Back Inflate Air Distribution

[divinh]

I have a Sopras Sub/Tek Compact Lite travel BC. It's a BP/W style, though with a soft, light backplate. It has trim weight pockets on both sides of the cam band.

Two issues...

1. Initially, I tried to see if there was the face-forward issue on the surface and indeed, there was. If I inflated the donut bladder, it would push my face into the water. After some reading here on scubaboard, I have countered the motion by finning a bit and leaning back. It's worked well and I am no longer worried about the issue. Further, once on my back, I can stay in that position without finning, just resting.

2. For underwater, currently, the weights are on either side of the cam band and below, around my waist via weight belt. This tends to make me vertical instead of horizontal. When I add air into the BC, the natural tendency is for the air to fill at the top of the donut bladder first. This further makes me more vertical, as air is at the top of the bladder and all my weights are below that point. So, my question is, underwater, what do I do to make the BC tend toward making me horizontal, which is the benefit of back inflate? Do I immediately dip my head lower to get the air to shift toward the bottom of the bladder?

 

[oly5050user]

I have a Sopras Sub/Tek Compact Lite travel BC. It's a BP/W style, though with a soft, light backplate. It has trim weight pockets on both sides of the cam band.

Two issues...

1. Initially, I tried to see if there was the face-forward issue on the surface and indeed, there was. If I inflated the donut bladder, it would push my face into the water. After some reading here on scubaboard, I have countered the motion by finning a bit and leaning back. It's worked well and I am no longer worried about the issue. Further, once on my back, I can stay in that position without finning, just resting.

2. For underwater, currently, the weights are on either side of the cam band and below, around my waist via weight belt. This tends to make me vertical instead of horizontal. When I add air into the BC, the natural tendency is for the air to fill at the top of the donut bladder first. This further makes me more vertical, as air is at the top of the bladder and all my weights are below that point. So, my question is, underwater, what do I do to make the BC tend toward making me horizontal, which is the benefit of back inflate? Do I immediately dip my head lower to get the air to shift toward the bottom of the bladder?

Sounds like you have too much lead on you. How much lead you have on, wet suit or dry suit, how thick a wet suit..did you do a proper weight check?

 

[divinh]

Sounds like you have too much lead on you. How much lead you have on, wet suit or dry suit, how thick a wet suit..did you do a proper weight check?

3/2 mm full wetsuit, 3 lbs. in each of the trim weight pockets and 3 lbs. on each side of my hips, so 12 lbs. total.

I did a quick 2 lbs. in each of the trim weight pockets (4 lbs. total) the first time I was testing and floated at eye level without air in the BC... though I should retest that. I'm not familiar enough with weight checks. My understanding is to use a full tank, empty BC and add enough weight to float at eye level, then redo with an empty tank to see the difference and split the difference? (I just did a search and some pages say to use a fully inflated BC. Huh. This appears to be the most comprehensive? Buoyancy Calculator: How to Figure Out How Much Weight You Need for Scuba Diving ?)

Even with proper weighting, the air tends to the top of the BC bladder. How do I compensate for that?

 

[runsongas]

depends on the tank too. for an al80, full tank/empty bc/exhale to check you sink and inhale to check you come back up, then add 4 pounds. or use a near empty al80 and just use enough weight to sink when exhaling and rise when inhaling.

 

[rsingler]

Even with proper weighting, the air tends to the top of the BC bladder. How do I compensate for that?

If you start your descent vertical, then the first burst of air will indeed float to the top of the bladder. But if you fold at the knees and let your fins rotate you to the horizontal position as you start to descend, the first bit of air will collect at the bladder's highest point. Check your bladder to determine its shape when horizontal. Many wings, for example, are rounded at the top, or trapezoidal overall, so that in the horizontal position, there is more flotation at the bottom of the bladder than at the top. If you start with a vertical tendency, then you'll indeed need to roll forward to redistribute a small bubble of air. But as you add more air to compensate for wetsuit compression, the air distributes more evenly, and flotation becomes bladder shape dependent.
Then other things become important - fin flotation, ankle weights?, trim weight position, tank position.
The Sopras has just a hint of rounding at the top, with minimal extra volume at the bottom. That means that with a modest amount of gas in it, it will be a horizontal bladder. Therefore, tank position will be a key initial determinant of trim. As pointed out above, your weight belt may work against you, though floaty fins like the Scubapro Seawing Nova can help counteract that. A couple of 2# trim weights removed from your belt and taped to your shoulder straps are another thought. Lots of options.
This is where spending some pool time completely motionless will give you tips about where to shift your weight. It's possible that certain tank combinations with certain bladder shapes are non-starters for trim. Hence, our endless discussions here on SB about equipment configuration and personal preference.

 

[Jim Lapenta]

I would also recommend getting with someone to do a proper weight check. It sounds like your initial instruction was sorely lacking. I checked the link to the way to calculate your buoyancy and was pleasantly surprised to see how it was written.

That's how I have my OW students check their weight at the beginning and end of the first pool session and continue to fine tune it in subsequent sessions. Much better than the BS advice to use 5,10, etc % of body weight. But it doesn't go far enough because it doesn't talk about distribution of the lead.
There is no substitute for actually getting in the water and not only checking for the correct amount but also where that lead should go.

If you need more over your lungs, put it in the back cam band pockets. Take some off the belt. With 12 lbs of lead I would say in an emergency you'd only need to drop 2-4 to be nice and positive on the surface. I'd take 4's and put them in the cam bands and 2's on the belt. Get the lead distributed correctly and it will help underwater and on the surface.

Also, work on horizontal descents and ascents. On the descent, once your head goes under the water get horizontal and float down in that position like a leaf or snowflake. On ascent, horizontal until the last 10-15 feet if there's the possibility of boat traffic, then get vertical to look around.

 

[divinh]

If you start your descent vertical, then the first burst of air will indeed float to the top of the bladder. But if you fold at the knees and let your fins rotate you to the horizontal position as you start to descend, the first bit of air will collect at the bladder's highest point. Check your bladder to determine its shape when horizontal. Many wings, for example, are rounded at the top, or trapezoidal overall, so that in the horizontal position, there is more flotation at the bottom of the bladder than at the top. If you start with a vertical tendency, then you'll indeed need to roll forward to redistribute a small bubble of air. But as you add more air to compensate for wetsuit compression, the air distributes more evenly, and flotation becomes bladder shape dependent.
Then other things become important - fin flotation, ankle weights?, trim weight position, tank position.
The Sopras has just a hint of rounding at the top, with minimal extra volume at the bottom. That means that with a modest amount of gas in it, it will be a horizontal bladder. Therefore, tank position will be a key initial determinant of trim. As pointed out above, your weight belt may work against you, though floaty fins like the Scubapro Seawing Nova can help counteract that. A couple of 2# trim weights removed from your belt and taped to your shoulder straps are another thought. Lots of options.
This is where spending some pool time completely motionless will give you tips about where to shift your weight. It's possible that certain tank combinations with certain bladder shapes are non-starters for trim. Hence, our endless discussions here on SB about equipment configuration and personal preference.

My impression of jacket BC's is that the bladder is tapered such that more air goes to the waist. When I use a jacket, I have lead only around my waist and it feels like the lead and bladder are evenly counteracting each other. My center pivot seems to be around my waist.

I'm new to BP/W. All of my readings have said that it will force you into the horizontal position and with the way it behaves on the surface, forcing my face forward, it does seem to be doing that... but I do notice that the air is concentrated at the top of the donut bladder at first, which says to me that it's not going to force me into full horizontal on its own. I would need to compensate by pointing down or going horizontal first to get the air to shift around.

The fins are slightly negatively buoyant in the pool, probably positively buoyant in salt water.

The trim weight pockets are about 1/3 up from the bottom of the BC.

Yes, I'm spending time in a pool to sort this out. I wish I could do some testing tonight with the feedback I'm getting, but I'm limited to Saturdays.

 

[divinh]

I would also recommend getting with someone to do a proper weight check. It sounds like your initial instruction was sorely lacking. I checked the link to the way to calculate your buoyancy and was pleasantly surprised to see how it was written.

That's how I have my OW students check their weight at the beginning and end of the first pool session and continue to fine tune it in subsequent sessions. Much better than the BS advice to use 5,10, etc % of body weight. But it doesn't go far enough because it doesn't talk about distribution of the lead.
There is no substitute for actually getting in the water and not only checking for the correct amount but also where that lead should go.

If you need more over your lungs, put it in the back cam band pockets. Take some off the belt. With 12 lbs of lead I would say in an emergency you'd only need to drop 2-4 to be nice and positive on the surface. I'd take 4's and put them in the cam bands and 2's on the belt. Get the lead distributed correctly and it will help underwater and on the surface.

Also, work on horizontal descents and ascents. On the descent, once your head goes under the water get horizontal and float down in that position like a leaf or snowflake. On ascent, horizontal until the last 10-15 feet if there's the possibility of boat traffic, then get vertical to look around.

My initial instruction gave me the basics, enough to know more information would be needed.

Yes, it would be nice to have repetitive trials in a pool with the same equipment to refine, but in my experience so far, with rental equipment, most of the time, I only get to encounter said equipment for at most, a week. (Most of the time, it's just a few days.) I do adjust my amount and weight distribution over the course of the week, then it starts all over again at the next dive shop. (Vacation diving.) This is my first experience with my own equipment and calibrating for it.

I have no problem being horizontal for descent and ascent with a jacket. My impression with BP/W is that it would tend toward horizontal without effort on my part (as jackets tend toward vertical without effort on the surface), but it seems I do need to treat it similarly to a jacket, go horizontal as soon as possible.

 

[divinh]

I finally got pool time today...

I first started with just the 3/2mm full wetsuit and did inhale/exhale to see my buoyancy without any other equipment. I found that a full lung kept my head above the water and an exhale dropped me to eye level. Holding only 2 lbs. made me sink instantly. From this, my guess is that I'm fairly neutrally buoyant with just the 3/2mm full wetsuit.

Instead of an AL80, I was given a steel tank, which I was told would be 5 lbs. (difference?) I skipped using a weight belt. I found that I could easily reach neutral buoyancy and skim just inches above the bottom with ease. I did have to put some air into the BC to compensate for the tank. I could definitely feel the tank pull downward when doing a roll. The steel tank made it very easy to stay on my back on the surface, as it pulled me back instead of face-forward.

From some reading, a steel is about 13 lbs. negative and aluminum 1.5 lbs. negative when full. Does this mean that if I want a similar starting point, I would need 11.5 lbs. in the cam band trim pockets? I also read that the aluminum will become 3 lbs. positive toward the end of a typical dive. Would this be what I would put on a weight belt? I don't plan to put 11.5 lbs. in the trim pockets for aluminum, as the steel tank was definitely negative and needed some air in the BC, but it does sound like I need to put quite a bit to have a similar experience as with the steel tank.

My next test was seeing how buoyant the BC was by itself. It was positive with 2 lbs. At 4 lbs. it sank. It seems to need about 3 lbs. of weight to be neutral?

The final test was 3/2mm full wetsuit and fins. The result was about the same as with just the wetsuit.

3/2mm full wetsuit - neutral
BC - 3 lbs. positive

steel tank, no weights - easy to reach neutral but had to add a bit of air
aluminum tank - what should my weighting be?

Then what to do in salt water?