Buoyancy with Monster Tanks, and Other Newbie Questions

[Vegan Shark]

I'm looking to purchase my first tank, after having only used rental AL80s so far. Despite reading as many tank guides as I could online, I'm still a little confused. Is it better to go LP over HP if you're not sure you're going to get good fills? And how do you calculate buoyancy?

I have my eyes set on a Saber HP149, or a HP130, since the height is about the same as the AL80s I use, and the weight isn't an issue. Being able to do 2 dives with one tank, or super long shallow dives, sounds like a blessing. The sheer negative buoyancy, and buoyancy changes of these suckers confuses me, though.

The buoyancy for the HP149 is -9.41 lbs full, and 2.35 lbs empty. The HP130 is -11.7lbs full, and -2lbs empty. Right now I am wearing 30lbs of lead with the AL80, so how much weight would I use for each of these tanks? Will a standard BCD with 29lbs of lift be enough to keep me from sinking to my doom?

Also, if a shop/boat will only fill an HP to 3000PSI, would a fully filled LP121 give me more air than the HP tanks?

Thanks for the help!

 

[dumpsterDiver]

The 149 is the same tank as a lp 121. They look the same anyway, I have both. If you use a steel tank, you will take about 6-7 lbs off the weightbelt. Other than that, I love the 149 in the water, but they are heavy and will feel very different than an 80 aluminum.

 

[MaxBottomtime]

Scuba Cylinder Specifications from Tech Diving Limited - 928-855-9400

This will help explain the different sizes and weights of various tanks.

 

[Vegan Shark]

The 149 is the same tank as a lp 121. They look the same anyway, I have both. If you use a steel tank, you will take about 6-7 lbs off the weightbelt. Other than that, I love the 149 in the water, but they are heavy and will feel very different than an 80 aluminum.

Good to know people are out there diving with 149s! I haven't seen or heard of anyone with tanks that big, so thought there must be some catch. So with the 149 you take off about 6 lbs? Wish more stores carried these tanks so I could try them out before buying.

This is probably a stupid question, but if I turn a tank into a nitrox tank, will I still be able to put air into it when I go on boat dives where no nitrox is available? I didn't fully understand all the details about o2 cleaning etc. when I did the nitrox class, since I didn't plan on owning a tank anytime soon.

 

[dumpsterDiver]

Most places use partial pressure blending; so they put about 500 psi of pure oxygen and then add air. Those tanks must be clean (they come clean when new). If you ever want to skip the first step and not put oxygen in and only add air that is fine... fine as long as the air that is put into the tank is the same quality as used to make nitrox fills. If the dive shop makes notrox fills by blending, their air is good. If they don't do it that way, you could theoretically have a problem.

 

[NAM001]

vegan

On those tank bouyancies, make sue that you are gettng the bouyancys WITH THE VALVE. The valve is another 1-3# depending on the valve. A 149 tank is a lot of air. probably best for use as you say for all day shallow dive. You could just as well us a lp95. a true 95 as opposed to the al80 which is i think 77.

 

[neonstingray]

Those are heavy tanks you're talking about. The guiding idea of trying to find a steel tank that's physically similar in size to an AL80 isn't necessarily the best one. The way a tank 'feels' underwater is related much more to its buoyancy characteristics. Having the 10 pounds of tank weight on your back, above your center of gravity, will likely make you feel less stable. Grant it, you're using 30 pounds of lead, so it may not be as noticeable for you.

I know you specifically asked about buying tanks, so please don't take it the wrong way if I suggest that you may want to back up a bit. According to you're profile, you're a newer diver. A tank is usually the last piece of equipment you should buy, and you should make sure you'll continue to do enough diving to justify the cost of maintenance - visual inspections and hydro. Also, air fills usually aren't much cheaper than just renting a filled AL80, so you don't save much there by having your own tank. Furthermore, by diving more and with different shops, you can come across opportunities to try other tanks. Often you'll just have to ask if the shop has a steel tank you could try other than the typical AL80. Grant it if they do though they'll likely just have steel 72's.

You also said you're using 30 pounds of lead. That's a lot of lead, and you'll likely decrease it significantly as you work on your trim and buoyancy. As you do so, your air consumption will improve, and you'll see an increase in your bottom time from that alone.

If part of your motive is being able to do two dives on one tank just because you don't feel like switching tanks, you'll quickly come to find that lugging around heavier tanks becomes more exhausting than the couple minutes it takes to switch out an AL80. It's like strapping a keg of beer to your back so you don't have to walk to the fridge to get another bottle.

 

[Vegan Shark]

Thanks for all the advice

I am a newer diver, but I've been diving weekly since I got certified this year, sometimes three days per week. Really love diving, so I don't mind splurging money on a tank. Plus it's the last thing I need to complete my gear, and having my very own setup sounds really nice. I know my air consumption will get better with time, but I guess my mindset is the more air the merrier. Dunno if I'll be dropping lead anytime soon, I wear a 7mm suit + 7/5 mm hooded vest, and when I tried dropping down to 28 lbs had a lot of trouble holding a safety stop even when holding onto kelp.

I'm not a tall guy, 5'10, so maybe the HP130 would be a better fit since it's 4 inches shorter than the HP149.

 

[KneeDeep]

I was in the same boat as you starting out on gear. Tanks, were actually the 2nd item(s) I bought, as I got a really good deal on 2x 130's. I'm 6' and 180lbs (now) and LOVE my huge tanks. if all you plan on doing is Rec diving... I would get 130's, I can do long shallow dives and do some fare about of time at deeper depths.

But, if you do plan on going into Tech, I would get 119's and they are great doubles.

 

[elan]

If HP149 is 2lb positive when empty you can only take a couple of pounds off your belt as the aluminum is around 4 lb positive. A downside of a big tank is you will have a lot of gas in the beginning thus will be hugely overweight. These big tanks are quite unstable in the water too. I you are willing to go through a trouble of hauling those beasts I would just think about moving towards doubles - Lp72 or lp85s

---------- Post added August 26th, 2013 at 08:35 AM ----------

You mentioned you dive 7+7 wet. Your 29 will most likely be insufficient or very marginal for these big tanks. Those suits lose a lot of buoyancy at depth IIRC around 15-20 lbs and the gas will weight around 10-11 lbs.