Cylinder Diameter and Trim

[DennisH]

Hi folks,

I'm a long-time lurker, first-time poster. I want to start by saying I've learned lots from these forums.

I am a new diver with just under 50 dives this year. I dive in cold water (Vancouver, BC, Canada). I own most of my own gear, with the exception of tanks. I am now looking at buying tanks, and need your thoughts.

First, some context: I am an air hog. I am a new diver, so this will (and has been) improving, especially as I've improved my buoyancy and trim. But, I am also a big guy. I'm working on improving my cardiovascular fitness, but I'll never manage "petite" to say the least. My SAC rate will always be on the high end.

My shop rents only steel 100s or aluminum 80s, so I can't rent other sizes to "try before I buy". I have been diving a HP100 steel. I get between 25-30 minutes of dive time, usually with a max depth of 80ft and much of the dive at 40-60ft. Even with a buddy on an aluminum 80, I am usually the one calling to turn the dive first. I would like more bottom time while still keeping a safe reserve.

I am debating between the HP 3442 120s and 130s. This is where I need your thoughts. The 120s have a cylinder diameter of 7.25 inches (the same as the 100s); the 130s have a cylinder diameter of 8 inches. If I progress to diving doubles later on, I will want the 130s. But, is an 8 inch diameter too wide for diving single tanks? How much does it affect drag and trim? Some people have said that with a 130 they feel more "tippy"...what are your thoughts and experiences?

By the way, I did search for this topic but was only able to find debates on steel vs aluminum tanks, and tips for improving air consumption -- all of which are interesting and helpful, but none of which dealt with the effects of cylinder diameter. If I missed a post, please tell me and I will delete this thread as redundant, with my apologies.

Ok...let my education...begin! And thanks!

 

[DA Aquamaster]

Check this post - it gives the basics on steel doubles, some that are most common and why they are logical choices rather than many of the other tanks that are available.

first doubles

X7-120's are considered to be a bit long by most people (28") but are 7.25" in diameter, -2.0 lbs negative when empty and weigh 38 lbs, putting them in the same weight ball park as LP 95's - just skinnier and 4" taller. If you can tolerate the trim issues with the longer tank, they are not a bad tank from a weight/volume perspective. If you really have to have a 120, but need a shorter tank, the X8-119 weighs 42 lbs but is 8" in diameter and back in the 24" tall range.

LP 121's are even longer at 29" and weigh 49 pounds making them a poor choice in my opinion.

Personally, I dive HP 100's and LP 95's. They are similar in lenght but the HP 100's are 7.25" diameter versus the 8" diameter of the LP 95's and I never notice any difference in trim.

In terms of single tank diving, the "tippy" feeling in a single 130 may stem more from diving a fairly heavy and negative tank (compared to an AL 80) in a less than wonderfully stable jacket style BC.

 

[diverrex]

I have two tanks, a HP 100 and a HP 130. I switch between them regularly, on beach dives I'll use the 130 first then switch to the 100 for the second dive. I actually don't even notice any difference with the tanks, except that I have tons of air with the 130!

 

[Web Monkey]

First, some context: I am an air hog. I am a new diver, so this will (and has been) improving, especially as I've improved my buoyancy and trim. But, I am also a big guy. I'm working on improving my cardiovascular fitness, but I'll never manage "petite" to say the least. My SAC rate will always be on the high end.

My shop rents only steel 100s or aluminum 80s, so I can't rent other sizes to "try before I buy". I have been diving a HP100 steel. I get between 25-30 minutes of dive time, usually with a max depth of 80ft and much of the dive at 40-60ft. Even with a buddy on an aluminum 80, I am usually the one calling to turn the dive first. I would like more bottom time while still keeping a safe reserve.

Just a couple of thoughts:

The tanks only give their rated volume when filled to their rated capacity. You might want to check with your LDS to see if they can actually do a 3442 PSI fill on a regular basis. Some can only do it if you catch them while their banks are full. If you want a big steel tank, you might want to go LP, since you can always get a good fill, and the size shouldn't be a problem for a big guy.

I'd also suggest working on your weighting and trim a little more before buying tanks. You might find that your consumption has improved dramatically and you don't need as large a tank as you think you do. While there's no such thing as "too big a tank" while underwater, but there's definitely such a thing when you're carrying them up-hill back to the car.

As for the diameter, I don't think you'll notice anything. I beleive LP 95's are 8.25" and my 45Cu Ft pool tank is maybe 5". There is no difference between either of these and an AL 80 for stability.

Terry

 

[TSandM]

I have dived LP95s, HP100s and HP130s, and I basically trim out the same in all three. HP 130s are WONDERFUL single tanks. They are definitely a bit on the heavy side to haul around on land, but they hold as much gas, basically, as a set of double 72s.

They are ugly monsters, in my book, as doubles.

But most people, if they ever go to doubles, end up with doubles and single tanks, because there are some charters you can't take doubles on, and there are some sites you don't WANT to take doubles to (long hikes, steep hills, and shallow sites).

If you have new-diver air consumption, buy yourself a couple of HP 130s. You won't ever regret them; as your skills improve and your dives get deeper and longer, they'll still work for you. If, in the future, you want to move to doubles, you'll probably more likely want a pair of HP100s, which are lovely, versatile double tanks at a manageable weight.

Unless you're diving caves.

 

[DaleC]

Hi Dennis, welcome to the board (officially that is)
I've done AL80's and HP 130's. The 130's were a little more tippy (floppy jacket BC) but not in an unreasonable sort of way and they do allow you to take some weight off the belt. Curt and Randy (whom you met at W.C.) and John (who you haven't met) all dive 130's and love them. They keep trying to talk me into getting one as well but I am a cheap b_stard and still choose to dive off old secondhand 80's.

Before you splurge out you may want to just see where your sac rate is going. I know I was a hog as well but have settled down to a much more reasonable level once I got comfortable in the water and stopped using my BC so dramatically. Using 130's on repetitive dives is also a great way to blow your NDL's if you aren't keeping an eye on you dive time. You just never seem to run out of air (me ).
See you in the parking lot,
Dale.

 

[rongoodman]

I think a cylinder's length has more to do with how it trims out than its diameter, which might have more effect on side-to-side stability. I like my LP85s, but the HP 130s are only an inch longer and feel pretty similar.

 

[PvtStash]

the steel LP 121 for my first tank purchase...

I know I'm not going to be able to afford a set of doubles for another 5 yrs or so but still wanted more backgas than a typical 80... I'm 6' 3" , 250lbs. so the size/wt. of the tank is manageable... I Was concerned with getting the Full fill in an HP tank... its usually very busy at my dive site's air station and often get shorted on even LP fills, a short fill on an HP tank being even greater... also by buying the tank through my LDS I get free fills there all year, and have a Good chance of getting 2800+ fills, giving me even more gas than that HP 130 would have anyways... I'm also hoping to get 2 dives off a single fill and economize a lil' more...

 

[The Kraken]

I'll have to cast my vote for the HP 130's.

I purchased 3 of the HP 119's, and if I had it to do all over again, I'd go for the 130's.

The 130's are only an inch or so longer than the 119's and the weights are almost identical. Each is an 8" diameter tank.

I'm 5'8", 175# and the 119, as well as the HP 100, trim out quite nicely.

As has often been said, "One can never have too much gas."

Only issue with the 8" tank is on the dive boat. Most operators have their tank holders set up for the 7.25" tanks.

the K

 

[DA Aquamaster]

The tanks only give their rated volume when filled to their rated capacity. You might want to check with your LDS to see if they can actually do a 3442 PSI fill on a regular basis. Some can only do it if you catch them while their banks are full. If you want a big steel tank, you might want to go LP, since you can always get a good fill, and the size shouldn't be a problem for a big guy.

Short fills on 3442 psi tanks aren't the issue they used to be as they are now pretty common tanks.

And from a size/weight perspective, you get about the same size, weight and internal volume when you compare any LP tank with a similar dimensioned HP tank. In other words, if you get short filled on a HP tank, as long as you have more than about 2800-2900 psi, you will have more gas and carry slightly less weight than a similar dimensioned LP tank plus filled to 2640 psi.

For example a short filled X8-130 filled to only 2850 psi holds 108.8 cu ft - The same as as the 3 pound heavier and 1" longer LP108 filled to 2640 psi. So the fill has to get really short before you lose any ground to an LP tank.

Plus, LP tanks only hold their rated capacity at the 10% overfill. Consequently if an LP108 is not plus ratred at the next hydro, it only hold 98.7 cu ft. at 2400 psi.

LP tanks really only make sense in cave country where you can get them filled to 3600 psi or in a case where you have your own small portable compressor that may work a little faster at lower tank fill pressures.