Weighting for descent vs bottom

[gfaith]

Beau,

Proper planning involves proper contingency planning. It may sound all nice and clean on the innerwebs when you say that a diver should never end their dive with less than 500psi but that is (a) incredibly naive and (b) does not address the *function* of that 500psi, which is for contingency at the tail end of the dive. If you are setting your ballast for 15ft AND 500# then you are compounding bad advice with bad planning.

R..

@Diver0001 You are incredible tolerant and absolutely correct. Best practice and reality don't always line up perfectly.

 

[mi000ke]

Why wouldn't you just fin up with a normal breath? Why use a larger breath?

Yes, could certainly do that. I tend to take long, deep diaphramatic breaths naturally (again, as a result of years playing a woodwind instrument), so when I say a "larger" breath, for me it is really just a normal breath. When I'm hovering I tend to take sightly shallower (not shallow, just slightly shallower) breaths. If I did my normal breath while hovering I may actually go up a down a couple of feet. My normal breath when relaxed is about 8-10 seconds in, 6-8 seconds out. You can travel a pretty good distance up and down in that amount of time. (Gives me a pretty good SAC rate for a novice, though.) If I take shallower breaths I can keep my depth more under control. Then when it's time to come up I go back to a normal (for me) breath, with a little finning if needed.

 

[mi000ke]

The discussion about planning/weighting for zero air vs 500 psi at end of dive is interesting, and makes sense. What would you all think if next dive (assuming benign conditions - I do most of my diving on Grand Cayman in warm, clear water) I breathe my tank down to about 100 psi at my 15' safety stop (I would inform the DM of my plan) and see if I can still control my ascent? Dumb idea?

 

[Grey Goose]

"What would you all think if next dive ...I breathe my tank down to about 100 psi at my 15' safety stop"

Just go to surface at end of dive, inform buddy/boat what you are doing, empty tank to whatever (e.g., 100psi), exhale, and see what happens (float vs. sink). Add/subtract weights accordingly and retry. This would be the weight you need in the exposure suit you are wearing (if in drysuit, consider how much loft you want at end of dive during SS and repeat).

 

[gfaith]

The discussion about planning/weighting for zero air vs 500 psi at end of dive is interesting, and makes sense. What would you all think if next dive (assuming benign conditions - I do most of my diving on Grand Cayman in warm, clear water) I breathe my tank down to about 100 psi at my 15' safety stop (I would inform the DM of my plan) and see if I can still control my ascent? Dumb idea?

I would say this is not a really good idea SPGs are not accurate in the 100psig range, and if you do empty the tank the DM is not going to be happy nor the dive op.

 

[gfaith]

@mi000ke do you hover @ 15 in a horizontal or vertical position you really should not need much finning to ascend that last couple of feet. All in all, I think you are very close to your appropriate weight for right now. Consider this DMs and instructors generally carry 2 to 4 lbs extra to give to divers that might be under weighted. If possible you might ask the DM if you could hand him a 2 lb weight at the end of the SS as a test.

 

[archer1960]

I use the neutral-with-empty-bc-at-end-of-dive-with-empty-tank method like others mention here, but I use 6' as my target depth because of all the shore diving I do. We typically start swimming in 4-5' going out, and swim back in to there coming in. So I want to be able to stay down at 6' without needing to fin myself down.

 

[Storker]

I want to be able to stay down at 6' without needing to fin myself down.

Also, and particularly if you're diving deep-ish and close to your NDL in cold water, it's very nice to do a slow, controlled ascent even through those last couple of meters. At least I feel a lot better post-dive if I do that rather than corking after the safety stop.

 

[ScubaJill]

I am pretty familiar with the issues involved in proper weighting – I actually constructed a spreadsheet for myself that computes my net buoyancy at beginning, midpoint, and end of dive at 0, 33, 66 and 99’ (accounting for suit thickness, compression, air consumption, net tank weight, body buoyancy, and equipment). ...

So (finally) my questions are, should we under-weight and work really hard (or use an anchor line) to get ourselves down? Is it supposed to require some work to get down? And for those divers who are able to do an easy descent, are they actually overweighed? Or is my situation that unique?

Weight is a function of the dive plan, not just an ideal number. Bob

I appreciate the thoughtfulness of your questions - I have something similar rattling around in my head. While I agree with the other posters to weigh yourself with an empty tank (and soaked wetsuit), the questions about how weighting affects your dives at different depths and with different exposure suits, and what you should really be accouting for, are important, too. I would like to know what is the standard for being overweighted? Is it a number or is it that it negatively affects your ability to dive? I think these finer points are lost in many discussions about buoyancy and weighting. Also, one thing I've learned is divers responding to questions on this board may have different "typical" diving experiences they are thinking about that may not apply to a situation being asked about (e.g., deep, shallow, ocean, lake, quarry, current, surge, visibility, temperature, rebreathers, steel tanks, aluminum tanks, doubles, etc.). I think @Bob DBF's comment about adjusting your weight for the dive you plan on doing is spot on, once you've figured out the range of weighting you may need on a particular dive.

To date, in my experience (90% of my dives have been ocean dives, warm water wearing bathing suit/shorty/3mm, AL80)
*I need to add weight on the first dive with a "dry" wetsuit or expect to duck dive down.
*If I'm heavy to begin with on descent, I only get heavier the deeper I go. I've had to learn to add in air to my BC at regular intervals as I'm descending, rather than wait until I've reached depth.
*I can usually tell what how much air I have used in my tank based on buoyancy changes.
*I need to add weight for shallower dives with surge conditions.
*Most of the time I surface with more than 1,000 PSI on a dive (aluminum 80), so I need to take that into consideration about how I felt about the weighting on a particular dive. That's what, 2 pounds in an AL80? It's a small amount, but if I've diving with 4 pounds of lead (plus whatever weight from my regulator, fins, camera, etc.) and I felt fine in terms of buoyancy, should I actually be adding 2 pounds of lead to account for a lighter tank?

 

[mi000ke]

@mi000ke do you hover @ 15 in a horizontal or vertical position you really should not need much finning to ascend that last couple of feet. All in all, I think you are very close to your appropriate weight for right now. Consider this DMs and instructors generally carry 2 to 4 lbs extra to give to divers that might be under weighted. If possible you might ask the DM if you could hand him a 2 lb weight at the end of the SS as a test.

I usually hover horizontal, and my finning is very minimal - and usually none for the last 5' or so.

Just go to surface at end of dive, inform buddy/boat what you are doing, empty tank to whatever (e.g., 100psi), exhale, and see what happens (float vs. sink). Add/subtract weights accordingly and retry. This would be the weight you need in the exposure suit you are wearing (if in drysuit, consider how much loft you want at end of dive during SS and repeat).

Makes sense, thanks. And have the DM there to help add/sub weight as you both suggest.