Steel 119 vs AL 80 cu ft capacity

[steveolevi]

I am used to diving with aluminum 8o and I know when to begin my ascent depending on my SPG. Turn around and head back at 1000 psi and begin ascent at 500 psi.
Wth a new steel 119 I have different air capacity so I worked these numbers out. Can somebody tell me if this is making sense?


Steel 119 3442psi / Aluminun 80 3000 psi

50% 1721psi 60 cuft. / 50% 1500 psi 40 cu ft.

25% 860psi 30 cu ft. / 25% 750 psi 20 cu ft.

10% 344psi 19 cu ft. / 10% 300 psi 8 cu ft.

So, if i adjust with the new steel tank I can rely on the same amount of air available with lower psi reading? Is this correct/safe?

 

[Big-t-2538]

yes & yes....

 

[Scubaroo]

For starters, your AL80 is probably only a true 77.4cf - it's a marketing label.

You might be better off doing a search on "rock bottom" - rather than just use an arbitrary figure like 500psi to end the dive with, you should be calculating what the minimum gas you should be turning the dive at, in order to allow a safe ascent for both yourself AND a buddy, in the event of an out-of-air/low-on-air incident, from the deepest part of the dive. Calculate it in cubic feet, then convert that into a relative PSI for whatever tank you are using. Your buddy should do the same.

You would need to rework your math using the correct figure of 77.4cf - if that's the correct figure. 77.4cf is by far the most common correct figure for an AL80.

Here's a tank chart which shows different AL80s and their capacity:

http://www.sportdiverhq.com/tkcht.htm

Here's another user's explanation of rock bottom - just realised there's a ton of posts to wade through:

http://jonnythan.com/rockbottom.html

 

[ScoobyDave]

Thanks for that link, Ben - I had been working through similar calcs and was pleased to see I was on the right track. The engineer in me added a 13% safety factor, accomodating things like if my buddy has to breath off my system, chances are he/she's a little excited and will consume more than the normal sac rate.

 

[deepstops]

steveolevi once bubbled...
I am used to diving with aluminum 8o and I know when to begin my ascent depending on my SPG. Turn around and head back at 1000 psi and begin ascent at 500 psi.
Wth a new steel 119 I have different air capacity so I worked these numbers out. Can somebody tell me if this is making sense?


Steel 119 3442psi / Aluminun 80 3000 psi

50% 1721psi 60 cuft. / 50% 1500 psi 40 cu ft.

25% 860psi 30 cu ft. / 25% 750 psi 20 cu ft.

10% 344psi 19 cu ft. / 10% 300 psi 8 cu ft.

So, if i adjust with the new steel tank I can rely on the same amount of air available with lower psi reading? Is this correct/safe?

Your 10% figure for the steel tank should be 12, not 19.