AL80 to 132ft (caution-contains math)

[BHB ScubaTroll]

Originally Posted by CoolTech
Venice Beach... Single HP 149. 4 1/2 hours. The only reason I surfaced... I got hungry. I still had 1,500 PSI left in the tank.

Yeah right.

I love people from out of state, who don't know the dive at Venice Beach, but certainly put their 2 psi in...

 

[Oddsnends]

A big two thumbs up to Lamont Granquist's guide, which AzAtty linked to.

GSDMan, it's commendable that you are working through the calculations. You and your dive partners are safer for it. Looking back through my PADI training, I just can't understand why this topic is not thoroughly covered. "Just take a class" doesn't work when the curriculum has gaping holes.

Having said that, a big thank you to the experienced divers and instructors that stoop to teaching us lowly recreational divers how to count our bubbles.

 

[NorthWoodsDiver]

.79 for a SAC seems way way high. on a bad day I might hit .55 and generally amin in the .3 to .35 range.

on a shallow dive an al80 will last me over 2 hrs with 750 left at the surface. A deeper dive to 140ft on a standard 80 I take about 9-10 minutes bottom time before starting back and always get back in the boat with well over 1000psi even after some minor deco obligations and a multiple stop ascent.

this particular profile seems far to conservative for even me and I generally fall on the overly conservative side. Just follow thirds and tables or a computer (or better yet use both). take the limiting factor to turn on. If you hit thirds before a suunto calls your dive or before even a standard table limits you there is an issue somewhere.

 

[dumpsterDiver]

GSDMan, kudos to you for working through this kind of problem.

I'm just back from a trip and a little short on sleep, but going through your math, I see that you are using 3 cfm as your gas consumption for your rock bottom. Why is this? It looks as though you are mixing up rock bottom with rule of thirds. We were taught to use a stressed gas consumption rate for calculating rock bottom, on the theory that someone who has gone out of gas and is having to ascend on someone else's supply may be blowing through gas as a result of adrenaline. What we use is 1 cfm for each diver; you might, at this point, want to use a bit more, simply because your unstressed SAC rate is so high, but I doubt you need to go to 3 cfm. However, what you have learned from the exercise is that rock bottom for significant depths in a small tank is a HUGE proportion of the gas you are carrying.

The bottom line is that both safe gas management and staying within decompression limits result in very short bottom times in the 100 to 130 foot range. As Mike Ferrara has often said, such dives are better planned and executed as staged decompression dives, with proper equipment and training, so that you can actually get a chance to enjoy what you went down there to see.

I agree with everything until the end. A dive to 130 feet can be made using nitrox, no-deco, an 80 cu-ft tank and you should probably take a pony bottle for redundancy. I sometimes make 3 dives a day to that depth and generally do no acrue decompression and if I do, it is usually less than 5 minutes.

Drawing the line at 100 feet for requiring staged decompression is way off track. With nitrox you can do a bunch of diving around 100 feet. I think I might be able to get around 29 minutes of no deco time on my computer at 100 feet with nitrox?

 

[GSDMan]

I see that you are using 3 cfm as your gas consumption for your rock bottom. Why is this?

Like you said, I started with 1.0 cu.ft. for my self and 1.0 cu. ft. for my buddy. Then, after thinking about my SAC being at .79, I began to think 1.0 cu. ft. for myself might be a bit low so I added another cu. ft. for good measure. Total - 3.0 cu.ft. on the ascent and safety stop. I'm sorry, I should have been clearer on how I got to that number.

 

[GSDMan]

rather than trust post from people you don't know and could be full of BS, why don't you take a class that teaches proper dive plaining?

Name the class and I will seriously consider it. I haven't found one yet.

 

[ucfdiver]

Name the class and I will seriously consider it. I haven't found one yet.

Most all tech training teaches this, or should.

 

[GSDMan]

Courtesy of Lamont: a walk through of how to calculate your rock bottom and some potential numbers to use.

Rock Bottom and Gas Management for Recreational Divers

Thank you for that. What an excellent article that is! I have it stored on my phone and read it at work when I get bored.

 

[GSDMan]

Consider including a deep stop in your rock bottom.

Can you help me out with some guidelines on this. How deep? How Long?

 

[GSDMan]

No, there is no harm and it will probably help you understand more...

Apologies for your question getting the off-track hijack... I didn't expect some would attempt to discredit me because they didn't agree with my statement (Then, find a single post from the past... which is correct and accurate, in an attempt to do just that without knowing the dive site...) Then, others would take my responses to the person (I quoted in the response, and one answer that was sarcasm... ) and attempt to use them as some answer to your original post.

So, please accept an apology for myself (and others) not allowing your question to continue on track.

No apologies are necessary, but are accepted none the less. Thank you.

Actually, the arguments, while not terribly helpful with the original question, are quite useful. They help me decide whose opinions and advice I will take seriously and who's just taking up room on my screen.