weighting question/buoyancy test

[Splitlip]

I've noted that some advocate doing the "float at eye level test" with a full cylinder (empty BC) while others recommend a near empty cylinder (empty BC). Compensating for weight of the gas usually means 4-6# and can mean as much 9 pounds with a single HP-120.
What is the right way to do it? The eye level test I mean. (I think the best way to do a buoyancy test is with a near empty cylinder at your safety stop depth). You know, like if say you had a brand new BP/W and wanted to get it close before you went out in the open water with a bunch of guys who think your a kook already because you have two perfectly good BC's in the house and now are gonna strap a piece of metal with a tiny little balloon on your back with an "uncomfortable" one piece web harness. You know, hypothetically.

 

[Whatgoesdown]

To avoid being too light at the end of a dive, you should be neutrally buoyant at the end of the dive (on the surface with an empty tank. That said, to ascend from a safety stop breathing out a little more than usual is no biggie so I tend to weight for the safety stop and prepare to completely empty my lungs from there if necessary. To further complicate things, I rarely use more than half a tank on a single dive and if you are properly weighted you'll find you use less air anyway, so don't have to try and perform the safety stop on an empty tank 99 per cent of the time. If you actually plan your gas consumption, you can also plan your weighting accordingly.

 

[scubafool]

I set my weighting with 300 PSI in my tank, completely totally thoroughly empty wing, saturated wetsuit, at ~4' deep. When I exhaled, I sank, when I inhaled, I started to float upwards.

 

[spectrum]

First when to do it....
Ideally at the end of the dive and the reason is stowaway buoyancy. This will vary with your gear and here's an example. My Sherwood Avid BC is a fairly unpadded porous fabric sort of thing. When I come home to rinse gear I put it in the dunk tank, vent any air and that's it. My wife's Diva LX has a lot of soft rubbery material and even places not intended to trap air old air and it seeps from the stitching like an Alka-Seltzer. The first time I saw this I thought her bladder was shot. If she were to do a bouyancy check at the start of the dive and addi more weight for her air depletion she woiuld be carrying weight for phantom byouyancy and be over weighted.

If you have a BP/W with a single ply wing you are in a better position to do it on the front end of the dive but I still feel the last word is at the end of the dive.

As for what to set your weight for.....
First it's all about how to end the dive right and with the least amout of weight. As mentioned you never know your ending air reserve ahead of time so IMO you need to plan for the near worst case, 500 PSI.

If you set it by hanging still plumb, empty BC bladder, at the surface with an average lung volume holding you at eyeball level to the surface at the end of your dive with 500 PSI in the cylinder you should have no problem staying at your stop or even swimming along the bottom to your exit depth on a shore dive. IMO you never want to be so light that there is a point where you will surface against your will. In the final 5 feet you may need to swim yourself down but you can still get down if you need to.

If you rarely get to suck you tank down to 500 PSI you can just do it at the end of the dive and allow .08 LB for each cubic foot of air beyond what 500 PSI represents in your cylinder. At least your gear is saturated, the rest is simply math.

The last paragraph and some of your statements bring us to the "why 500 PSI" question since you with a 120CF cylinder have 50% more gas than many divers but that's another thread.

Pete

I've noted that some advocate doing the "float at eye level test" with a full cylinder (empty BC) while others recommend a near empty cylinder (empty BC). Compensating for weight of the gas usually means 4-6# and can mean as much 9 pounds with a single HP-120.
What is the right way to do it? The eye level test I mean. (I think the best way to do a buoyancy test is with a near empty cylinder at your safety stop depth). You know, like if say you had a brand new BP/W and wanted to get it close before you went out in the open water with a bunch of guys who think your a kook already because you have two perfectly good BC's in the house and now are gonna strap a piece of metal with a tiny little balloon on your back with an "uncomfortable" one piece web harness. You know, hypothetically.

 

[Splitlip]

Thanks for the "phantom buoyancy" info. Makes sense.
Agreed, it should not matter as much with my wing which is bladderless, but answers a lot if questions I had with my other BC's.
I am not diving the HP 120's ..yet. I do dive pumped up LP 98's. And that too belongs in another thread.

First when to do it....
Ideally at the end of the dive and the reason is stowaway buoyancy. This will vary with your gear and here's an example. My Sherwood Avid BC is a fairly unpadded porous fabric sort of thing. When I come hole to rinse gear I put it in the dump tank, vent any air and that's it. My wife's Diva LX has a lot of soft rubbery material and even places not intended to trap air old air and it seeps from the stitching like an Alka-Seltzer. The first time I saw this I thought her bladder was shot. If she were to do a bouyancy check at the start of the dive and addi more weight for her air depletion she woiuld be carrying weight for phantom byouyancy and be over weighted.

If you have a BP/W with a single ply wing you are in a better position to do it on the front end of the dive but I still feel the last word is at the end of the dive.

As for what to set your weight for.....
First it's all about how to end the dive right and with the least amout of weight. As mentioned you never know your ending air reserve ahead of time so IMO you need to plan for the near worst case, 500 PSI.

If you set it by hanging still plumb, empty BC bladder, at the surface with an average lung volume holding you at eyeball level to the surface at the end of your dive with 500 PSI in the cylinder you should have no problem staying at your stop or even swimming along the bottom to your exit depth on a shore dive. IMO you never want to be so light that there is a point where you will surface against your will. In the final 5 feet you may need to swim yourself down but you can still get down if you need to.

If you rarely get to suck you tank down to 500 PSI you can just do it at the end of the dive and allow .08 LB for each cubic foot of air beyond what 500 PSI represents in your cylinder. At least your gear is saturated, the rest is simply math.

The last paragraph and some of your statments bring us to the why 500 PSI question since you with a 120CF cylinder have 50% more gas than many divers but that's another thread.

Pete