having some trouble deciding on tank size

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I will be taking technical foundations this coming spring and will need a larger capacity steel tank. I have been particularly interested in either LP120 or HP120 sizes. What are some pros/cons on the pressure rating differences?

Thanks,
Jason.

Hi Jason,

If you are purchasing Worthington steel cylinders you can look at the specifications for the tanks here: XS Scuba Worthington Steel Cylinder Specifications.

I like to look at tanks from two perspectives. The first is how they behave in water and the second is how they behave out of water.

In water you want to think about the buoyancy and the size. How the weight is distributed will depend on part on the size. A long, thin cylinder will distribute the weight differently than a short, fat cylinder. The LP121 is an 8 inch diameter and over 29 inches long. As a reference, an AL80 is typically 7.25 inches and 26 inches long. With a really long cylinder you might find yourself foot heavy or head heavy. If you centre the cylinder you could find it is hitting your legs. If you move it up to avoid leg hit, you could become head heavy. Only using the cylinder will be able to tell you how it will work for you. Two people 5'10" might have different weight distribution and therefore react differently to the same cylinder.

The other in water factor is buoyancy change. If I have a cylinder which will lose 10 pounds of weight as it empties, I must be 10 pounds heavy at the beginning of the dive to avoid being too light at the end of the dive. A cylinder which only changes 5 pounds in buoyancy would be better. Looking at the Worthington cylinders, you are typically looking at 6 to 8 pounds change. So this is not a factor when comparing two Worthington cylinders.

Also, if you are used to weighting yourself for AL80, the steel cylinders will typically be 5 pounds heavier in water. This means you can remove 5 pounds from the weight belt.

Out of the water there is the weight and the height. If you are short person or have long arms and no upper body strength, a tall cylinder can be a pain. My former instructor is short. She cannot carry two AL80s easily. She needs carry one at a time and tilt sideways to stop it from dragging on the ground. On the other hand, an HP100 is 2 inches shorter and she has no trouble carrying two HP100 cylinders. An LP121 would be monsterous for her to carry. The other thing is the weight out of water. An AL80 is 31 pounds. An HP120 is 38 pounds and an LP121 is 50 pounds. The HP120 is 7 pounds heavier BUT you dropped 5 pounds from your weight belt, so total kit is only 2 pounds heavier. Not bad. On the other hand, the LP121 is 19 pounds heavier. Even when you subtract the 5 pounds, you are 14 pounds heavier out of the water. This might not be a problem if you are boat diving as it is steps to the back of the boat. However, if you are shore diving and there are waves, this could really tire you out.

Finally, check with the agency you are training with. I have heard some agencies or possibly some instructors require specific gear. You don't want to buy HP120s only to find out the instructor insists on HP130s.
 
If you are thinking of doubling your tanks up in the near future, I'd recommend HP 100s. I can't imagine you needing more gas than that in a single tank, for an Intro to Tech type of class. And HP100s make a good set of doubles down to the 150 foot range.

We have a couple of 120s, and they are sort of amazingly long. I tried diving one, I'd be unable to sit down anywhere!

I do dive the HP130s, and they are wonderful single tanks for deeper and longer dives. But they're very heavy, and even my dive buddy, who is 6' tall and enormously strong, has figured out that diving smaller tanks doubled up makes life much more pleasant.

scubadiver888, that's a lovely analysis, but I'd like to address one thing:

Looking at the Worthington cylinders, you are typically looking at 6 to 8 pounds change. So this is not a factor when comparing two Worthington cylinders.

Swing, full to empty, is entirely a function of the gas mass in the tank. Air weighs approximately a pound for every 13 cubic feet, so an 80 cf cylinder will typically have about 5 lbs of gas in it that you intend to exhaust into the water. A 130 will have about eight pounds of gas in it that you would intend to use. So 5 to 8 lbs of swing is going to be roughly the range for any set of recreational cylinders. It is not a phenomenon of Worthingtons.
 
was about to say that about the worthington tanks...

If you're getting singles for a fundies type class, a HP130, or the 3442psi 130's is a good tank to keep around. Great for diving singles, and good for keeping in the house for regulator testing etc, a LOT of my friends have them doubled up, but most of my friends are bears like I am.

If you want the 120cf gas capacity, the HP119's are good, they're shorter 8 inch versions of the 120's. Same size as LP95's, and the 95's can usually be had pretty cheap. Lots and lots of the 95's running around, not so much on the 119's
 
scubadiver888, that's a lovely analysis, but I'd like to address one thing:

Swing, full to empty, is entirely a function of the gas mass in the tank. Air weighs approximately a pound for every 13 cubic feet, so an 80 cf cylinder will typically have about 5 lbs of gas in it that you intend to exhaust into the water. A 130 will have about eight pounds of gas in it that you would intend to use. So 5 to 8 lbs of swing is going to be roughly the range for any set of recreational cylinders. It is not a phenomenon of Worthingtons.

I might have thought the same thing but something about the specifications leads me to believe otherwise. The HP120 is 120.6 cu.ft. of air and it has a weight change of 9.0 pounds from full to empty. The HP119 is 123.0 cu.ft. of air and it has a weight change of 8.9 pounds. Shouldn't 123.0 cu.ft. of air weight more than 120.6?

If I go through all the HP cylinders taking the change in buoyancy divided by the cubic feet of air when full, I get slightly difference numbers for all the cylinders.

I believe because buoyancy is all about weight and displacement, the shape and size of the cylinder also plays into the change in buoyancy.
 
we also know how brilliantly accurate those tank spec charts are... so taken with a grain of salt.
Would have nothing to do with Worthington specifically, the 100cf tanks are all within half a pound of each other as far as weight swing, that's AL and steel, just depends on internal volume. They may be doing their buoyancy calculation on the 119cf instead of actual, who knows
 
I believe because buoyancy is all about weight and displacement, the shape and size of the cylinder also plays into the change in buoyancy.

But empty weight and displacement for a given tank are constants, they cannot change. Any change in buoyancy could only be due to the weight of the compressed gas in the tank.
 
My instructor said that I will need at least a 100cf tank for the dives we will be doing. I am still pretty young and i know doubles will be in the future for me, I just dont know how far or near in the future at the moment.
 
Thanks for wording it that way tech3324. The displacement isn't changing. That makes sense. Which confirms the specifications for the tanks are not very accurate.
 
you can go measure tank heights though and they'll often be slightly different, why it's usually important to get two tanks that came together for doubles instead of just buying one tank then finding a similar size one later.
If I were the OP, I'd get a 130 or 119 now for a good single tank, then get a dedicated doubles set later...
 
you can go measure tank heights though and they'll often be slightly different, why it's usually important to get two tanks that came together for doubles instead of just buying one tank then finding a similar size one later.
If I were the OP, I'd get a 130 or 119 now for a good single tank, then get a dedicated doubles set later...


It's like you just read my mind!! I will be doing that.
 
https://www.shearwater.com/products/perdix-ai/

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