Buoyancy with Monster Tanks, and Other Newbie Questions

[flots am]

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?

While you didn't ask it, I'll mention this anyway:

Most new divers are limited by the amount of gas they carry, and it's difficult to exceed the No-Decompression limit.

If you're dving a 130 or a 150, you're going to have to actually plan your dive or watch your computer to make sure you're not running out of no-deco time.

Diving until your 80 is empty and then ascending isn't particularly elgant, but is probably more-or-less safe. Doing the same thing on a 130 can be dangerous.

flots.

 

[dvankirk]

I love HP100's. They trim out great and can easily be managed as doubles.

 

[CT Sean]

Another thing to consider, while perhaps of lesser importance, is how your huge tank impacts buddy diving. A 149, filled appropriately, is close enough to 2 AL80's, but if you can only get it filled to 3000 (if you statement is true about shops only filing to 3k), then it's a different story. If it is filled correctly but your buddy is using a bigger tank, perhaps a 100, then you've used "too much" of your tank to get a full second dive out of it.

 

[neonstingray]

I love HP100's. They trim out great and can easily be managed as doubles.

I agree. I'd recommend going with something like an HP100 (FX100 by Faber). This is one of the most common tanks for doubles, so if you get into more serious diving you can easily double them up. Doubles have advantages over a single larger tank, but that's a discussion that drifts from your original question.

 

[halocline]

You might consider looking into dealing with your weight situation a bit more before buying a humongous tank. Since you're okay with spending money.......

First consider a steel backplate/wing type of BC. Look into the deep sea supply set ups; they have bolt-on weight plates that are very convenient and can really help trim you out if you're carrying that much lead. You might be able to drop half your weight.

Second, for me personally, if I were using 14mm of neoprene, I'd be dying to get a drysuit. That's a big expense but I have never once heard about anyone regretting buying one. I think between the backplate and the drysuit you'll find a huge improvement in your overall dive comfort.

For a tank, I agree that the HP100s (either faber FX or the XS scuba/worthington) are very versatile with good buoyancy characteristics and a good size for many types of diving. Diving with a huge cylinder on your back sucks and there is some truth to the idea that if you're diving in such a way as to need 140 cft of gas on a single dive, chances are good that it's a technical/deco type dive and you'd want doubles for the redundancy anyhow.

 

[danvolker]

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!

In addition to what the others have said so far....a 120 or 140 will literally "roll" you over if you slow down or try to hover...which you "should" want to be able to do...since this can often happen when you see something really spectacular. Think of your center of gravity as you are in the horizontal swimming postion....
With an 80 cu foot Al, or even double 80's with a bp/wing, there is not much weight of the tanks in water, and very little weight up high enough to have any rolling effect at all...In fact, with Double 80's, due to the light tanks, and the catamaran style effect of the wings--these well actually help to hold you flat--and resist rolling side to side....THEN, consider a lp 120....and there are plenty worse..but consider these---the tank is heavy in water, and larger diameter, and you have alot more weight up high off of your back.....and this has an effect like someone has a lever on you, and they are trying to roll you to one side or the other....if you keep up a decent cruising pace , they actions of your fins will correct for this, and you probably will not notice....but if you stop--to take a picture, to lay some line, to consider a penetration, whatever, all of a sudden your feet have to get busy COUNTERACTING the rolling effect of the big single tank. Even divers swimming slowly with these tanks will feel this happening.

HP 100's dont hurt your trim so much....they are a good compromise if the al 80 is not enough gas.

Even with the double 80's, don't expect the 2 dives on your rig concept to work well..... You end up shortchanging yourself on either the first dive, or the second, and it is really annoying.....now you could buy two sets of double 80's, but then you need to find charter boats that are big and roomy, and that have plenty of room for you to store the 2nd set of doubles.

Choice of Diveboat IS a big deal in diving...one frequently ignored by new divers....there are some very big differences between the experience you would have on one charter versus another...

 

[victorzamora]

I have to agree with most of the above posts. Especially the one about waiting. Also, the one about doubles. Another mention: Sidemount. Two smaller tanks, redundancy, and you can always double them up with a manifold and bands if you decide to go that route. If you get doubles, you can always break them apart for sidemount. There's a lot of flexibility with owning two tanks, including going back to singles if you want to lend one to a buddy.

Now, the weight difference between alu tanks and steel tanks cannot be summarized by simply saying "drop 6-7lbs". Check alu tank specs. Normally they're 3-4lbs positive while empty. If your steel 149 is +2# while empty, then you can only drop 1-2#. If you're talking about a tank that is -3# while empty then you can drop 6-7#.

About which has more gas, short-filled HP tank or full LP tank....do the math yourself. I'll teach you. There are two main ways to do the math. They're the same thing with different ways of thinking about it. I'll mention both:
(Tank Factor)149 cu ft @ 3600PSI means 149ft3/3600psi. That's 4.14 cubic feet per hundred PSI. That's your TANK FACTOR. You need to remember it for future maths. Then multiply that by 30 (3000psi = 30*100PSI) to show that with a short fill you end up with 124.2ft3.
(Ratios) This is a pressure ratio. You can nearly assume pressures to volume is linear. That was also assumed in the previous calculation. So, if your tank is 130ft3 at 3600psi and you're filling to 3000psi, you're only at 3000/3600 of your rated tank capacity. 3000/3600 simplifies to 5/6 which is .8333. That ratio times your rated tank capacity is 108ft3.

That math was assuming they're rated for 3600psi. They're probably rated for 3442psi. So, check the math yourself without me there to do it for you and I'll just give you the numbers if that's the case: 149 gets 129.9ft3 and 130 gets 113ft3. So, an LP121 beats the shortfilled 130 in both cases, and slighly loses to the shortfilled 149 in both scenarios. Also, at 500psi, the LP121 has: 22ft3, the HP149 has: 21.6ft3, and the HP130 has: 18.9ft3 left.

Remember, air weighs ~.0765 pounds per cubic foot. So, multiply that by the amount of cubic feet in your tanks and you'll get the weight of the air. That is the true difference in buoyancy between a full and an emty tank.

 

[BabyDuck]

what do you have available in way of fills? if you can't fill hp's, no need to get them. and what do your buddies dive? if you're diving giant tanks & they're diving 80s, you'll still be coming up after an 80 amount of gas has been used & the extra is...well, not 'worthless', extra gas is never 'worthless', but definitely overkill.

but basically, borrow several different tanks & try them. you'll know the right one for you right now.

 

[tracydr]

I'm not going to discuss the tank,specifically, although I do think that you will find a steel tank more to your liking when using heavier exposure suits and weight,due to the absence if positive buoyancy at the end of the dive.
My comment is with regards to your 30 pounds of lead and a 29 pound lift wing. Are you sure that you have enough lift in your wing? There are some calculators available to make sure that your wing has enough capacity. I'd hate for your to get into a situation where you really needed that lift and you didn't have it.
I dive an HP 80, because I like the size and buoyancy of the tank. Unfortunately, it rarely gets filled to capacity. This is fine, most of the time,because I used to dive an AL 63 but there are times when the extra gas would come in handy. Consider this when considering an HP tank. If your shop is unable to fill above 3,000 psi you will be getting short fills and an LP tank would be the way to go.

 

[Vegan Shark]

My mistake, my BC (Sherwood Avid CQR3) has 32 lbs lift. Leisurepro's site says 29, so I was going off that number, but the BC itself says 32. Using doubles/going into tech diving eventually is something I'm interested in, so maybe I should plan with that in mind. I hadn't heard of larger tanks rolling you when you slow down, that would be a huge downside. I'll be diving a steel 95 this weekend, so I guess if that feels pretty good then a HP100 would be the way to go.

I'm saving the drysuit for when I am more advanced and diving in colder waters. Looking like a badass astronaut sounds wonderful, but all the repairs, maintenance, and extra care compared to a wetsuit is putting me off more than the cost. Water temps around here are just 50-55, so neoprene serves pretty well.