Should I buy this HP120 doubles setup?

[tbone1004]

if the hp100 and hp120's are both PST 3500psi tanks, they have identical buoyancy characteristics fwiw.

HP120 is only 9.6lbs of air, so if you are diving a balanced rig it shouldn't be that bad depending on how much wetsuit you are wearing. Part of the reason a lot of GUE recommendations are for AL80's has nothing to do with their buoyancy characteristics, but more to do with the fact that they limit the amount of air you can carry so instead of 9.6lbs+ whatever you lose from your wetsuit, it is only 6lbs+ whatever you lose from your wetsuit.

You can't dump the air in your tank quick enough for it to matter, but yes your math is approximately correct at 22lbs. With fins you should be able to swim it up, but at the surface it might be a bit difficult to keep it there depending on how good your surface sculling techniques are especially with those blade fins which aren't designed for that type of work. Always dive with redundant buoyancy if you are remotely concerned about not being able to surface with the rig. In the ocean that means dsmb, in a cave it usually means small ish lift bag.

 

[stuartv]

if the hp100 and hp120's are both PST 3500psi tanks, they have identical buoyancy characteristics fwiw.

The HP100 is a 3442 tank. An E7-100. The chart I found says it's -1 #, empty.

The HP120s are Sherwoods, I gather. They are 3500 psi tanks. The chart says they are -1.3 #, empty.

So, yeah, I figured I would dive them using the same amount of ballast, no matter which tank I use.

And, back to Dan's point, as long as I can swim them up, it's not really so important how long I can tread them right? Because the worst case scenario is being deep, in a 7mm suit, and having 100% wing failure. In that case, once I swim them up (even part way), my suit will help and, at the surface, assuming proper weighting in the first place, my suit will provide its full buoyancy, so treading water will be to keep a neutral rig at the surface, not hold -22 #.

For that matter, in that scenario, even an inflatable safety sausage should be enough to assist me in getting to the surface and then staying there. Right?

 

[danvolker]

Dan, thank you for bringing this up. And I do mean that sincerely. I do not know the answer to your question... yet.

All my new-to-me tanks (AL30, HP100, 2 x HP120) are in for Hydro/VIP/O2 right now. When I get them back, I am planning to take the 100 and a 120 to the pool to do some work. I want to sort out exactly what weighting I need and how to distribute it. And I will attempt to determine whether I can swim a 120 up, too.

Once I sort out my weighting with my 7mm suit, what I would do is get in the pool with the full 120, no wetsuit, and the weighting for my 7mm suit. If I can swim that up and tread water to keep myself afloat, without undue difficulty, that should be a reasonable test and make up for being only 12' deep, right?

I'm 48 years old, but I'm 6' 1", 215 #, and in fairly decent shape and I do go to the pool usually 2 or more times per week to swim 1,000 yards in the morning before work. And I have, currently, Atomic Blade fins. I think I can do it.

I was in the pool last weekend doing training as part of my U-Boat Diving course. I wore my 7mm, to try and get an idea on weighting, with an HP100. I had on 4# of weights and it was definitely enough. I think it was actually too much, really. I drained that HP100 down to 55 psi before I got out and I was still able to sink myself. Unfortunately, my class didn't afford me time for experimentation, so I'll just assume 4# as worst case, for now.

From what I have found, the HP120 will be 10.3 # negative, when full. So, I would be attempting to swim up (6 # BP + 4 # weights + 2 # regs + 10 # tank =) 22 #. In real life, I might also have an additional 2 - 5 # of lights and/or reels.

Swimming up 22 # seems like a pretty much worst-case scenario. In reality, my suit won't lose ALL its buoyancy, even at 130', right? If necessary, I can also dump most of my tank air, to decrease my negative buoyancy by at least 8#, too. So, the realistic worst case is probably more like having to swim up 15 # or less. Really, probably 10 #. Right?

And if I determine that it IS too heavy for me, then do I need to sell the tanks? Or would a reasonable alternative be to always dive with an SMB that would give 30# of lift?

Tbone gave you a great answer....and there are many divers that will dive seriously negative tanks, with an SMB on them, so they have redundant lift--just in case.
It sounds like your fitness should allow you the power to swim these tanks up easily, provided the fins can handle your power. Many fins like Scubapro Novas, or twin jets, may fail miserably with this--they cant take the power that a strong diver has....they are really made for weak or relatively sedentary divers. I am not familiar with your fins--test them--see for your self. If they are bending like crazy, and not giving you much thrust, different fins are in order. ...Whether Scubapro Jet fins, Hollis F1's, or high tech with Force Fin Extra Force or Excellerating Force fins with whiskers( they channel more volume of water into the cupped blade---under high power, it is more like the torque potential of Freedive fins, but without the long length). Of course, carbon composite freedive fins can develop vastly more torque for a strong athletic diver than is possible with Jets or F1's--but they are not intelligent for significant penetrations into shipwrecks--it is too easy to hit the ceiling with the tips, and reverse kicking ( backing up) long freedive fins, tires you out much faster than you would like

I myself, would not consider using tanks I could not swim up, and the presence of an SMB is not relevant to me for lift. Tech divers have died on deep air with heavy tanks...on the bottom, when their BC could not get them up, and the SMB got away from them. It's just not the right solution. Lighter tanks would be the solution.

New divers get SOLD big heavy and huge tanks, with the idea that this will allow them to stay down as long as the highly experienced divers, that are using Al80's....Instead of gaining experience, and working on SAC rate and Trim, the heavy tanks allow the new diver to be deeper, longer....but they can also help PREVENT this new diver from the learning curve they ought to be allowed to try, and the better technique that could make the Al 80 be just fine for them. For the money of the 120's, you could take a Freediving course, get spectacular in the water, drop your Sac rate to .27 or so....and then the 80's would be plenty....and cheap.

And if we are talking tech and deep....and if you cant do an ocean dive with double 80's...maybe there are other shallower sites more practical to do!

---------- Post added April 7th, 2015 at 02:06 PM ----------

The HP100 is a 3442 tank. An E7-100. The chart I found says it's -1 #, empty.

The HP120s are Sherwoods, I gather. They are 3500 psi tanks. The chart says they are -1.3 #, empty.

So, yeah, I figured I would dive them using the same amount of ballast, no matter which tank I use.

And, back to Dan's point, as long as I can swim them up, it's not really so important how long I can tread them right? Because the worst case scenario is being deep, in a 7mm suit, and having 100% wing failure. In that case, once I swim them up (even part way), my suit will help and, at the surface, assuming proper weighting in the first place, my suit will provide its full buoyancy, so treading water will be to keep a neutral rig at the surface, not hold -22 #.

For that matter, in that scenario, even an inflatable safety sausage should be enough to assist me in getting to the surface and then staying there. Right?

True...and if you were really getting tired, you could always ditch the heavy tank and wing...or....you have a snorkel attached to a chest strap or in a pocket, and you pop it on, and stay horizontal on the surface, swimming to keep a "plane" up...this could allow you to carry a great deal of weight...Swimming vertical is very wasteful of energy, and hard to get your head far out of the water without a great deal of excess lift.

 

[jcaplins]

If I may point out... DanVolker is in south Florida and probably dives with little to no exposure protection. An HP120 would probably be too much for a balanced rig, for him. (yeah, I know, I'm making some assumptions.)

I think you would have no problems while wearing a 7mm. The hp120 will not be that much different than the hp100.
If you needed 4 pounds in freshwater, you may need 0 to 4 pounds with the 120 in freshwater. That is your worst case scenario. Head out to the ocean and you'll need more lead. If you can ditch that lead in an emergency, even better.

I've dove a 5mm wetsuit with HP120's and still needed lead. 14lbs. But I did't have a SS backplate. I'm 6', 185 lbs, I don't swim for exercise, and I could easily be neutral at 120' with little air in the wing, control my safety stop, and swim everything up easily. If I dropped the lead, I would have been bobbing along the surface. (I was using a 23lbs wing and in photos it looked empty)

I seriously doubt you will have any problems with the tanks. If you ever plan to do tech diving and double the tanks back up, you are pretty much guaranteeing you will be over weighted and will need redundant lift (double bladder wing, drysuit, lift bag, or whatever...). Until then, enjoy!

 

[stuartv]

If you ever plan to do tech diving and double the tanks back up, you are pretty much guaranteeing you will be over weighted and will need redundant lift (double bladder wing, drysuit, lift bag, or whatever...). Until then, enjoy!

Yep. My long-term plan is to get through Adv Nitrox and Deco, so I can go where I want to go, see what I want to see, and stay longer while I'm there! They'll go back together as doubles then.

I completed TDI Nitrox this past weekend. Got a 99.9% on the Final. I missed the very last box on the last problem, for calculating CNS % on a repetitive dive. Somehow I read 21% off the table and the instructor said the correct answer was 23%.

When I finish my current U-boat Diving course, I will have all the prerequisites for starting TDI Adv Nitrox and Deco. Well, except for having the money to pay for the course.... So, probably next year. The rest of this year, I'll be happy to just rack up more Rec dives and experience. Using a wreck reel to run a guide line in the pool on Saturday already showed me very clearly how much more practice I need on my buoyancy....

 

[danvolker]

I dive with a 2.5 mm freedive wetsuit....it provides essentially zero lift at 100 feet, maybe 2 to 3 pounds of positive lift on the surface if any... Because my cycling training( bicycle ) is part of my dive training, and I use big carbon composite freedive fins, I can swim the 120 up without wing lift, with very little exertion. I only need the wing so that I can be neutral when swimming close to the bottom, or so that I don't waste energy on an hour long dive, lifting the tank the whole time--being dead neutral is much better for your SAC rate

The Fin selection is the X factor, assuming fitness.

 

[TSandM]

Again, to go back to a question that not many have answered . . . when we were taking our tanks into shops and leaving them for fills, I neatly lettered my name in calligraphy in permanent marker on the tank shoulder. That way it was obvious that they were mine, and it satisfied my desire to have the identification easily seen in the dive shop, but not obtrusive in photographs underwater.

 

[Leadking]

I am completely ignorant on tank tech. Assume the singles valves are Sherwood. Assume they've been sitting in a box in the seller's garage for a few years.

Am I likely to need new seats?

What is the seat? A piece on the bottom of the valve that contacts the top of the cylinder?

Is there some way I can look at them myself and tell whether they are good, bad, or maybe?

Is it as simple and obvious as I'm thinking it would be (given my ignorance)? Look at the bottom of the singles valve and make sure it looks flat and smooth, with no chips/gouges/dings, so it will mate correctly to the top of the cylinder?

These are7/8" valves and no longer made in that thread configuration. I believe the valve supplied to PST when these tanks were made were Thermo, but may have been changed out.

 

[tbone1004]

I think Lee is correct on Thermo supplying to Genesis. Call John at Northeast Scuba, he should have the parts for the singles valves in stock, those are still available, it's the Sherwood Manifold parts that aren't available, such a shame too, that manifold is properly brilliant.

 

[stuartv]

I think Lee is correct on Thermo supplying to Genesis. Call John at Northeast Scuba, he should have the parts for the singles valves in stock, those are still available, it's the Sherwood Manifold parts that aren't available, such a shame too, that manifold is properly brilliant.

AFAIK, I am good to go. The tanks were setup as doubles when I bought them, so I have a fully functional (and even new-looking!) doubles manifold setup. And the guy gave me the original singles valves, too, which also looked like new. I took off the doubles manifold/valves and tank bands and put the singles valves in myself.

Now the tanks are at my LDS and I'm just waiting to hear that they passed hydro and VIP and I'll be ready to get 'em wet.