"proper" weighting vs Rock bottom

[TSandM]

I learned a lesson the hard way about weighting for neutral at 10 or 15 feet. I had a dive where we had a problem at 100 feet that caused us to stay there longer than we had originally planned, so I ended up at the end of the dive with about 600 psi. We had planned for me to shoot a bag at the beginning of the ascent, and I did. I got to 10 feet with 500 psi in the tank and learned something important -- You can't keep a bag upright on the surface unless you put downward tension on the line. You can't put downward tension on the line unless you can be a little negative. I spent the ten foot stop swimming vigorously downward to try to avoid popping to the surface from keeping tension on the line -- of course, I could have let it get loose, but then you get entangled in it and that's a PITA.

I've had a couple of instructors (DIR instructors, to boot!) tell me that they prefer to dive a couple of pounds overweighted just for this sort of reason.

 

[JeffG]

I learned a lesson the hard way about weighting for neutral at 10 or 15 feet. I had a dive where we had a problem at 100 feet that caused us to stay there longer than we had originally planned, so I ended up at the end of the dive with about 600 psi. We had planned for me to shoot a bag at the beginning of the ascent, and I did. I got to 10 feet with 500 psi in the tank and learned something important -- You can't keep a bag upright on the surface unless you put downward tension on the line. You can't put downward tension on the line unless you can be a little negative. I spent the ten foot stop swimming vigorously downward to try to avoid popping to the surface from keeping tension on the line -- of course, I could have let it get loose, but then you get entangled in it and that's a PITA.

I've had a couple of instructors (DIR instructors, to boot!) tell me that they prefer to dive a couple of pounds overweighted just for this sort of reason.

I did a dive with my double 130's where with my old underwear I never needed any weight. So I then, tried some new DUI undies and I was waaaaay underweighted.

Spent the last part of the dive holding onto the downline to keep me from floating to the surface.

Had to be one of the most uncomfortable/dangerous dives I have ever done, and it all started with 1 bad assumption.

 

[TeddyDiver]

You can be a couple pounds light and still stay down by breathing towards the bottom of your lung capacity. So another possible strategy is to be neutral at 15', 500psi, full lungs; and neutral at 3 or 4', 0psi, and breathing shallow. With a 5mm full wetsuit, these are about the same for me.

IMO, if you can make a controlled ascent to the surface with a near empty tank, then you have enough weight, even if you need to change your breathing pattern a bit for the last 5' of the ascent. With this weighting, you should also be able to redescend by simply exhaling hard, even with a near empty tank.

:wave-smil

 

[jepuskar]

I weigh myself to be slightly negative with 500psi in my tank at the surface. As stated before being neutral at 15ft with 500psi does no good with the amount of pressure change between 15ft to the surface. Between 10ft and the surface is where I am constantly dumping air from my drysuit and ensuring I dont find myself going up to quickly.

At the surface with 500psi I should be able to descend to atleast 5 feet with a nice good exhale with an empty wing and all the air I can get out of my drysuit. Once at 5ft I stay there for a couple breathing cycles to ensure I'm close to neutral..if I am then I know im ok...if I sink really hard at this point I am overweighted.. I'll hand up a pound or two to my buddy and do it all over.

Regarding changing your breathing to compensate for any minor weight issues...that is a valid point and one that is affective. However, I tend to get all worked up or a little stressed if I feel underweighted and need to compensate with my breathing. I would much rather add a pound or two to shy away from this.

J

 

[limeyx]

Things aren't lining up correctly in my head on this. There are several schools of thought on weighting:
Neutral at 15' with 500psi
Neutral at surface with 500psi

Then we have Rock Bottom pressures. Using this you should never be at 15' or the surface with only 500psi unless you had to share air.
This seems to me like it should be easy but every time I hear someone talking about where you need to be neutral they bring up the ease or difficulty of ascending the last 15'.
I personally weight for neutral at 15' at 500psi but am generally at about 750 - 1000psi when I get there (depending on depth).

I don't know what I wanted from this other than to mention it. It could almost go into whine and cheese but it is more scuba related than most of those.

Joe

"Rock bottom" is calculated so in the worst case you actually surface with zero psi not 500. You are free to pad that value, but that's the definition of RB. So neither of your cases above are technically correct.

In practice, you can't get a check with zero PSI, so do the check with 3-500 and add 1-2 pounds.

If your exposure protection changes buoyancy with depth, then you need to be aware that if you are neutral at 15 feet, you may not be able to do as slow an ascent from 15 feet as you'd like, so you have to add more weight to allow for that.

 

[Sideband]

"Rock bottom" is calculated so in the worst case you actually surface with zero psi not 500. You are free to pad that value, but that's the definition of RB. So neither of your cases above are technically correct.

In practice, you can't get a check with zero PSI, so do the check with 3-500 and add 1-2 pounds.

Those aren't "my" cases, but what you are saying was the point of my post though.
Joe

 

[KMD]

You can't keep a bag upright on the surface unless you put downward tension on the line. You can't put downward tension on the line unless you can be a little negative. I spent the ten foot stop swimming vigorously downward to try to avoid popping to the surface from keeping tension on the line -- of course, I could have let it get loose, but then you get entangled in it and that's a PITA.

Of course another method would be to look at your team mate and sign, "YOU - HOLD - THIS"


As RB is calculated with an emergency SAC rate of 2 Cuft/min for two people, under normal ascents I am always surfacing with 400 to 500 psi still in the tank. Under more stressfull situations (but still no air share) I have gone as low as 300.

 

[limeyx]

"Rock bottom" is calculated so in the worst case you actually surface with zero psi not 500. You are free to pad that value, but that's the definition of RB. So neither of your cases above are technically correct.

In practice, you can't get a check with zero PSI, so do the check with 3-500 and add 1-2 pounds.

Those aren't "my" cases, but what you are saying was the point of my post though.
Joe

So your question is answered

- You can indeed have less than 500 psi @ 15 feet, and need to plan for it (whether you use Rock bottom or anything else -- you can always go OOA)
- If your exposure suit expands and gains buoyancy with a decrease in depth, you'd do well to account for it or you might have an undesirably fast ride to the surface.


what else is there?

 

[TSandM]

Of course another method would be to look at your team mate and sign, "YOU - HOLD - THIS"

Didn't Joe just spend three days trying to pound this concept into my head?

 

[JKSteger]

I did a dive with my double 130's where with my old underwear I never needed any weight. So I then, tried some new DUI undies and I was waaaaay underweighted.

Spent the last part of the dive holding onto the downline to keep me from floating to the surface.

Had to be one of the most uncomfortable/dangerous dives I have ever done, and it all started with 1 bad assumption.

I too had something similar happen to me just this past weekend. I dive dry and this time I decided to try layering some polartec outfit under my normal undies. I also tried some real "drysuit socks", etc.
Guess what?
I was underweighted by the end of the dive! I was actually carrying a rock that I picked up on the bottom until I got to the up-line, then at 15' had to hold on to the line or I would not have been able to hold the 15' stop!
I learned a vaulable lesson...better to be a FEW pounds overweighted when diving dry. This would be very important if doing deco dives.
Disclaimer....I said a FEW, not 20 pounds overweighted!!!

Jeremy