BC doesn't have enough lift

[Mark Vlahos]

Has anyone ever run into a situation where an XS bc doesn't have enough lift to float a diver using and AL80 tank wearing a 3mm wetsuit and no weights? I haven't seen the actuality of it but hoping to this weekend.

Here is my scenario...

I was undergoing training recently and was at Blue Grotto in Williston, FL. I was wearing a SS Backplate with double steel hp120's, Al80 stage bottle, Al30 stage bottle, 7mm wetsuit, and no extra lead. With the bladder (50 lbs of lift) full I was ever so slightly heavy, when I would inhale I would become neutral. I am on the platform at 20 feet in fresh water, and I am to simulate a failure and I need to swim to the surface with the bladder completely deflated. For the purpose of instruction I was to leave the stage bottles in place and swim up and when at the surface I was to inflate the bladder manually.

This was an EXCELLENT training tool and demonstrated the need for redundant lift when dealing with this type of technical gear, but trust me, this is not the kind of situation I would "hope" to see. It SUCKED! It was really hard work just to get to the surface, fortunately I was allowed to keep a working regulator in my mouth for the entire time. I am a big guy and was exhausted when I was finished, I would not want to see a small diver (implied in your example above) have to lift a smaller, but proportional, amount of weight to the surface.

For me as a training exercise it was hard, but not dangerous, remember I was only SIMULATING a failure, in a real and unexpected situation it could turn ugly really quickly.

Mark Vlahos

 

[Sideband]

Mark,
I'm new and I'm trying to figure out where all the "trouble" is with buoyancy. I'm in no way saying what you said is untrue. I just don't understand it.

Your backplate and steel 120's are about what, 23 - 25 pounds negative? Maybe 3 to 5 lbs for other gear? (not including stage bottles and such.) I'd think that you would be just a bit negative and your 50lb lift wing should be plenty. Where is the other -25 plus lbs coming from?

In my 7mil with an al80 I need about 25 lbs of lead. I'm not a small guy, 5'11" 230lbs, but my suit doesn't have THAT much more rubber in it. Heck, with just the suit and no lead I float with my arm pits out of the water (no bc or tank). I think my BCD has about 35-40lbs of lift. I can't imagine having enough gear that my BCD wouldn't be enough.
In my first pool session I had a terrible time getting down on my knees in the shallow end and they kept giving me 2lb bags. I know being new and breathing deeper and all, but still... if I were to dump my weight belt I'd be on the surface pretty quickly. When I bought my BCD I made sure it had weight pockets so all of my weight wasn't on the belt just in case I had to ditch.
I guess I'll have to try a BP/W doubles set up to see the difference for myself.

Joe

 

[Mark Vlahos]

Mark,
I'm new and I'm trying to figure out where all the "trouble" is with buoyancy. I'm in no way saying what you said is untrue. I just don't understand it.

Your backplate and steel 120's are about what, 23 - 25 pounds negative? Maybe 3 to 5 lbs for other gear? (not including stage bottles and such.) I'd think that you would be just a bit negative and your 50lb lift wing should be plenty. Where is the other -25 plus lbs coming from?

In my 7mil with an al80 I need about 25 lbs of lead. I'm not a small guy, 5'11" 230lbs, but my suit doesn't have THAT much more rubber in it. Heck, with just the suit and no lead I float with my arm pits out of the water (no bc or tank). I think my BCD has about 35-40lbs of lift. I can't imagine having enough gear that my BCD wouldn't be enough.
In my first pool session I had a terrible time getting down on my knees in the shallow end and they kept giving me 2lb bags. I know being new and breathing deeper and all, but still... if I were to dump my weight belt I'd be on the surface pretty quickly. When I bought my BCD I made sure it had weight pockets so all of my weight wasn't on the belt just in case I had to ditch.
I guess I'll have to try a BP/W doubles set up to see the difference for myself.

Joe

Hi Joe,

First, remember that in my case this was a controlled training exercise, and I did have available to me enough lift to maintain bouyancy.

Each steel 120 would be about -18 pounds when full so there is about -36 pounds, then figure in -6 pounds for the backplate, another pound for the bands, the Aluminum 80 when full is worth about -4 pounds, and the Aluminum 30 is another -3. So now we are up to about -49 pounds. Doubles and two stage bottles means 4 regulators, lets say each one is -1.5 pounds, so figure in the -6 pounds, and we are at about -55 pounds. Don't forget the cannister light and at least one backup light, two computers, and other little miscellaneous clips for the lift bag and spool, etc... lets call all this junk -2 pounds. So now we are at -57. The bladder was rated to 50 lbs of lift but I did not do any testing to see how accurate they were in their advertising. So if you add up those numbers we are a little heavy. Yes I was wearing a wetsuit, it is only about a year and a half old but has several dives below 100 feet so it is at least a little compressed, and at this point is probably comperable to a 5mm or slightly better suit, so now we are back closer to even. And if you are not familiar with Blue Grotto, it is fresh water not salt water.

All of the weights above are really rough estimates so don't hold me to them as gospel, and I might have made a simple math error, it is 2AM here as I write this, but that is the general idea.

But remember, all of that aside the main comment I want to make is not what junk I was wearing, but rather that in enough gear I was heavy, and I would not want to see any other diver, like was implied in the original post that started this thread, to find him/herself heavy. It is a really unpleasant position to be in.

Mark Vlahos

 

[Sideband]

Mark,
Thanks for the reply. I didn't realize the 120's were that negative. That all makes sense to me now as you spell it all out. I think I'll stick to straight rec. diving. Besides, it's cheaper.

I understand too what you were saying about not wishing that situation on anyone. I know I wouldn't want to have to struggle to the surface and then have to fight at all to stay there.

Joe

 

[Don Burke]

The bladder was rated to 50 lbs of lift but I did not do any testing to see how accurate they were in their advertising.

My limited fiddling with BCs seems to indicate that BC lift is expressed as the lift with no restrictions on inflation. Put a body in front and a couple of tanks behind, and the lift is quite a bit less.

Since the manufacturer has no idea how you are going to restrict the BC, unrestricted inflation is probably the only way to go for the ratings.

Your exercise demonstrated where the general guidance of "wet with aluminum, dry with steel" comes from. It isn't always exact, but it's normally pretty close.

 

[The Chairman]

Hey Mark...

have you thought of going to an aluminum back plate and/or a drysuit? Your configuration gives me the willy nillies.

 

[Mark Vlahos]

Hey Mark...

have you thought of going to an aluminum back plate and/or a drysuit? Your configuration gives me the willy nillies.

Hi,

I do not own the backplate or the tanks, it was used as part of my Advanced Nitrox, and Decompression Procedures class. If I were to own a backplate it would almost certainly be aluminum, and a tall one. I did have an adequate amount of lift available to me to maintain neutral bouyancy, plus I was able to ditch the stage bottles if I needed to. Again this was a controlled environment and training. I would not want to use this as my normal system. When I went on my deep dive in the ocean I had dual bladders with more than adequate lift. Yes if I find myself doing more technical diving redundant lift with either a dual bladder or more preferably a dry suit is the way to go.

I don't want this thread to dwell on the gear I had on a training dive; I only used it as an example (admittedly an extreme one) about the need for adequate lift. In my case I had to kick to the surface and manually inflate probably -45 pounds, and this was really hard. I was prepared for it, if I was not ready for this situation and it was in an open water environment it would have been, shall we say, extremely unwelcome. A diver that is surprised by an inadequate amount of lift will first notice this the instant they jump in the water and start to sink rapidly, they had better be really good at clearing or they will start to have the serious problems add up and the task loading will only make each problem worse.

Lift is important and as was implied by the post that started this thread, I would not want to go on a dive knowing that I did not have an adequate amount of lift to at the very least maintain neutral bouyancy.

Mark Vlahos

 

[Lead_carrier]

That last sentence was a bit strange....

So is the situation. I was just in the pool today with a 2 mil shorty. With no other gear on I needed 8 pounds of lead to get neutral. So, if an AL80 and all gear is even 5 lbs. negative *I* would still need lead to get neutral, let alone negative and I still wouldn't have any air in my BC.
Acording to Dive Rite, the Luxfer AL80 is -1.4lbs when full.

Something is really off in what you are saying...

Joe

It is a former student that is having the problems. I just got a phone call last week and asked about it because she swears it is happening. I was hoping to be able to get in the water with her this weekend but things didn't work out. Since she leaves on a trip this week it will be a couple of weeks before I can see the whole story. I couldn't believe it myself when she told me but I am very anxious to get in the water with her and see what is happening.

 

[Rick Murchison]

It is a former student that is having the problems. I just got a phone call last week and asked about it because she swears it is happening. I was hoping to be able to get in the water with her this weekend but things didn't work out. Since she leaves on a trip this week it will be a couple of weeks before I can see the whole story. I couldn't believe it myself when she told me but I am very anxious to get in the water with her and see what is happening.

There's some weight hidden in her system somewhere... perhaps it's gold
Rick

 

[Sideband]

It is a former student that is having the problems. I just got a phone call last week and asked about it because she swears it is happening. I was hoping to be able to get in the water with her this weekend but things didn't work out. Since she leaves on a trip this week it will be a couple of weeks before I can see the whole story. I couldn't believe it myself when she told me but I am very anxious to get in the water with her and see what is happening.

BTW,
I didn't mean to imply that I thought you were lying or dumb or anything like that. (not that you took it that way) I just meant there is some info missing someplace like you inferred above. Words sometimes appear colder than intended when reread a few days later. I'll be watching to see what you come up with.

My best guess so far is that she is part avian and has been eating gravel to help with digestion.

Second best is that she has water in her BC and isn't getting full inflation.

Joe