Which size steels for my purpose?

[mfigaro]

I do nodeco dives with a rear inflate BCD and an aluminum 80, mostly in lakes. I also plan to begin to dive oil rigs in the Gulf of Mexico as well. I like for each individual dive to last about 1 hour. After that I get tired or bored or both. If I do a square dive at 50' generally I get about 50-55 minutes out of my 80. However, if I start at 100-110' and use most of my nodeco time and then ascend to 50', I only get about 30-35 minutes out of my air. I'd like for those dives to last longer. I don't want to use doubles yet, so my plan is to buy a steel cylinder. Right now I am considering the Worthington high pressure x7-100, x8-119 and the x8-130. I am considering these three options because their length is no longer than an aluminum 80. I don't think the x7-100 is enough extra gas but I think it might be my best choice for when/if I decide to use doubles. However, I'm thinking I might use aluminum with weights on the tank or backplate when I decide to go doubles. That leaves the x8-119 and the x8-130. They both weigh about the same. Since the 130 is still shorter than an aluminum 80 so I think I might purchase it. As for the future, I have no desire to dive caves but I might do easier wrecks some day. I might get into some simpler decompression diving but probably want to keep it to the use of air. So, what does the board think would be my best choice? It would be nice if the equipment was appropriate for use on some of my stated plans but it is not a major factor in my decision. Thank you for your thoughts. p.s. I am 6' tall and about 180 lbs.

 

[mpgunner]

I have a similar number of dives but one thing I would recommend, after my rescue diving cert, is to also consider having extra air in case of an emergency. It is one thing to plan about what you need but another thing to plan if you have to bring someone up from below.

 

[DevonDiver]

What you need to know is your Surface Air Consumption (SAC). You can calculate this an average over your next few dives.

litres per min SAC = bar used x total cylinder capacity litres / minutes
...................................... (depth in meters / 10) /10

OR

cf per minute SAC =(psi used / working pressure) x total cylinder capacity / minutes
. .............................................(depth in feet / 33) / 33


Then you can plan your gas requirements for any given depth and duration of dive (remember to include a 1/3 reserve if doing deco).

ACTUAL litres required = (minutes x SAC rate) x ((depth in meters / 10) / 10)

OR

ACTUAL cf required = (minutes x SAC rate) x ((depth in feet / 33) /33)

THEN

TOTAL gas require (incl reserve) = ACTUAL gas requirement x 1.5

Once you know what your gas requirements will be, you can start to determine the size of cylinder/s you will require for your diving.

Cylinder capacity is worked out by;

Designated volume in litres x pressure in bar

OR

Actual pressure in PSI / working pressure in PSI x capacity in cubic feet.

 

[The Kraken]

Well, if you've narrowed your choices down to the 119 and the 130, go with the 130.

They're approximately the same weight, have approximately the same negative buoyancy characteristics when depleted, and are relatively the same cost and both are 8" tanks.

Just keep in mind that many dive boats don't have tank clips that will acommodate 8" tanks very easily, so carry a couple of extra bungies with you.

the K