit compression at depth. Closed cell neoprene will not normally go negative due to compression at normal open circuit SCUBA depths, so that side of the swing is not necessary to compute. To determine the maximum possible change in your exposure suit buoyancy you need to determine the near surface buoyancy of the suit. This can be done in a pool with a mesh bag and a weight belt with multiple small weights on it. Take the bag into the pool empty, and then add the suit to the bag underwater, being sure not to trap air in the suit as you put it into the bag. Tie the bag to the end of the belt, and see what amount of lead comes off the bottom with the suit and bag still fully submerged. This is the net positive buoyancy of the suit at the surface. You need to add this amount of negative buoyancy to your base load, since you'll be wearing lead or other negative gear to counteract this at the deco end of your dive.

Other things that can change your base load during the course of the dive. One is the escape of air trapped in your wetsuit suit when you first put it on. Another is the escape of, or development of, methane in the gut. Avoiding chili, bratwurst & kraut, bean soup and other methane producers before diving is the simple solution to that one. One common with "branded" BCs is the trapping of air in the BC outer bag between the BC bladder and the case. This air will work out of the case during the dive, yielding a more negative result at the end of the dive than would otherwise be expected.

Picking the rest of your gear wisely, taking into account your base load and buoyancy "live loads", can minimize the lift required for your BC. In any case you need enough lift to enable you, at the first of your dive with the maximum possible negative "live load", to swim up to the surface from the maximum dive depth POSSIBLE in your area. IOW If the bottom of the wall you dive is at 230' you had better be able to swim up from there, even if you never intend to get that deep! The world occasionally turns brown, so it's best to have the gear setup to handle it. The difference in cost is minimal, but the difference in result can be huge. There have been several instances where people have drowned from not being able to swim their rig to the surface.

All other things being equal choosing the rest of the gear you carry is an exercise in minimizing the amount of lead you need by judicious selection of the rest of your gear. If diving in warm water you may want to select a tank that counteracts your base load so all you need to deal with by BC flotation is your minimal exposure suit changes and changing gas load. This could be as small as a 10 pounds of flotation, or as high as 40 pounds.

I use my BC for other things besides buoyancy compensation quite often. This is NOT suggested by most instructors, but then again most instructors have never tried to turn an over 7' long speared AJ heading to the bottom in 800+' of water... They have their agenda; I have mine.

FT

 

[scubafool]

Good writeup, fredt Think I understand the whole weighting thing just a little bit better now. Posts like yours are the reason I keep returning to this board again & again. All newbies such as myself should check out this board on a regular basis. And if you're not a newbie, post your opinions & experiences. We love the free info! Thanks all, from a grateful beginner.

 

[GoBlue!]

OK, so I know I need 5# to make "me+suit" neutral at the surface. Let's estimate that'll be 8# in salt-water (rough guess...no ocean nearby). So, if I used a FredT 11g large backplate (6#) + light STA (2#) + harness (1#?) and an AL80, which seems to be about +4# at 500psi if I read the websites right, that would put me at +12# buoyant & 9# equipment weight, thereby needing 3# as extra weight.

So, at the beginning of the dive, I'd have all the same characteristics except the tank would be 2# weight instead of 4# buoyant. I'd be carrying 6# net weight at the beginning of the dive (volume of the tank) with 3# ditchable.

Correct so far?

Ok, now if I used an aluminum plate (2#) but the same STA+harness (3#) with an AL80, at 500psi I'd need 7# on the belt (7+5 offsets the 8# from suit + 4# from empty tank). I'll still obviously be 6# heavy at the beginning of the dive (since the only difference is weight of gas from beginning to end), but now I'm with 7# ditchable weight.

Have I done these calculations correctly? Seems like I'd rather go with the SS plate with this setup.

Jim

 

[SeaJay]

Doesn't sound right to me.

If you and the suit needs 5 pounds in freshwater (let's keep it "fresh" at first) and intend to use an AL80, then you'll need -8 pounds to get neutral (AL80's in fresh, empty, are +3).

That, to me, sounds like a 6lb FredT and a 2lb STA; no ditchable necessary.

Add 3% of your total weight (with all of your gear on) for salt water. Most people are around 200 lbs with all of their gear on (empty tank), and so most have to add 6 lbs to their gear to have the same characteristics in salt water as they do in fresh with the above rig.

So... You'd add six somewhere. I find that six on a belt or on the harness is ideal.

The most common AL80's (not all of them are the same) swing from -3 to +3 in freshwater, and -2 to +4 in salt.