Article: Are you Diving with the Right Tank?

[ScubaJohn65]

Sheri,

Your article neglected to mention a significant downside of diving with PST, Worthington, and Faber (and other) high pressure steel cylinders. A recreational, open water diver should be weighted such that, with a full cylinder and a completely empty BCD, he or she should be able to establish *positive* buoyancy at the surface by simply dropping his or her weight belt. However, when full, these high pressure steel cylinders are so negatively buoyant that a diver just might not be able to do this—especially if he or she is wearing a thin, or no, wetsuit.

And, of course, this has obvious implications for recovering a non-responsive diver from depth, too.

Note: This is less of a concern when using the old steel USD (PST) 72's (71.2 cu ft at 2,250+10%=2,475 psig)—especially when diving in salt water.

Safe Diving,

rx7diver

Propering weighting for neutral buoyancy is near empty tank, empty BCD, holding a full breath, you should float at eye level. Reasoning is that you will still have enough lead to maintain neutral buoyancy for a safety stop at the end of your dive and in an out of air emergency, you can establish positive buoyancy at the surface simply by adding one-breath of air to your BCD.

All things being MOL equal, you will use about 5 lbs less weight in your weight system (belt or integrated BCD) using a steel tank instead of an aluminum. As rx7diver pointed out, diving in warm water with little or no exposure suit, you may not need even 5 lbs of lead when diving an aluminum tank. In 80+ water temp, I dive using a 2mm shorty with an aluminum tank and only 2# of lead (lbs not kg). 10 years ago, and about 30 lbs of table muscle ago, I could dive a 3mm full suit without any lead when using an aluminum tank. I would have been a rock on the bottom with a steel tank.

In CA, I dive a drysuit, Stainless steel BP/WG with a steel 100. I use 4 lbs of lead on a weight belt. On colder days when I add my thermal vest in addition to my Bare SB Systems Mid-layer thermal, I might jump up to a 6 lbs belt

 

[rx7diver]

Propering weighting for neutral buoyancy is near empty tank, empty BCD, holding a full breath, you should float at eye level. Reasoning is that you will still have enough lead to maintain neutral buoyancy for a safety stop at the end of your dive and in an out of air emergency, you can establish positive buoyancy at the surface simply by adding one-breath of air to your BCD.

ScubaJohn,

I don't agree. A *recreational* diver needs to be able to establish *positive* buoyancy at the surface in an emergency situation in the event that his or her BCD does not function at the *beginning* of a dive when his or her cylinder is at it's most negative buoyancy (i.e., when it is full), by simply dropping his or her weight belt.

For recreational diving, a safety stop is optional.

Your rule for proper weighting is not sufficiently conservative for recreational diving, imho.

Safe Diving,

rx7diver

 

[SeahorseDeb]

I love my 100hp steel. Drysuit diving mostly in cold water, so when I go to warmer waters and see those AL's lined up I give them them the stink eye and add an extra 4lbs of lead.

 

[bostontechdiver]

Could someone chime in on the steel LP85's that are stamped 2400+. Does the + allow a percentage of "overfill" and how much? How would this compare to a Steel HP85 at 3500?

 

[flyandive]

Could someone chime in on the LP85's that are stamped 2400+ Does the + allow a percentage of "overfill" and how much? How would this compare to a HP85 at 3500?

The + allows 10% or 2640psi. In theory a LP85 at 2400 would have 85 cu.ft, same as an HP85 at its rated pressure (3500 or 3442 depending on who makes it). A LP85 at 2640psi should have about 93.5 cu.ft ([85/2400]*2640=93.5). The reality is no two tanks are the same and the actual volume can vary somewhat significantly between manufactures of tanks. A very important note is that a lot of manufactures consider that 10% in the given volume. A LP95 may only have that 95cu.ft at 2640psi. Meaning at 2400psi they will really have 77cu.ft.

Generally speaking, comparing a similar VOLUME LP tank to a HP tank the LP will be bigger and heavier. Comparing a similar PHYSICAL SIZE LP tank to a HP the HP will hold more gas (at the rated pressure), but is often significantly heavier, but not always. The question that often comes up when the two are debated is what about overfilling? What if that LP is filled to high pressure as is often the case in certain parts of the country? Especially in some of the larger tanks the additional volume can be significant.

 

[eelnoraa]

+ means 10% over fill. In case of a 2400 LP tank, + let you fill it 2640psi, and only at that pressure, you have the air specified to your tank. In you case, LP85, you get 85cf at 2640psi. If you fill it to 2400psi, you will only have 77cf. For HP80, you get 80cf at 3442psi.

The + allows 10% or 2640psi. In theory a LP85 at 2400 would have 85 cu.ft, same as an HP85 at its rated pressure (3500 or 3442 depending on who makes it). A LP85 at 2640 should have about 93.5 cu.ft ([85/2400]*2640=93.5). The reality is no two tanks are the same and the actual volume can vary somewhat significantly between manufactures of tanks.

this is incorrect

 

[flyandive]

+ means 10% over fill. In case of a 2400 LP tank, + let you fill it 2640psi, and only at that pressure, you have the air specified to your tank. In you case, LP85, you get 85cf at 2640psi. If you fill it to 2400psi, you will only have 77cf. For HP80, you get 80cf at 3442psi.



this is incorrect

I corrected my post, my additional point is that sometimes the 10% is included but not always. I was assuming it wasn't. It's not incorrect, just incomplete.

 

[bostontechdiver]

Thank you for the clarification. I am looking for a second tank to go with my steel HP100. The dive shop says I would be better off with a LP85 because I would get more air when it is "over filled". The bottom line is that I think I need consistency between my tanks more than the so called advantages of overfilling a LP steel tank, so I will stick with another steel HP100.

 

[empressdiver]

For a lot of divers, the best bang for the buck is the older steel 72's. Girls can usually get almost the same bottom time as a guy on a larger steel tank. Divers like the lighter weight and smaller size as well as the much lower price.

The original K valve can be swapped out for a 200-bar DIN valve to take both yoke and DIN regs.