High Pressure vs. Low Pressure Steel?

[raposarose]

Hello all,

I am looking to buy a tank but I am unsure about the advantages and disadvantages of high pressure vs. low pressure steel tanks.

I want steel so I can take a little weight off of the belt. Also, I want to probably get a steel 100 since when my wife and I dive 80's I finish my air (500 psi) when she still is at 1000 psi.

So from what I gather:
-HP requires a DIN valve whereas LP can use either DIN or yoke
-HP will be more negatively buoyant
-HP will possibly be harder to get filled to the full amount (though maybe this won't be a problem because I probably will do most of my diving in Monterrey where there are many dive shops)

What I don't know is which tank is heavier on land, and what the respective dimensions of the tanks are.

Any suggestions/opinions are welcome.

Thanks!

 

[NorthernMelody]

LP will also last a lot longer as there is no pressure (compared to an hp) around the neck. That's what I was told by the dive shop...I'm looking at getting a steel as well. Thanks for posting in an informative manner like this

 

[wmspdi]

I have 4 PST E-7 100 ft3 tanks and they are great. I had the same problem with running low on air before my dive partner and wanted a little more gas. I dive wet (5-8 mill) and wanted to get 6 pounds off of my weight belt. The 100's did both for me. The "E" series has sort of made the LP/HP debate a moot point. PST is phasing out the LP & HP tanks in favour of the "E" series which could best be descibed as "variable" pressure tanks. They are rated for 100 ft3 at 3442 PSI, 89 ft3 at 3000 PSI, and 80 ft3 at 2640 PSI. They are also the same diameter as a AL80 so you won't need to readjust the BC tank strap when switching back and forth between E7s and AL80s. Tank neck threads are standard 3/4" making selecting a valve/minfold system easier if setting up doubles. No need to worry about getting a "+" hydro for an overfill. The tanks are shipped with a Thermo DIN/K valve. The valve has DIN threads with a screw-in insert that allows it to work with yoke regs as well. Leave the insert in place for a yoke reg, or unscrew it (with a hex wrench) to use a DIN regulator. The tanks are also hot dip galavanized instead of painted (better rust protection). The down side is that the demand for these tanks has driven the prices sky high, created a shortage, and long delivery dates. All the specs are available at the Fill Express web site:

http://www.diveriteexpress.com/gas/steel.shtml

 

[Sideband]

I have a Faber LP95 and love it. I get a lot of fills from a cascade system on our shops truck. With my AL80 I was only getting 2/3 of a fill. Now I get pretty close to a full fill until the last day of diving when the tanks are all low. I can't complain either way because I'm not paying for those fills, but it is nice to have a full tank.
I also like the 8 lbs of lead I got off my belt.

Joe

 

[MXGratefulDiver]

LP will also last a lot longer as there is no pressure (compared to an hp) around the neck. That's what I was told by the dive shop...I'm looking at getting a steel as well. Thanks for posting in an informative manner like this

Actually, this is a non-issue with steel cylinders ... they are designed to last longer than the diver using them if properly cared for.

Pressure cracks around the neck of the cylinder are a potential issue with aluminum cylinders, not steel ones ... regardless of whether they are LP or HP.

The biggest issue you have to consider with steel cylinders is rust and corrosion ... mostly internal. Even tiny amounts of moisture will cause them to rust. You'll also need to be careful with handling ... especially painted cylinders. Chips or dings in the paint will lead to rust and corrosion on the outside of the cylinder.

Also, if you are interested in using steel cylinders for many years, avoid chronic overfills. It's not a short-term problem, but over the course of years your cylinder walls will lose their elasticity ... which will cause them to eventually fail hydro. Avoiding overfills minimizes this as a potential issue.

... Bob (Grateful Diver)

 

[MyDiveLog]

I'd still buy steel tanks, but I'd get an LP 80 instead of the HP 80's that I currently have (LP Steel 80's weren't available then). I love the bouyancy characteristics of the HP 80's, but the cost about 2.5x as much as the AL80's and I have to be careful not to run down the tank pressure due to rust.

Having said that, there are two problems with my HP80's that the LP80's would solve:

1) Chronic underfills .. my tanks hold 80 cubic feet at 3500 PSI. Unless I fill them myself, I typically get 3000 - 3300 PSI fills at the dive shop. Most boat compressors can't even 'theoretically' fill sto 3500 PSI, so the problem is worse. I have good air consumption, so this hasn't been a problem, and I usually hit NDL limits way before air limits .... but it still bugs me.

2) The HP 80's are slightly bigger in diameter than the AL80's, and the boats 'tank wells' are usually designed only for AL80's. The LP80 is the same diameter as an AL80 (but a bit shorter).

NWgratefulDiver is also right about the neck weakness: it's an AL problem, not a Steel problem. Because of this we have to electronically check the necks of AL tanks when doing a VIP, but not the necks of steel tanks.

One other thing: I prefer the DIN connection over the Yoke connection. The DIN valves are stronger (withstand more bumping, etc.) and "lock' the o-ring into place; you can't knock it out of place. I know of one diver that was diving with a steel tank/yoke connection. He bumped the yoke on the platform when entering the water & the tank pressure dislodged the o-ring, resuting in a fast air loss and a cancelled dive. For a nominal cost you can get a DIN-to-yoke converter that you hand-install in seconds.

 

[rjchandler]

it isn't necessarily bad for your bddy to come back with more air than you if you generally consume at a faster rate. That's YOUR OOA reserve.

 

[MXGratefulDiver]

What I don't know is which tank is heavier on land, and what the respective dimensions of the tanks are.

I have found these tables to be reasonably accurate (not all those you will find on the Internet will be) ...

http://www.huronscuba.com/gear/cylinders.htm

... Bob (Grateful Diver)

 

[wmspdi]

it isn't necessarily bad for your bddy to come back with more air than you if you generally consume at a faster rate. That's YOUR OOA reserve.

They don't see it quite that way. Besides my OOA reserve is a 19" pony for deep dives (but I don't plan it into my available gas for the dive... only for OOA emergencies).

 

[northcoast_diver]

PST has tanks available now that are rated for both, either high pressure or low pressure for the same tank. Generally this would be your ideal choice, in a tank size that meets your needs with a low pressure fill.

Some dive shops do not fill to high pressure. Some boats do not fill to high pressure either. Those are the only drawbacks to having your own high pressure tanks.