What size cylinder should I buy?

[The Chad]

LP is 2250 unless it has a + then 2400 basically. HP is 4250psi or somewhere in the ballpark. fogive me if I'm off a hair

 

[lowviz]

Yeah, however if you consider HP you might run into a problem depending on where you dive getting it topped off fully

True now and then. At the worst case, you get just short of 70 cuft with a 2400psi fill. (She will still beat a 'hoover' on an 80.) HP fills aren't usually a problem, other than paying a bit more for the fill.

Nice tank, it doesn't go buoyant when empty.

 

[mikemill]

LP is 2250 unless it has a + then 2400 basically. HP is 4250psi or somewhere in the ballpark. fogive me if I'm off a hair

Eh? LP has a service pressure of 2400psi with a + rating allowing for 2650. HP is basically > 3000 psi with HP steels in the US commonly around 3442 psi.


Now as to the real difference: HP tanks have a higher service pressure than LP tanks. Remember back to the ideal gas law: pv = nrt. So you pump up the pressure and you can reduce the volume for the same amount of gas. So HP tanks are typically smaller in physical size and lighter.

For example a HP 80 vs a LP 80 (both steel):
HP 80 is 20 inches long with a diameter of 7.25 inches and an empty weight of 28 lbs, buoyancy ranges from -9lbs to -3 lbs.
LP 80 is 25 inches long with a diameter of 7.25 inches and an empty weight of 34 lbs, buoyancy ranges from -7lbs to -1 lbs.

numbers pulled from XS Scuba Worthington Steel Cylinder Specifications
HP 80: X-Series 80
LP 80: LP Series 85 (since it's 2400 service pressure would have about 80 cuft)

 

[The Chad]

lol like I said I may be a little off...I guess my 2250 came from O2 bottles. I must be narced or in Europe or something

 

[Dive Alaska]

Okay, up to this point (141 dives) I have been renting tanks. It's time for me to stop throwing money down the drain with rental fees and get my own. Over the past couple of years I've worked hard on attaining a neutral and trim position in the water BUT with an AL 80 I have to secure it pretty high so that my head hits the valve in order to help with my tendency to be "bottom-heavy." In fresh water, wearing a 3ml, I use 4#s of nonditchable weight on my cam band. I am about 5'4" and weigh about 125#s. What is the best cylinder for me to purchase so that 1) it is shorter and and therefore has less negative weight on my bottom half; 2) I would really like to get rid of my need for any extra lead weight in warm water conditions; 3) My sac rate is pretty good as I am usually just hovering over one area- (I don't like to kick much unless I actually have somewhere to go). In other words, my air consumption is equal to or better than my buddies so I don't have a need for massive gas volume. At the present time I use a back inflate BC but hoping to get into a BP/W soon. I would appreciate any suggestions so I can see if it helps with my trim. Thanks!

You actually eluded to your own solution right in your original question! Your trim (bottom heavy) problem is probably more a function of your BCD than your tank. A BP/W will allow you to trap gas along the full length of your tank and buoy up your feet. You should be able to trim out an aluminum tank OR steel tank because of this design feature. When selecting a wing, however, such things as tank size and composition will come into play. When wet suit diving in a thin wet suit, an aluminum tank may lend itself to a properly "balanced" rig. Dry suit diving on the other hand might favor steel tanks since they're generally more negatively buoyant. Ultimately, my recommendation is to buy a Halcyon 30-lb Eclipse BP/W then get a couple of Aluminum 80 tanks and use trim pockets on the tank straps to trim yourself out. last thing...have a blast!

 

[lowviz]

.... I love carrying around stupid amounts of gas, though.

Here is where a single smiley would do nicely, but I need 5 characters to post.

 

[WD8CDH]

lol like I said I may be a little off...I guess my 2250 came from O2 bottles. I must be narced or in Europe or something

You must be showing your age. 2250 was the service pressure of the LP71 tanks.

 

[dannobee]

+1 on the HP80's. My daughter loves them. They're a bit too short for me, but I've been known to use them on occasion.

Service pressure on the LP tanks is 2400+. You'll only get the rated volume if you fill them to 2640psi.

 

[rstofer]

Are you sure you want to get rid of all your ditchable weight?

The very first thing a rescuer wants to do is get a victim positive at the surface. Maybe the BC works, maybe it doesn't. But ditching weight is always a step in the right direction.

If the incident occurs at the beginning of the dive when an HP 100 is 10.5# negative and the wing is blown, there might not be a way to get the diver positive other than ditching the entire rig. This then removes one tow method from the list of possibilities. It also means there are less options for providing air to the diver because their rig just sank to the bottom of the ocean. Of course, there is the time lost trying to figure out why the diver keeps sinking...

In a way, cold water divers (wetsuit) have it made. We always need a lot of weight so we can put some on cambands or use heavy backplates and still have several pounds of ditchable weight. Steel tanks are ideal for cold water divers.

Warm water divers have it made, too! I LIKE warm water diving. But I like it better with a less negative tank and perhaps 4-6# of ditchable lead. If I were diving in warm water again, I would be using an Al 80 but I would consider a steel LP 72. I think I would be looking at tanks that are definitely positive at the end of the dive to ensure that I needed some ditchable weight.

Just something to consider...

Richard

 

[hydropunk]

Triple 30's...

good.