Rock Bottom Calculation Practice

MaverickNH · Feb 22, 2012

I've been reading up on Lamont Granquist's and Bob Bailey's articles on Rock Bottom Gas Management. Great stuff. But like many such new skills, I need some practice.

Would any knowledgeable diver(s) be will to pose a series of problems on this post, wait a day or so, then present & discuss answers? As a "Basic" subforum, probably limiting problems to beginner-intermediate dive scenarios, like AL90/HP100 single tanks, depths to 100ft, turn-arounds & difts, no overhead, no-deco only, a single 3m/15ft safety stop, etc.

I will eventually find live instruction on the topic, but always like to do the book-work upfront.

Peter Guy · Feb 22, 2012

I'm not sure I understand what it is you are trying to do. Don't look at this as trying to find RB for 85 feet vs. 70 vs. 90 -- just do a few simple calculations and be done with it.

100 feet -- RB 40 cubic feet (or 31 if following PADI RDP guidelines)

60 feet -- RB 25 cubic feet

30 feet -- 500 psi

K.I.S.S.

You might want to look at my cheat sheet:

http://www.tsandm.com/gm/gm.pdf

TSandM · Feb 22, 2012

Okay: You are diving an Aluminum 80. You did a dive to 80 feet, with an average depth of 40. The dive was 30 minutes long, and you came up with 500 psi. How many cubic feet of gas did you use, and what was your SAC rate for that dive?

The next day, you are going to dive a HP100 (working pressure 3442). You plan a dive to 60 feet. It's a wreck, so the profile will be square. Assuming your gas consumption is the same as yesterday's, which is going to limit your dive, gas or no-deco time? (Don't forget you have to account for rock bottom!)

15 minutes into the dive, you look at your gauge, and you have only 1700 psi left in your tank. Are you going to be able to do the time you planned? (In other words, is your gas consumption on track with the day before's?) If not, what things might you think about as to why it is different?

Is that the kind of thing you had in mind?

MaverickNH · Feb 22, 2012

TSandM:
Is that the kind of thing you had in mind?

Yup. I'll work on those.

MaverickNH · Feb 22, 2012

Peter Guy:
You might want to look at my cheat sheet:

http://www.tsandm.com/gm/gm.pdf

That's very helpful, thanks.

lowviz · Feb 22, 2012

TSandM:
...//... It's a wreck, so the profile will be square. ...//...

-and you get to pick how far you swam from the tie-in.

Captains hate it when divers surface off the line.

<<Hint: you can't just come up anywhere, you plan to get back to the tie-in.>>

Jax · Feb 22, 2012

Girlfriend drops her expensive glasses off the pier in 100' water. You have an AL80. How many minutes can you spend looking for the glasses?

What is your descent rate?

What is your ascent rate?

Jax · Feb 22, 2012

You go to dive the Blue Heron Bridge in West Palm Beach. (Avg depth 15') You have an AL80.

For how long can you dive?

What is your thermal protection if the water is 78F?

DiveNav · Feb 22, 2012

Jax:
..... How many minutes can you spend looking for the glasses?

it depends ... on the girlfriend :cool2:

Alberto (aka eDiver)

Rock Bottom Calculation Practice

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MaverickNH

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