2 more upper keys dive fatalities, 8/6/2011

[MXGratefulDiver]

Personally, I dive with 12L steel tanks (about 102 cu ft at 3500 PSI).... with a 1mm full, empty BC, and 6 lbs, I'm about 4lbs negative at the beginning and near neutral with 500 PSI.

Hmmm ... I'm thinking that should be about a six-lb buoyancy swing ...

... Bob (Grateful Diver)

 

[Tortuga68]

no desire to begin yet another DIR/anti-DIR thread -- they have some stuff dead-on and other stuff is somewhat silly, yet none of them ever die even with the whacked out stuff they do...

Don't know where you got that from - nobody is immune to mistakes & disasters

 

[MXGratefulDiver]

This is one area where the DIR folks have it right.... (FYI -- no desire to begin yet another DIR/anti-DIR thread -- they have some stuff dead-on and other stuff is somewhat silly, yet none of them ever die even with the whacked out stuff they do, so go from there....)

Actually, the WKPP had a fatality just a few weeks ago ...

... Bob (Grateful Diver)

 

[TSandM]

Diving 8-10 lbs negative is a bad idea under ANY circumstances. You should be BARELY negative with a near-empty tank. If that translates to 8-10 lbs negative with a FULL tank -- you need different tanks. I've seen people end up in this situation with AL 100s (worst thing ever invented, IMHO) Usually smaller, muscular people end up so buoyant at the end of a dive with the 100 cu ft blimp on their back that they need that much extra lead.

You are so out of trim in this condition that (forget even safety for a minute) that you end up consuming much more air than needed and your "attitude" in the slipstream is a mess. This is one area where the DIR folks have it right.... (FYI -- no desire to begin yet another DIR/anti-DIR thread -- they have some stuff dead-on and other stuff is somewhat silly, yet none of them ever die even with the whacked out stuff they do, so go from there....)

I really don't understand all of this. What is wrong with starting the dive 8 lbs negative from gas? So long as you can either swim up the weight in the event of a BC failure, have ditchable weight, or have redundant buoyancy, all this means is that you have a lot more reserve gas. If an Al100 is strongly positive at the end of the dive, then you have to wear weight to counter that, which means you can put that weight in a place that will cause the tank to balance properly. Now, I don't LIKE aluminum tanks in cold water, because of the weight penalty, but I don't think they are utterly pernicious.

I frequently dive HP130s for deeper recreational dives, because I like the extra gas reserve. I carry 31 lbs of ballast with those tanks, of which 20 is easily ditchable, and I have redundant buoyancy as well (dry suit). I see nothing wrong with this at all.

 

[DoNotDstrb]

I should have said none of them "hardly" ever die -- but that's all the DIR discussion I care to get into....

Re:

I really don't understand all of this. What is wrong with starting the dive 8 lbs negative from gas?

The more mass you have to carry in order to counter a positive condition, the more mass you have to move around underwater. 8 lbs does not sound like a lot, but over time it adds up and you pay for it in gas consumption and being less "nimble" underwater. Ideally, you would dive different tanks with a dry suit than "naked" in order to be as well trimmed as possible..... The least amount of lead you need to be SLIGHTLY negative with a near-empty tank is the right amount..... the other "variables" (exposure protection, etc) should dictate the "correct" tank for the dive and therefore the weight.....

Maybe saying like this will make better sense: In a perfect world with a perfectly-sized tank for your exposure protection and body build, you would need NO weights at all to achieve that slightly negative (i.e. almost neutral) condition with a near-empty tank. Of course, this statement does not take the safety of ditchable weights into account -- just the physics of the best "trim"..... Adding safety into the equation should make you slightly POSITIVE with a FULL tank and way positive with a near-empty one with NO weights........ so you select a tank that would achieve these characteristics and add a SLIGHT amount of weight (6lbs works for me) to get you back to the negative/full, slightly negative/empty condition..... I have steel 133s, Steel 12L (about 102) in single and a pair as doubles, and a steel 80. In warm water, I wear a 1mm with a single 12L -- this is most of my diving. With heavier exposure protection, the 133 is more appropriate... "naked", the 80 is great. The doubles, I use for tech diving usually in a 3mm or 5mm and carry NO extra weight but DO carry redundant buoyancy in the form of a lift bag/reel. This is what works for ME and my body build.

Re:

Hmmm ... I'm thinking that should be about a six-lb buoyancy swing ...

The 12L european tanks (they call them Dumpies in the UK -about as tall as an AL 63 and 8" thick) are slightly heavier than their 100cu ft american counterparts..... I don't know where the extra pound or so comes from (the neck "meat" around the 25mm valve threads seems thicker than my American 133 with standard 3/4 threads???). The swing is about 4-5 lbs depending on how much air is exactly in them when "full" ---- Ocean Divers in KL seems incapable of achieving 3500 PSI for some reason..... 3300-ish is what I usually get from them.

 

[Splitlip]

I really don't understand all of this. What is wrong with starting the dive 8 lbs negative from gas?

A lot of us infered something else from DD's first post (which he later clarified).

"the diver WILL need to carry an extra 8-10 lbs of lead (over and above what PADI would say to use in their recommendations)"

 

[Blue Sparkle]

The more mass you have to carry in order to counter a positive condition, the more mass you have to move around underwater. 8 lbs does not sound like a lot, but over time it adds up and you pay for it in gas consumption and being less "nimble" underwater.

I don't understand this, but I'm fairly new at it so there is probably something obvious I am missing. What I don't see is how you have to carry "more mass" underwater with an aluminum tank.

Okay, so say you have an HP100 vs. an AL100.

100 cu. feet of gas will swing about 7.5# between full and empty, so that seems the same for both tanks. In other words, with either tank you will end the dive (presuming you use all/most of your air) about 7# lighter than when you started.

Where I see the difference is that you can use some of the negativity of the steel tank to provide all or part of that 7.5#. But whether that weight is in a steel tank or strapped on a belt, you are carrying it underwater, right?

The HP 100 (PST) starts out at -8.8, whereas the Luxfer starts out at -4.34; so doesn't that just mean that you can "use" about 4.4# of the steel tank instead of other added weight (say on a belt)? So you are still carrying it, but just "built in"?

(I think an AL tank will get a bit "butt light" as you use gas, but then you can use some of the added weight to trim with, right?)

One place I *can* see a huge advantage is on land. The HP100 weighs 34.1# (empty) as compared to the AL100's 40.86# (empty). So, if you need to add an extra 4-1/2# of weight-belt ballast, plus you are carrying an extra 6-1/2# of tank weight, that is an additional 11# on your back on land. That I get. I also see that the tank is physically larger. But I'm not sure I understand that it makes you "move more mass" underwater.

Okay, I'm ready to be embarrassed by the obvious factor I overlooked :blush:

(Not only do I not have very many dives, but they have all been in warm water. I can certainly see why people -- especially those who dive in cold water -- prefer steel tanks.)
34.1

 

[boulderjohn]

A lot of us infered something else from DD's first post (which he later clarified).

"the diver WILL need to carry an extra 8-10 lbs of lead (over and above what PADI would say to use in their recommendations)"

I don't think Lynne was talking about that post. I think she was talking about this one:

Diving 8-10 lbs negative is a bad idea under ANY circumstances. You should be BARELY negative with a near-empty tank. If that translates to 8-10 lbs negative with a FULL tank -- you need different tanks.

I don't understand it either.

If I am barely negative with near empty tanks at the end of a dive, and I plan to breathe through 8-10 pounds of gas during the dive, how can I dive unless I am 8-10 pounds overweighted at the beginning of the dive?

Let's say I am diving my steel 108 doubles and plan to use about 12 pounds of gas during the dive. What can I do differently so that I am not at least 12 pounds negative at the beginning of the dive? How can switching tanks help? If I am going to use a certain amount of gas, I am going to lose that weight of gas during the dive, no matter what kind of tank that gas may be in.

 

[TSandM]

DoNotDstrb, none of what you are writing applies very well at all to cold water diving. Were I to try to find a tank that would make me slightly negative when it was empty, you could park a schoolbus in it . . . and I'm not sure I even agree that the ideal tank is one that leaves you slightly negative when it is empty, in warm water, because if the trim characteristics of said tank are bad, you might WANT to have to carry some weight to balance it out.

But I've dived an HP100 with a 3 mil suit, and it works great. No ditchable weight at all, which doesn't bother me. 6 1/2 lbs negative to start the dive, which is manageable. And they balance perfectly.

 

[Splitlip]

I don't think Lynne was talking about that post. I think she was talking about this one:



I don't understand it either.

If I am barely negative with near empty tanks at the end of a dive, and I plan to breathe through 8-10 pounds of gas during the dive, how can I dive unless I am 8-10 pounds overweighted at the beginning of the dive?

Let's say I am diving my steel 108 doubles and plan to use about 12 pounds of gas during the dive. What can I do differently so that I am not at least 12 pounds negative at the beginning of the dive? How can switching tanks help? If I am going to use a certain amount of gas, I am going to lose that weight of gas during the dive, no matter what kind of tank that gas may be in.

Ahhh. Well I always start a dive ~10# negative. ~7# is gas in my 100CF and a couple or three helps me keep my sausage upright when I hang with a near empty cylinder.