When to choose a lower capacity tank

[Diver003]

I've recently been toying with the idea of purchasing my own tanks. I only do recreational, single tank diving and originally had planned to purchase some HP100's. After some re-thinking, it seems that HP130's would be a more logical choice because I'd have to get used to trim, weighting, etc. regardless with new tanks - might as well have more bottom time, right? (Plus fill costs are usually the same price 100 vs. 130 where I am.)

I also recently read the account of the Edmund Fitzgerald recently posted by ianr33 in another account (I can't post links due to < 5 posts). The author stated that his buddy used double 120's as main bottom gas and he used double 104's as main bottom gas. Max depth stated was 530 ft. and I don't need to mention other tanks, etc. used. Anyways - why not used double 130's as main back gas? The extra 20-52 cu. ft. seems like it could be critical on a dive such as that.

What am I missing that prevents people from spending the extra $50-100 when purchasing tanks and just going with 130's all the time? I know in many cases it's overkill (shallow reef dives) for my purposes, but if you're going to go with one tank, why not get something good in all scenarios?

Thanks in advance!

 

[MXGratefulDiver]

I've recently been toying with the idea of purchasing my own tanks. I only do recreational, single tank diving and originally had planned to purchase some HP100's. After some re-thinking, it seems that HP130's would be a more logical choice because I'd have to get used to trim, weighting, etc. regardless with new tanks - might as well have more bottom time, right? (Plus fill costs are usually the same price 100 vs. 130 where I am.)

I also recently read the account of the Edmund Fitzgerald recently posted by ianr33 in another account (I can't post links due to < 5 posts). The author stated that his buddy used double 120's as main bottom gas and he used double 104's as main bottom gas. Max depth stated was 530 ft. and I don't need to mention other tanks, etc. used. Anyways - why not used double 130's as main back gas? The extra 20-52 cu. ft. seems like it could be critical on a dive such as that.

What am I missing that prevents people from spending the extra $50-100 when purchasing tanks and just going with 130's all the time? I know in many cases it's overkill (shallow reef dives) for my purposes, but if you're going to go with one tank, why not get something good in all scenarios?

Thanks in advance!

It's common practice among tech divers to overfill their low-pressure tanks. Not that I'm encouraging such practices ... but if you pump a set of 104's up to 3500 psi, you have 130's.

There's a saying among tech divers ... "When your tanks are full, it's time to turn the dive" ...

... Bob (Grateful Diver)

 

[Rainer]

There's really no "one size fits all" cylinder. Else I wouldn't own over 20.

I have a few single HP130s (and even some doubled up). They're nice cylinders, but they're heavy. If I'm doing a dive in 20-30', personally, I'd prefer to take a small cylinder (e.g. LP72). If I need to hike down (and later up) a large hill, or across a lot of uneven terrain, often a smaller cylinder is more practical.

If you're really only going to buy one cylinder, though, it's a decent choice.

That said, what makes the most sense is to actually work out how much gas you need for the dives you're doing and the dives you want to do. It's a rare (deeper, multi-level dive on nitrox) recreational dive where I can actually make use of the HP130 vs. a HP100. That is, for much of my diving, I'm usually NDL, runtime, cold, or boredom limited, not gas limited.

 

[andymeng]

Some things to consider. Your profile doesn't say where you are from but shallow reef dives = someplace tropical.

Straight answer, weight, size considerations. And do you really need that much air for a shallow reef dive?

If you are doing a charter, you are going to be assigned a max bottom time, usually 50 minutes on Nitrox and a computer, less for air, and follow your tables otherwise. I dive with LP108's (more on lp vs. hp in a sec) and for the 20-40 foot dives I get 2 dives on a tank. I used to only get 1 dive per tank at that depth. You may want to work on your air consumption rate if you're burning through it that fast.

LP vs. HP. You may want to look at a LP tank instead as many shops will overfill a LP tank to 3000PSI (if you're ok with overfills) or beyond. A 3000 fill is a 13% overfill of a 108, giving 122 CF. By contrast, a HP fill of 3442 is much less likely to get an overfill. My shop can only go to 3500 on their banks. But get a hasty fill, and that 3500 cools to 3200 and you are now in an underfill. That's a 9% underfill and your 130 has 118 CF. So check with your local shop to see what they do and make a plan from there.

Also remember not only will a charter limit your time, but you need to dive within your buddy's limit so you'll be wasting a lot of air anyway.

Twin 130s? 100 or so pounds of tank. Consider that when getting in and out of boat, standing up and walking, etc. It probably trims out nicely though. Then, consider the cost of the 2 tanks and you can do a s-load of dives on a more normal tank, or unless you dive a lot, a rental tank. Don't forget to factor the yearly viz and 5 year hydro. If you're not using them a lot, you're probably better off renting.

Also consider that if you have this megatank and can't use all the air, but can't get 2 dives out of it, you're not getting any bonus on the fill anyway.

Just some things to consider.

 

[Mr Carcharodon]

Well I have a HP100 and a HP130. The HP100 weighs a lot less and often times has plenty of gas. I have done 100 minute dives on the HP100 at some sites which was about as long as I wanted the dive to be any way. For a dive that is twice as deep the HP130 is a better choice.

One thing to consider is what your buddies are diving. It is certainly nice if your gas supply and theirs lasts a similar amount of time. Of course you have to consider capacity and SAC.

 

[Diver003]

andymeng...just to clarify the shallow reef dive was not the usual, rather an example of overkill going with the 130.

I live in North Carolina and frequent wrecks 80-120 ft. deep. The boat operator I use most often allows going to the no deco limits - no time restraints. So with 30% nitrox the NDL's are pretty good at those depths and it seems the 130 would let me ride them out.

I hadn't considered the LP's because I've never used them. Nor did I think too much about my buddy limiting my time drastically who usually just goes with 80 or 100 (like me) - I'd have to talk him into getting 130's too At least I'd get two dives per tank on quarry dives!

Thanks for the input so far, I've found it all very helpful!

 

[iztok]

Then there is a factor of having spare gas to boot to share in case of emergency even after you reach your NDL.

 

[Akimbo]

Here is another philisophy I recently heard. This guy figured the cost of air fills, the hassle of hauling around tanks, hydros, and inspections. He decided that that he would rather pay for a dive charter boat — all the boats in his area include tanks and fills. Since he can afford it and enjoys boat dives so much more than shore dives, it made a lot of sense for him.

 

[Jim Baldwin]

One of the things you definitely need to consider is the size factor. I'm 6'1" and have
HP 100's and HP 120's that I routinely dive with.

The 100's work great and have the same balance as a LP 95. I can maintain good trim
and don't even know the tanks are there. I teach in these tanks and do the majority
of my diving with these.

I bought the 120's years ago when I did my tech training and used them doubled up.
They're just too tall for me. With the HP 120's they are about 3 inches longer and
weigh more. I have to really work to have good trim. Yes they hold a ton of air.

I was in Pensacola in early May diving the Big O as a planned deco dive. I dove the 100's and put 8 lbs on my weight belt and stayed comfortable in my drysuit. I had the
120's along for a buddy who just uses more air than me. The bigger tanks worked out perfect for him and we both finished with good gas reserves.

Price difference is really not that much. But bigger is not always better. This past weekend I sold my 120's to a taller and larger diver. They will be perfect for him.

It's what fits you and remember that tech divers carry a lot more air than they actually need to accomplish the dive. Murphy's Law and the "Oh xxxx" factor and gas reserves have to be planned for.

 

[ianr33]

What am I missing that prevents people from spending the extra $50-100 when purchasing tanks and just going with 130's all the time? I know in many cases it's overkill (shallow reef dives) for my purposes, but if you're going to go with one tank, why not get something good in all scenarios?

Thanks in advance!

LP 104's and HP 130's are *essentially* the same tank. 104's hold 104 cu ft of gas at 2640 psi, 130's have 130 cu ft of gas at 3442psi.
Pump a LP 104 to 3442 and you have 136 cu ft of gas (Numbers may not be exact)

104's are commonly filled to 3600 psi in some parts of the country.

Only downside to such big tanks is they are heavy. They also have enough gas for even the heaviest breathing diver to get well into deco.