"proper" weighting vs Rock bottom

[mislav]

The way weighting was explained to me is this..."you completely DEflate your BCD. and you should have enough weight to make you float at eye level in the water". You do that, and the weight should be perfect. I use this method and have never had a problem with over or underweight.

I agree with this if you add the words "with an empty tank" somewhere in the equation.

This makes me part of the 'neutral at the surface with an empty tank' camp. Never had a problem with this method.

 

[spectrum]

Well first the weighting.. IMO you need to in control all of the way to the surface and you want to be able to make a slow and orderly ascent as well as get back down if needed. 500 PSI is often mentioned simply because it's the "back on the boat" resort version of gas planning and it represents the lightest tank a diver will commonly have. I have never checked my buoyancy at 15 feet but I'm sure that I have a some air in my BC at that depth. I am able to creep to the surface without excessive lung volume manipulations.

I use the bobbing plumb at eye level, with an average breath, your legs still, 500 PSI in your cylinder and an empty BC method of weight evaluation. Very deep inhales and exhales should sink and surface your entire mask and then some.

For perspective you need to include exposure protection. If I were in trunks or even a lightweight wetsuit I could be neutral at 15 feet and control myself all the way to the surface. In a heavier wetsuit or drysuit the buoyancy increase in the lfinal 15 feet will overtake your lung volume control and up you go! So you can see that the who, what and where of this one is very different.

Regarding rock bottom you need to consider the range of dives. Rock bottom is fine and dandy on a deeper dive. Let's say you are making a shore dive, you head out, explore some ledges, and you or your buddy reaches a turn pressure and you start making your way back. You are back in the cove in 15 feet of water it's bright warm and critters are everywhere and you each have 750 PSI in your cylinders. In that situation what's wrong with exploring the cove until you are down to 500 PSI or even a little lower? You have an alternate air source well within reach and baring entanglement your bigest risk is a slight surface swim.

The right plan for the right dive.

Pete

 

[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.


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?

I guess more or less what I was after waws, I see people talk about proper weighting. I see the same people talk about rock bottom. They seem to talk about them exclusively and not how they work together. For example, if they are discussing proper weighting they will tell someone to weight themselves for neutral at the surface with 500psi or they will never be able to make a controlled ascent past the safety stop. Unless there was a OOA you would have more air in your tank than that and would not be too light. Likewise, if you weight that way and there is an OOA then you actually need a bit more weight because you are going to be aproaching 0psi and your buddy is already at 0psi. That means you are both light by about 2 pounds each and ascending as a single unit (except for those long hose users).
I, personally don't have any problem with my weighting or using RB. What was getting me was the way I see others talk about the 2 subjects.

Joe

 

[TSandM]

Pete, even the strict concept of RB would allow you to explore the cove until you were at 500 psi. RB is always calculated relative to your depth; if you plan a dive to 100 feet, you do your gas planning based on a reserve for 100 feet. But that doesn't mean you have to get out of the water with that much gas in your tank. Should you ascend to a shallower depth before you have hit RB, you are permitted to use gas to the RB value that corresponds with that depth. Planned consumption below 500 psi is discourged because of the inaccuracy of gauges in the extreme ranges.

 

[lamont]

generally i start with being able to descend from 0 fsw with 500 psi in my tanks. then i go diving and make sure that when i'm low on gas and at 5-10 fsw that i'm still comfortably in control -- best thing to do here is a skills dive where you wind up trying to do tasks (OOA, bottle drills, etc) at 10-20 fsw midwater with not a lot of gas left... i threw on some more layers last week and did a skills dive to see what i needed to do to my weighting and wound up throwing on 3# because i had to shake and dance at 20 feet to get comfortable enough to hold stops...

 

[Sideband]

There isn't much in the way of shore diving in these parts. I'm in wreck territory!
I wish I had the $$ to go where I could shore dive. Here, you hit your turn pressure and you start up the line.

Joe

 

[PacketSniffer]

<snip>So I then, tried some new DUI undies and I was waaaaay underweighted.<snip>

Yeh, those 400g Thinsulate undies can eat some lead. I don't dive them much but was out in the North Channel Islands and Catalina Island recently and used them. I was really surprised by how much lead I needed.

I also prefer to be slightly heavy to avoid the underweighted anxiety. Being underweighted is not a cool feeling.

 

[spectrum]

Pete, even the strict concept of RB would allow you to explore the cove until you were at 500 psi. RB is always calculated relative to your depth; if you plan a dive to 100 feet, you do your gas planning based on a reserve for 100 feet. But that doesn't mean you have to get out of the water with that much gas in your tank. Should you ascend to a shallower depth before you have hit RB, you are permitted to use gas to the RB value that corresponds with that depth. Planned consumption below 500 psi is discourged because of the inaccuracy of gauges in the extreme ranges.

I know that. My point was to the origanal posters question of why you would still be out in the water with only 500 PSI.

I agree with your planned consumprion below 500 OSI comment to the extent that you need that gas. Teasing it below 500 in the shallows where there the burn rate is slow and egress is readilly available is a reasonable thing to many. We could also get into the relative cubic foot contents of different cylinders at a given pressure.

Pete

 

[Blackwood]

t
it is also taught in recreational diving that as you ascend, you should purge or release air from your bcd,

It's also often taught that as you do your initial descent, you should purge all your air (hose up and extended) and that you should add it again when you get close to your target depth.

This can cause "dropping like a rock" and mislead people into thinking that they're overweighted when they may, in fact, be UNDERweighted.

[Negative descent = wrong]

Don't purge it all, just purge enough to make a reasonable, controlled descent.

[Neutral descent = right]


[[IMO]]

 

[Melicertes]

[Neutral descent = right]

I'm confused. Neutral descent? :huh:
The phrase "contradiction in terms" comes to mind here for me [IMHO]