Negative Buoyancy Tanks

[ScubyDoo]

As a newbie just learning the intricacies of scuba, there is much I really do not understand, so I need the advice of those who do. I'd like to get my own tank so I can go on dives at local lakes at my leisure. As a new diver, I'm somewhat concerned about buoyancy on tanks which are getting low on pressure. I have heard about tanks which are neutral buoyancy when empty. I would think this type of tank would be great since this would be one less thing to worry about when ascending. Granted, they are heavier, but they will hold a bit more pressure.....3300 PSI as opposed to 3000. I would like to know what everyones thoughts are on a beginner getting this type of tank as opposed to just getting a standard AL80. I hope this isnt a really stupid question.

 

[TwoBitTxn]

Scuby,

Around here there are no stupid questions.

You can buy what you want. If that is what you want then pick it up. Keep this in mind. One valuble resource that you should keep is your log book. In your entries you should write what equipment you wore, exposure protection, and weighting for future reference. If you go somewhere and do not carry your tank your weighting will change.

Otherwise, if you gots the $$ get what you want and enjoy it.

Come on south and dive with us sometime.

Tom

 

[buff]

I think it is generally a good idea to incorporate a percentage of your total ballast into your equipment. Buying negatively bouyant tanks is a good start.

One concern I have is the percentage of weight that is not "ditchable" that a diver carries.

It is pleasant to dive with weight that is "integrated" into your equipment-e.g. a negative tank, heavy backplate, weight bolted to backplate. But a basic principle of diving(at least when I took my open water in '77) was that your weight was to be ditchable.

I can see carrying a certain percentage of weight that is not ditchable but the quest to eliminate all ditchable weight in order to create a more "streamlined profile" is unsound. The point of a weight belt or integrated weights is that they are not only ballast but are ditchable.

I would keep this in mind as you set up your equipment arrangement.

I wish I would have bought steel tanks. Maybe if I would have not been afraid to "ask a stupid question" I would have.

 

[Walter]

It is a myth that steel tanks are heavier than aluminum. While there are exceptions a steel tank is usually lighter than an aluminum tank of similar volume. Forget about PSI as a measurement of which tank holds more air. Let's take 3 "80's" for an example, The Luxfer AL 80 which is full at 3000 PSI, the Pressed Steel 80 which is full at 3500 PSI and the Taylor-Wharton 80 which is full at 2400 PSI. They all hold about the same amount of air (the Luxfer holds slightly less 78.2 cu ft) at different pressures. The Luxfer is the only aluminum of the three, yet it is the heaviest of the three at 35.2 lbs. The Taylor-Wharton weighs in at 34 lbs while the Pressed Steel is the light weight at 27 lbs. The Luxfer is 2.2 lbs positive when empty, the Taylor-Wharton is 1.89 lbs negative and the Pressed Steel is 1 lb negative.

I agree it's ideal to be neutrally buoyant with a nearly empty tank and some weight should be ditchable, but 1 - 2 lbs of nonditchable weight is not, IMHO, an issue.

DSSW,

WWW™

 

[Lost Yooper]

I can't see anything wrong with Walters post other than relavance to a certain degree. How much a tank weighs on land is irrelavant (to a degree) as well as a tanks buoyancy empty (to a degree). The big deal is the buoyancy of a full tank.

PST HP 80 full is about -9lbs
PST LP 80 full is about -7lbs
Luxfer LP 80 full is about -4lbs

None of these, as singles, should effect your ability to swim up since additional weight would probably be required to offset a wetsuit. It is critical that you be able to swim your gear up from depth with an empty BC to simulate a BC failure at the worst possible time (full tank at depth -- 130' is a good test). You should be able to do this by ditching some amount of weight and kicking up until your suit and the air in your BC take over.
Where people have gotten in trouble is doubling up on LP & HP steel tanks and using thin wetsuits. Of course, they don't need a weight belt and therefore have no ditchable weight. Most people won't be able to swim up full double steel tanks with a relatively thin wetsuit (which is paper thin at depth). There has been more than a few people who have croaked by doing this. I realize this doesn't necessarily pertain to you now, but that's part of the reasoning on the subject.
All of this may be important to you in setting up a dive equipment plan, in which you buy equipment now that will work for future diving (i.e. tech diving if you're so inclined). If you're a tropical wetsuit diver, then an Al 80 may be the best choice now, and if you ever get the itch for advanced diving, get another Al80 to make doubles. If you're a cold water diver, then a LP tank (I'm not a huge fan of HP tanks) may be the best choice because you'll probably get a dry suit someday with which double LP steels can be safely used.

Take care.

Mike

PS. You're not likely to hear advice regarding future dive equipment plans from a local dive shop. The more you advance and update your gear, the bigger the smirk on their face and rightfully so, I guess.

 

[Walter]

Lost Yooper,

Concerning relevance, my post was in response to:

"Granted, they are heavier, but they will hold a bit more pressure.....3300 PSI as opposed to 3000."

Merely dispelling the myth.

I agree with the concept of being able to swim up with a deflated BC.

DSSW,

WWW™

 

[buff]

I agree with the Yooper. The BC failure with wetsuit is the expample I was thinking about but I didn't have the energy to write it all out.

 

[Lost Yooper]

Absolutely Walter.

My duh.

Mike

 

[watergal]

Hi, ScoobyDoo, and welcome.

I have never seen a stupid question. Now some of the responses, that's a different story. :tease:

I wonder if we all didn't jump the gun a bit. In re-reading your post I wonder if you were asking not about to steel, but about the "neutral" aluminum 80cf tanks, which would fit your description being 3300 psi and heavier (at least the Catalina).

If that is what you are considering, I am not sure that would be helpful for the type of diving you are talking about. The ascent issues you are discussing relate to the change in in-water weight from beginning to end of the dive, not whether the tank by itself is positive or negative. Ascent issues involve the combined inwater weight of the diver, weight belt, gear, tank etc, so having a tank be near neutral doesn't matter if you are strapped to it with other stuff.

I if I were choosing between a "standard" AL80 and a "neutral" one for myself in a cold lake, I would choose the standard one. As described, there is no benefit to ascent control, and there are some drawbacks. If you are going to put gear on/take it off in the water, somewhat positive is better. Hopefully you will not have an empty tank in the water, so your "neutral" one will actually be quite negative at the beginning of the dive and somewhat negative at the end. That can be a nuisance sometimes (ie if it slips out of a band). And again, since it is really negative in use, not neutral, you have those "swimming it up in case of emergency" issues previously discussed.

I think I have heard an authority talk about "dangerously buoyant AL80s" (...let's see, where was that... ), but that is when they are doubled and in salt water--not an issue for you at this time.

Having said that, I wholeheartedly agree with the previous posters that if I were buying tanks to use as a single scuba cylinder in for cold, fresh water diving, I would get steel. The in water weight characteristics are better as described previously and having more weight on your back is more comfortable and gives you better position in the water.


Warhammers page http://user.intop.net/~greggg/tanks.html has a lot of tank specs.
I have used theis one http://www.sportdiverhq.com/tkcht.htm and found it accurate.

As always, just my 2¢. Buy whatever you want, but definitely do have a great time diving!

As you've noticed, we like to talk, so if we still didn't address what you needed, let us know what we can ramble on about next!

 

[foothillsdiver]

Hey Scubydoo,

A couple of questions that new divers alwasys ask:

How much weight do I need ?
what thickness wetsuit do I need to buy ?

Now that everyone has stopped laughing.

The tank issue is one that falls into the same trap.

Answer >>>>>> I don't know.

most divers start with Al80s because of cost, availibility and that's what everyone else is using. My suggestion is to see if your LDS (or a buddy that has a LOT of EQ ) has different tanks that you can try out. Don't buy till you try.
In reference to the ascent, the tank should not make any difference in a normal acsent provided you are properly weighted. An Al80 will be slightly lighter at the end of a dive and "properly weighted" means you have taken that into account with your weight system.
I personally use steel tanks whenever possible. My favorite being a steel 72. I will sometimes use larger tanks just for the convenience of carrying just one instead of two. Different types of diving may require the use of larger and or multiple tanks, just remember that once you find the kind of diving that appeals to you to read all you can about it and seek out the proper training and practice practice practice.

BTW I gradgeeated Hs just up the raod from you . Go JAX Red Devils.
Do you frequent the Dive Shop in North LR ?
Mike McCrory was the owner. If you see him tell him Reese says hello


Happy diving
foothillsdiver


OH, I am going diving tomorrow. How much weight do I need ??