High Capacity Tanks - which one?

[rickc]

I am thinking about a higher capacity tank (than standard al 80) to increase my bottom time.

I am considering:

aluminum 100

low-pressure steel 95

high-pressure steel 100

Both the steel tanks are fairly pricey. However, I can latch on to an aluminum 100 (used) at a very attractive price.

What are the pros and cons these various tanks options? My diving is 75% shore diving and 25% boat dives.

rick.

 

[roakey]

I'm guessing salt (correct me if I'm wrong). Diving dry or wet? Think you'll get into technical diving anytime in your future?

Roak

 

[Arduous]

Hi Rick:

I have never used high capacity aluminum , but I do use high pressure steel. I have been using steel 100’s for several years now and am very happy with them. All my diving is done in the cold waters of New England, which means heavy exposure protection, which means lots of weight to get down. I find that using the steel 100’s I can take some of the lead weight off. In fact with my current set up ( when diving wet ) I do not use a weight belt at all. All the weight I use for the dive is in my BC. Most of my diving is done at around a depth of 70 to 80 fsw and I find that the steel 100’s serve me well for this purpose. Also an added bonus is that the steel tanks do not become positively buoyant at the end of the dive when the tank is low on air. This makes doing safety stops and or decompression stops easier.

Arduous

 

[scorpiofish]

You've got to answer Roak's questions before you can get a recommendation.

First consideration, the AL100 and HP steel need to be filled to a higher pressure to get the stated capacity. Can you get 3300 or 3500 psi fills? I know I can't.

Second, buoyancy characteristics. My local diving is fresh water with varying levels of neoprene wetsuits. Even thoroughly wrapped in neoprene, I only need 8lbs of lead with an AL80, none without a wetsuit. Increasing the negative buoyancy by way of a steel tank would make often make me very negative, not a good thing.

Third, an HP steel will require a DIN fitting. Is your reg a yoke? Do you really want to get an adaptor? Do you want to convert your reg? Not that big a deal, but added cost.

An LP 95 is usually the easier choice for more gas. Easily filled to 2760. No DIN conversion necessary. Of course, it's one heavy piece of gear, if that matters.

If diving fresh with little lead on the belt, Faber is a good choice. If diving salt, I'd choose a Pressed Steel, galvanized and more negatively buoyant.

 

[The Chairman]

I have always been able to get my PST hp 120s filled here. I know that when aluminum first came out, that 3000 psi was beyond most shop's equipment. Thats why the first Al tanks never caught on and died a quiet death. That was waaaayyyyy back, and luckily things change. Most shops in our area sell those 3300 and 3500 monsters and consequently can fill them as well. Just check first!

And as for buying a used tank... check out the thread "An Expensive Education" for an eye opener.

 

[large_diver]

A few quick thoughts --

As mentioned above, take a look at the buoyancy characteristics of your choices. I'm also in new England wearing thick wetsuits, so steel helps me get some weight of the belt.

LP steels are a good choice if you have trouble getting high pressure fills in your area. HPs typically are more negative in terms of buoyancy. They are also lighter and physically smaller than LPs of the same gas capacity. Since it looks like you do primarily shore diving, HP would probably be a better choice (assuming the fill issue is not a problem).

Steels can rust.

Finally, if you are looking at steels, I would strongly consider Pressed Steel -- well made and tough exterior finish.

One more thought -- take a look at the various posts that already exist on this board regarding the AL vs Steel choice -- there are many!

Good luck.

 

[Lost Yooper]

I'll wait until you reply to Roakey's thread to actually answer your question, but keep in mind that extending your bottom time may have consquences that may have to be dealt with. Also keep in mind that an additional 20CF of air is only about 10 extra minutes (assuming a good SAC) at deeper depths (80'+). One last thing, a larger tank shouldn't be the answer to poor air consumption. Often there are several things one can do to improve air consumption before a bigger tank is needed.

Take care.

Mike

 

[rickc]

I was told that the "real weight" is tank empty weight + bouyancy (say at 500 lbs).

So, an aluminum Catalina S-80 is 31.7 + 2.7 = 34.4

an aluminum 100 might be 46.1 - 1.2 = 40.8

normally, with a al-80 tank and gear for cold water, I use 30lbs of weight. with this hypothetical al100 tank, what would be on my weight belt? I am not sure how to do the calculation?

Also, I am assuming that, walking down to the beach, my total equipment weight will be heavier, i.e. 46.1 - 31.7 lbs + an extra pound or so for another 20 CF of air?

can anyone shed some additional light on this.

Rick

 

[Lost Yooper]

Rick,

A Catalina S80 is +4lbs positive empty and a C100 is neutral when empty, so you should be able to take off 4lbs. Yes, your equipment will be heavier, but you would have 4lbs less on your belt too. So, you would be roughly 11lbs heavier (46-31-4=11lbs assuming empty tanks).

Mike

Are you a dry suit diver?

 

[SubMariner]

I've been diving HP steel for years & only revert to AL if forced to

So I'd like to address a few of the issues scorpiofish put forth:

First consideration, the AL100 and HP steel need to be filled to a higher pressure to get the stated capacity. Can you get 3300 or 3500 psi fills? I know I can't.

If you make a point of asking for a high pressure fill AND give most shops enough time, they will deliver. A big factor in getting a true HP fill is if they are rushed, it will be a "hot fill" which results in losing 300-500 psi once the tank cools down.

Second, buoyancy characteristics. My local diving is fresh water with varying levels of neoprene wetsuits. Even thoroughly wrapped in neoprene, I only need 8lbs of lead with an AL80, none without a wetsuit. Increasing the negative buoyancy by way of a steel tank would make often make me very negative, not a good thing.

No offense, but yours is an exception and not the rule. Temperate diving in freshwater usually means 5-7mm of neoprene, which, for the average diver, cannot be offset by only 8lbs of lead.

Furthermore, just how much more negative do you think a steel tank is over an AL? In actuality, an AL tank becomes about 4lbs POSITIVELY buoyant at 1000 psi or less, while a steel tank will remain approx 4 lbs NEGATIVE throughout the dive. IMHO the latter is more desireable.

I started diving in the Great Lakes in Ontario. When I went from a 1/4" wetsuit to my first drysuit (also 1/4" neoprene) I wouldn't have been able to keep diving without my HP steel 80s!

Third, an HP steel will require a DIN fitting. Is your reg a yoke? Do you really want to get an adaptor? Do you want to convert your reg? Not that big a deal, but added cost.

The cost on converting from Yoke to DIN on a reg is minimal. The benefits of the more secure DIN connection (vs yoke) outweigh this small price difference. And if you need to connect to a traditional k-valve tank, adaptors are available. (I happen to carry a DIN fill adaptor and DIN to yoke adaptor in my dive box. They don't take up a lot of room or add much bulk/weight.)

BTW, to address the rust issue: if you keep at least 300-500 psi in your tanks REGARDLESS of what they are made from, it will keep rust at bay. Any possible problems will be caught at the annual VIP. My tanks are now 7 years old and it's only THIS YEAR that they may need a tumbling at the end of the season.

Just my $0.02's worth,

~SubMariner~