Weighting for Steel 90 vs Al 80

[DBailey]

Thus far I have only been diving with Aluminum 80 tanks. I have my weighting pretty much nailed in saltwater with these tanks.

In an upcoming live-aboard trip, they only supply Steel 90 tanks.

Anyone have an estimate of how much weight I can drop by using the Steel 90 instead of the Al80? Granted, a proper weight check will determine how much weight I can drop, but I just want to get an idea of how much I can expect to drop.

 

[DA Aquamaster]

There are steel 80's 85's 95's and 98's but I am not aware of any 90's. Well...OMS sells a set of twin 45's..that is kind of a 90.

 

[DBailey]

Hmmm, the Nekton website specifically says steel 90's in the fourth bullet point:

http://www.nektoncruises.com/Liveaboards/Features.aspx

Steel 90 cubic foot tanks (with standard "K" valves for use with standard yoke regulators) provided for each guest. (Limited number of aluminum and shorter tanks also available)

Well, for discussion, lets say they are steel 85 tanks. What like of wieght difference could I expect?

 

[mweitz]

You actually need to know the manufacturer and model of the tank they use. Different tanks have different characteristics.

As a general rule from what I’ve seen (and I’m not that experienced a diver) is that steel tanks are negative when empty. Al80’s are positive. Generally speaking 4 – 7 lb would be what you could drop. You have to look at the difference between the buoyancy of the tank in the water when it is empty, and subtract the difference.

Lets say that your AL80 are +4 when empty
Lets say that the steel tank is -2 when empty

You could probably drop 4 to 6 pounds for that scenario.

I’ve seen some pretty wild differences between different tank manufacturers though.

Mark

 

[DBailey]

I knew that it would vary between manufacturers. Thank you for the ballpark value. While the trip isn't until the summer, I am looking forward to using less lead.

 

[scubafanatic]

....I know a lady who did a Nekton Belize cruise last summer.......she said they used steel LP 95's....their WWW site has a typo, there's no such thing as a steel 90.

...the steel 95/98 specs are here:

http://www.omsdive.com/cyl_spec.html

.........the 95/98 is the same tank......Faber the manufacturer calls it a 95......OMS the retailer calls it a 98.

Karl

 

[DA Aquamaster]

They could be using older no longer plus rated Fabers or PST 95's that would be hold around 86-87 cu ft at a 2400 psi service rating rather than the 2640psi plus rating.

 

[scubafanatic]

Hi DA Aquamaster,

........my Nekton lady friend told me she got 2600 -- 2700 psi Nitrox fills, so they got 95 cu. ft. fills.

Karl

 

[Geezer]

I had the same question after checking out the Nekton website - never heard of 90's, and couldn't find out any other information.

I'll be diving the Roqual next week at Isla Mona - will post tank details after my return.

 

[Geezer]

The Rorqual uses LP Steel 95s - the tank spec numbers referred to above are accurate for weighting. We had 16 of 30 divers using Nitrox, with fill pressures ranging from 2,600 to 3,000 psi. The tanks appear to be PSTs, but some of the markings were hard to see under the paint. One of the DMs said they paint them with a heavy coat of gray marine paint every year to try to keep ahead of the rust. They have had exposure issues by being immersed in salt water for hours every day, and being splashed by salt water regularly when they are out of the water - according to the DM.