Using "fuller" tanks.

[Storker]

this Scubapro cylinder is *almost* the "preferred" cylinder I spec-ed. Problem is, it is too heavy, relatively. Because it weighs 26.0# (rather than the 21# that I spec-ed), its buoyancy is -2.3# empty (rather than the neutral empty buoyancy that I spec-ed).

I guess it depends on how much exposure protection you need. For us who dive in cold water, tank weight only matters when you're hauling it into the shop for a fill, lifting it out of the trunk or stowing it in the boat, off the rig. When it's rigged, any weight savings will have to be compensated with more lead, either on the rig or on the belt.

Even when I was using my almost-25-kg beast of a 15x300, I had to have some weight in my weight pockets or on my belt. Why should I pay big €€€ for a light tank made from unobtainium when my rig would weigh the same anyway?



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[rx7diver]

I guess it depends ...

Yes, I spec-ed my "preferred" cylinder for me and the type of diving I do these days: rec diving only, fresh water only, sometimes diving dry, sometimes diving wet (3/2 O'Neill).

As an example, with adjustment I can (and do) safely dive my 2,250+10% steel 72 whether I'm diving wet or dry. However, I don't believe I can safely dive (and I don't dive) my 3,500 PST HP 120 when I'm wearing my 3/2 O'Neill.

And, as I've written on here before, when I dive my 3,500 PST HP 80 when wearing my 3/2 O'Neill, I make sure to never forget how negatively buoyant this little tank is. The fact is, with this tank pumped full, I cannot make myself positively buoyant at the surface without putting air into my BC (whether my Scubapro Classic Stab Jacket or my Halcyon Pioneer 27)—a state recommended *against* for any rec diver!

The cylinder I spec-ed above will satisfy me for the type of diving I do.

Safe Diving,

rx7diver

 

[Storker]

with this tank pumped full, I cannot make myself positively buoyant at the surface without putting air into my BC (whether my Scubapro Classic Stab Jacket or my Halcyon Pioneer 27)—a state recommended *against* for any rec diver!

I'm not sure I'm following you. Are you saying that any rec diver should be positively buoyant with a full tank and an empty BCD, or that you didn't have any ditchable weight in that config, and no ditchable weight is recommended against for any rec diver?

I first read you as the former, but that just didn't make any sense. So thinking again I could read you as the latter. But IMO it wasn't crystal clear, at least to me.


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[rx7diver]

I'm not sure I'm following you ...

Yes, in this configuration in fresh water, even if I ditch my (albeit modestly weighted) weight belt at the surface, I cannot make myself positively buoyant at the surface without putting air into my Scubapro Classic Stab Jacket BC. Note: This wasn't the case when I dressed in my full 1/4" farmer John. But, significantly, it is the case when I dress in my 3/2 O'Neill.

 

[Eric Sedletzky]

If it was "fantasium", you would expect it...want it, to be either dead neutral at the end of the dive, or slightly positive.
Purposely diving tanks which remain negative means far more reliance on elevator technology of the BC or wing--which is a convolution in thinking.
The ultimate gets us to lower drag when swimming, no need at all for a wing or BC because no buoyancy shift occurs and we are using exposure suits with slick ultra low drag surface material, and with a non-buoyant insulated fabric combined with electrically heated underwear layer ( such as Thermalution Heated Undersuit -70M (Shortsleeve) ) or even more evolved wetsuit undergarment.

None of us should really want a bc or wing--it just slows you down and adds huge bulk to the kit.
So the next order of business, is finding a method of compensating for changes in tank buoyancy as it goes from full to near empty.
This could be chemical reaction --on the order of how a sperm whale changes buoyancy near the end if it's dive by CO2 changing the buoyancy of the wax like substance in it's large head. We could have a layer of "something"...unobtanium, that can line the exposure suit, and as the scuba tank loses pressure throughout the dive, the suit can get just slightly negative..... Best would be NO stinking BC!!!

I've been working on a few prototypes of hollow plates made out of carbon to address the very issue of compensating for the depletion of gas throughout the dive and how to balance this out. I have ideas of having an airspace in the plate that slowly floods as the dive goes on keeping up with perfect neutrality of the gas used. This would be enough for diving in warm water with minimal to no exposure protection. The only issue beyond that would be compensating for suit compression, but in addition to a compensating backplate, I'm also working on a bc vest that goes under your wetsuit to do the same thing a wing does now. Basically it would be something like a drysuit concept with added gas to stop suit squeeze but the side advantage is added buoyancy. With a BC device inside your wetsuit you could keep the slick streamlining of a wetsuit without sharply affecting water flow like these modern jacket BC's do. I don't think it would take much, and if a BC is used correctly it is primarily used only at depth to compensate for getting heavy, not as an elevator or surface buoying device to stay on the surface.

 

[rx7diver]

In the spirit of what good thread doesn't include an anecdote and an out-of-focus jpeg, consider the attached photo:



The two yellow, rigged cylinders are OMS Faber 2,400+10% 46's (OD = 5.5"). For perspective, the two grey/silver cylinders are, respectively, a PST HP 3,500 120 (OD = 7.25", height without valve = ~28") and a PST 2,250+10% 72 (OD = 6.9"; height without valve = ~25").

For the past three summers I've given my young daughters scuba lessons in my parents' backyard pool during our summer visit. Previously we've used a rented Luxfer 63 or a rented Luxfer 80 (both are positive when empty). My girls are older and stronger and, significantly, more buoyant now, so this year I've decided to use these OMS 46's (neutral, rather than positive, when empty) during our upcoming vacation. I just so happen to be configuring things this weekend.

The rigged scuba presents such a tiny, elegant scuba package, no? Now imagine if the yellow cylinders were only slightly wider (6" OD rather than 5.5") and slightly taller (i.e., as tall as the PST 72), but lighter in weight than the PST 72 (21# rather than ~28# iirc), and with a capacity of ~80 cu ft rather than the 72's 71.2 cu ft.

Now *this* would be something I would happily pay a bit more for! My back-of-the-envelop calculations above seem to suggest that such a tiny, though necessarily high-pressure (probably > 3,500 psig, though probably < 4,500 pig = 300 Br) cylinder is possible without too much pain. Maybe.

This final photo is from three summers ago. In it one of my twins is wearing a rented Al 63:



Safe Diving,

rx7diver

 

[Eric Sedletzky]

In the spirit of what good thread doesn't include an anecdote and an out-of-focus jpeg, consider the attached photo:

View attachment 210066

The two yellow, rigged cylinders are OMS Faber 2,400+10% 46's (OD = 5.5"). For perspective, the two grey/silver cylinders are, respectively, a PST HP 3,500 120 (OD = 7.25", height without valve = ~28") and a PST 2,250+10% 72 (OD = 6.9"; height without valve = ~25").

For the past three summers I've given my young daughters scuba lessons in my parents' backyard pool during our summer visit. Previously we've used a rented Luxfer 63 or a rented Luxfer 80 (both are positive when empty). My girls are older and stronger and, significantly, more buoyant now, so this year I've decided to use these OMS 46's (neutral, rather than positive, when empty) during our upcoming vacation. I just so happen to be configuring things this weekend.

The rigged scuba presents such a tiny, elegant scuba package, no? Now imagine if the yellow cylinders were only slightly wider (6" OD rather than 5.5") and slightly taller (i.e., as tall as the PST 72), but lighter in weight than the PST 72 (21# rather than ~28# iirc), and with a capacity of ~80 cu ft rather than the 72's 71.2 cu ft.

Now *this* would be something I would happily pay a bit more for! My back-of-the-envelop calculations above seem to suggest that such a tiny, though necessarily high-pressure (probably > 3,500 psig, though probably < 4,500 pig = 300 Br) cylinder is possible without too much pain. Maybe.

This final photo is from three summers ago. In it one of my twins is wearing a rented Al 63:

View attachment 210068

Safe Diving,

rx7diver

The HP 80 3442 steel is 7.25" dia. and pretty short . I've heard them called "bowling balls". In my opinion they are kind of a fugly little thing, too short and too wide. The only thing worse would be an 8" tank of similar height which I don't know if anybody ever made.
a longer skinnier version like you said would be a very elegant solution and a streamlined thing of beauty for the right petite person.
I had a pair of Heiser doubles for a while kicking around the shop. They were free to me. They were 200 bar with a doubles manifold that had DIN inserts, one post on one valve, a second post I the middle with it's own valve, and a "J" lever over on the right side. The first hydro stamp was 1983.
they were small, probably around 65 cubic feet each. I never got the purpose of those tanks and never dived them. I was going to split them up and find valves for them and a kid could use them, but apparently they were some sort of metric thread because a regular 3/4 valve would thread about half way down then stop.
long story short the bottoms were cut off and they became yard art bells.

 

[rx7diver]

The HP 80 3442 steel is 7.25" dia. and pretty short . I've heard them called "bowling balls". In my opinion they are kind of a fugly little thing, too short and too wide.

Eric,

I think you're thinking of the HP 65. The first "Sherwood Genesis HP 3,500 psig" cylinders introduced were the HP 65, the HP 80, and the HP 100, iirc. (The HP 120 wasn't available right away, I don't recall.)

Here's a pic of the HP 80's with some other cylinders. It's an old pic I uploaded a long time ago to a thread pertaining to "baby doubles." The cylinders, from left to right, are: HP 80's, OMS 46's (the ones pictured earlier in this current thread, my first deco cylinders), Luxfer 40's, Luxfer 80's, and HP 120's. No, I don't think the HP 80 is an ugly cylinder at all. It really does seem "jewel-like" to me!




Safe Diving,

rx7diver

 

[danvolker]

Our opinions differ on that, I prefer to be ever so slightly negative at 50 bar. Although I rarely get that low. In anycase the buoyancy can be generally controlled by breathing and or just a touch of gas in the BC/wing.

I did think about the possibilities of changing tank buoyancy - but that would require a larger tank and something else to go wrong. And in reality what benefit will it achieve?

I suppose you could start streamlining gear - give surfaces the plate technology replicated from sharkskin and used in the banned sporting swim suits - but given generally in scuba we try not to move too fast - what gain would streamlining achieve in the real world?

All kind of a sideline to the original post of how to get more gas for similar sized tanks?

Tuna....barricuda....sharks...even groupers and most reef fish, move effortlessly through the water....They appear to have benefited from having the ability to move quickly from A to B, when the need or desire arises....Evolution accomplished this, so there had to be extreme benefits to speed.

The typical diver with high lift BC, is more like an inflated pufferfish, except the diver has no real predation to eliminate the slowest and least able to find food.

Spearfisherman, because they "hunt", tend toward freedive fins and slicker gear configs. Typically they swim more than twice as fast as your normal dive instructor or advanced OW diver. If the spearfisherman switches to hunting with a video camera, they will continue to HUNT more effectively by occaisional use of speed and efficient motion, getting from point A to point B.... The reality is that every inch of reef bottom is not equal...there are often great collections of life...like an Oasis, and they can be 50 feet apart, or hundreds of yards apart....the diver with efficient propulsion, that can reach speeds several times higher than the "pufferfish" divers, will be far more effective at hunting things to shoot video of...or to enjoy seeing. Obviously scooter divers will agree with this range issue and the visual opportunities they get with scooters.

The big "but", is that with seriously negative tanks, and the need for a bulky high drag BC, it takes a massive amount of power to move through the water at anything much over the slow pace of 1 mph. The largest source of this horrific drag, on a diver wearing a wetsuit, is the huge BC or wing they use to counter the negative buoyancy issues they face with the tank --and with the wetsuit...The wetsuit buoyancy changes are easy enough to fix with the new high tech insulating fabric alternatives, that don't change buoyancy. This leaves the tank...

With the right carbon composite freedive fins and low drag gear and exposure suit, a scuba diver can swim as fast for an hour long dive to 95 feet or so, as scooter divers---and see everything they get to see. I did this yesterday on the outer fingers reef of Boynton, with three divers on Gavin Scooters. In fact, I had to wait for them occasionally.
And whenever we would see something we were looking for, we would stop....

Personally I think too many divers are like a 30 foot yacht, with a 2 horsepower motor.....they might look good, but they can't really get anywhere, especially if the current is going against them. This discussion of tanks is relevant, because a small tank with 80 to 120 cu feet of gas, with very low drag, and no need for a BC due to no buoyant changes, will allow the average diver to swim over twice as fast as they could with the pufferfish style jacket BC ( or work half as hard, and have the tank last much longer) .

See the far more efficient movement through the water --from our dive yesterday
[video=youtube;s1kc13CBOGA]https://www.youtube.com/watch?v=s1kc13CBOGA&[/video]

 

[Patoux01]

I'm also working on a bc vest that goes under your wetsuit to do the same thing a wing does now. Basically it would be something like a drysuit concept with added gas to stop suit squeeze but the side advantage is added buoyancy. With a BC device inside your wetsuit you could keep the slick streamlining of a wetsuit without sharply affecting water flow like these modern jacket BC's do. I don't think it would take much, and if a BC is used correctly it is primarily used only at depth to compensate for getting heavy, not as an elevator or surface buoying device to stay on the surface.

I feel like that already exists. SWISSUB - Produits