Floated Up and Couldn't Get Down

[WetSEAL]

This is a misleading statement that never seems to go away. The tank material has absolutely no bearing on the CHANGE in buoyancy during a dive, which is what we use a BC for. All tanks get more buoyant as gas is used from them. 80 cf of gas used will cause the exact same change in buoyancy whether it comes from an aluminum tank, a steel tank or one made from any other material. How much weight needed to compensate for the tank material does vary depending on the tank material but once that is determined, it remains a constant throughout the dive.

Thank you for this insightful comment. To verify this, I looked at the Faber manufacturer specs, computed the buoyancy delta (the difference between empty and full buoyancy) and divided by tank capacity, and I get almost the same value for all tanks.

I do notice however that there is a slight difference between LP tanks vs HP tanks, however. The buoyancy delta of LP tanks is about 0.072-0.074 lbs/cf, whereas the buoyancy delta of HP tanks is about 0.078-0.079 lbs/cf. I realize this is a minor pedantic difference but might be interesting to tech geeks? Presumably this pressure related differential would increase even further when comparing a LP tank that was not certified for 10% overfill to a HP tank.

 

[MichaelMc]

I do notice however that there is a slight difference between LP tanks vs HP tanks, however. The buoyancy delta of LP tanks is about 0.072-0.074 lbs/cf, whereas the buoyancy delta of HP tanks is about 0.078-0.079 lbs/cf. I realize this is a minor pedantic difference but might be interesting to tech geeks? Presumably this pressure related differential would increase even further when comparing a LP tank that was not certified for 10% overfill to a HP tank.

At higher pressures gases behave less like ideal gases. I do not know if that is the cause of your results, or just volume figures that are off.

Calculating SCUBA Cylinder Capacity | Dive Gear Express®
Edit: Compressibility factor - Wikipedia

 

[frogman63]

Check the s header valve in your BC inflated hose. Had the same problem, followed anchor line to 80 feet as soon as I let go I started to ascend, unable to stay down. After surfacing added weight and same thing again. Repeated the same up to adding 30 lbs of lead. No go..... found the the s header valve in the BC inflator hose had sand in it holding it partially open.
Schafer valve is basically a tire valve stem, you have to have a tool to remove it. Usually a tire valve stem tool does the job.

 

[frogman63]

Check the s header valve in your BC inflated hose. Had the same problem, followed anchor line to 80 feet as soon as I let go I started to ascend, unable to stay down. After surfacing added weight and same thing again. Repeated the same up to adding 30 lbs of lead. No go..... found the the s header valve in the BC inflator hose had sand in it holding it partially open.
Schafer valve is basically a tire valve stem, you have to have a tool to remove it. Usually a tire valve stem tool does the job.

Dang spell check should be schrader valve not s header

 

[OWDiver3]

Looks like a lot of great advice here. I'll definitely have to keep RayfromTX advice in mind for one of my dive buddies... it seems like that may be a likely solution for the lack of buoyancy if its not the tank. Enjoy your trip!

 

[kelemvor]

I acknowledged that in my last line, proper weighting Is necessary regardless of tank material. Swapping between tank materials almost always requires a change in weighting, just as changing tank size does. To be completely accurate, not all AL tanks end up positive and some steels go positive so even saying steel always remains negative and alum always becomes positive is not a true statement. But even if that were true, the fact remains that the swing in buoyancy is the what we use a BC to compensate for (among other things) and that never changes for a given volume of gas no matter what the tank material is.

Interesting. Thanks for the education, insert quip about assumptions here.

 

[Katie L]

Same thing happened to me this spring, at about the same number of dives. It was embarrassing! I thought I was getting good! Popped right up like a cork. I believe the trapped air in the BC was the main factor.

I have noticed that with rental BCs (the only kind I have ever used), which are “unisex” but let’s face it, that means men’s by default, I am often in between sizes. Sometimes when the chest and hip straps fit on land, (usually the larger of two sizes) the body of the jacket is boxy and everything floats up when underwater. The shoulder straps are already pulled tight, the hip strap rides up and becomes loose around the waist and my tank starts floating up towards my head. I think air gets trapped easier at the upper back because the vest isn’t close to the body. The slightly-small size is easier to pull closer to my body and therefore easier to empty.

 

[rongoodman]

That's why they invented crotch straps.

 

[KristenK]

Same thing happened to me this spring, at about the same number of dives. It was embarrassing! I ththought I was getting good! Popped right up like a cork. I believe the trapped air in the BC was the main factor.

I have noticed that with rental BCs (the only kind I have ever used), which are “unisex” but let’s face it, that means men’s by default, I am often in between sizes. Sometimes when the chest and hip straps fit on land, (usually the larger of two sizes) the body of the jacket is boxy and everything floats up when underwater. The shoulder straps are already pulled tight, the hip strap rides up and becomes loose around the waist and my tank starts floating up towards my head. I think air gets trapped easier at the upper back because the vest isn’t close to the body. The slightly-small size is easier to pull closer to my body and therefore easier to empty.

Yes, yes, yes!!! That's exactly my situation. I went with a medium but should have had a small. Never even knew about crotch straps until finding this site. I hope to purchase my own regulator and BCD before Curaçao. And I'd like to get one built for us chicks.

 

[Katie L]

I hope to purchase my own regulator and BCD before Curaçao. And I'd like to get one built for us chicks.

I learned about back-inflate BCDs thanks to this site, or using a backplate and wing instead, so it’s totally adjusted by straps you connect to the plate and has a crotch strap. I would definitely like to compare those with a women’s specific vest style BC to see how they each fit. Seems like the body composition differences between men and women could really lead to a lot of specific buoyancy considerations. I have to read the women’s perspectives part of the site more for tips.