Are Single 80's a Smart Choice Deeper Than 60 fsw?

[MXGratefulDiver]

One can quibble with the numbers, perhaps ... but I'm just pleased as can be to see gas management being discussed on a public forum.

My personal opinion about aluminum 80's is that they make great beer containers. For actual scuba diving, I prefer something a bit larger ...

... Bob (Grateful Diver)

 

[LiteHedded]

One can quibble with the numbers, perhaps ... but I'm just pleased as can be to see gas management being discussed on a public forum.

My personal opinion about aluminum 80's is that they make great beer containers. For actual scuba diving, I prefer something a bit larger ...

... Bob (Grateful Diver)

me too. can't remember the last time I used one aside from stages
I try and snag steels for single tank diving

 

[Blue Sparkle]

I'm reading the post about a 100' dive, but then the title is about "dives deeper than 60'." Granted, it's all a continuum, and the post is just one example, but since "deeper than 60'" could often include 60'-80'.... I'm feeling a bit of a mis-match between the example and the title (?). On the one had what is posted about the 100' dive seems really correct and reasonable, but then the title makes it seem like it extrapolates it to most/all dives deeper than 60'

I guess what I'm saying is that I understand that there is no "demarcation line," and that I should be calculating minimum gas for each dive/tank combination, but... the title seems a little bit all-encompassing compared to the example.

(I'm a little bit hesitant to post this as I know the OP is an instructor and I'm just a beginner, and learning about gas management beyond shallow, "simple" dives; so I'm probably missing something obvious. Which I should know about.)

 

[MXGratefulDiver]

It seems like the post is about a 100' dive, but the title is about "dives deeper than 60'." Granted, it's all a continuum, and the post is just one example, but it seems like there might be a bit of a mis-match since "deeper than 60'" could often include 60'-80'.... right? On the one had what is posted about the 100' dive seems really correct and reasonable, but then the title makes it seem like it extrapolates it to most/all dives deeper than 60'

(I'm a little bit hesitant to post this as I know the OP is an instructor and I'm just a beginner, and learning about gas management beyond shallow, "simple" dives; so I'm probably missing something obvious. Which I should know about.)

I believe it's about commonly accepted "limits" in recreational diving ...

- the recommended max depth for OW certification is 60 feet
- the recommended max depth for AOW certification is 100 feet

Most folks get their AOW reasonably soon after OW ... and most often because they want to increase their depth range. Quite often, these divers who are suddenly "certified" to go to 100 feet are still quite new, still working on the basics of buoyancy control, still struggling with a rather high air consumption rate, and haven't yet had anything resembling gas management training except "watch your SPG" and "end the dive with 500 psi".

While those are reasonable things to expect a new diver to be able to do, they do not in any way prepare you for handling something going wrong with your gas supply at 100 feet ... even if you and your dive buddy are completely comfortable with doing an OOA ascent (which is also rarely the case with newer divers, since they've only ever done it once, in class, from about 20 feet).

The key concept to get any recreational diver to learn is that they should always ask themselves the question ... "If my buddy needed air at the deepest part of the dive, would I have enough to get us both to the surface safely?" And, of course, provide them the requisite tools to be able to answer the question ...

... Bob (Grateful Diver)

 

[GrumpyOldGuy]

To add to the confusion of 60 vs 100ft, the question of "Is it a smart choice" is also unclear.

If we are asking, it it the optimum (best) choice, then NO is a pretty good answer for deep dives. More air is almost always better and steel tanks have other advantages.

If we are asking, is it acceptable then the answer becomes more dependent on the divers. After you calculate rock bottom (I would calculate 1500PSI based on an SAC of 1.0 at 100'), then is the balance available enough to make the dive worthwhile. If you are not willing or able to calculate rock bottom, then personally I don't think you should be diving to 100' without an instructor at your side regardless of tank size.

 

[Insta-Gator]

Insta-gator, that works out to about .4 cfm, which is a fairly low gas consumption rate for an adult male, but not unknown. It suggests you are either very fit, an efficient and relatively slow-moving diver, or skip-breathing

TSandM, I am very comfortable in the water and prefer moving slowly. There's more to see if you take your time. Also, I dive in clear, warm water, not cold and dark. I'm sure that makes a difference. My highest SCR (working) was 21 psi/min and my lowest resting SCR was 10.8 psi/min. Obviously, I'm VERY relaxed in the water.

Here's a worksheet I like to use just to analyze where I am on Gas Management and Rock Bottom, since it changes with SCR. The figures in the worksheet are not actual dives. But it points out that you must have a very good SCR to hit the NDL @ 60'. At SCR 15 I can only remain at 60' for 45 minutes, not 60 minutes and still not violate my RB.
Edit: I also should point out that I prefer multi-level dives, which may skew my SCR a little.

 

[Blue Sparkle]

One reason I took notice of this thread is because, so far, all my diving has been in Florida - mostly in the Keys. There, rental AL-80's seem to be the the most common tank with dive ops. Before my last trip, most of my dives were on shallow reefs, where an AL-80 was more than ample. But then I started to make some deeper dives, so I was learning to calculate things like minimum gas, RMV, and etc.

It just seemed like there was a big difference between the 60' of the title and the 100' of the example, without a lot of mention of the area in-between. I do understand the example though, as it seemed very similar to some of the wreck dives in the Keys (i.e. around 100', square profile) that I was planning or thinking about. I probably wouldn't have piped in if the title had been something like "How to determine safe dive limits with the ubiquitous AL-80."

I'm enjoying the thread though. This type of gas plannng was missing from my "standard" training - which was bothering me - but with some looking I was able to find it (albeit outside of "normal"/standard instruction channels).

Blue Sparkle

PS: NWGratefulDiver, your published material was very helpful, as was that of some other regular posters here.

 

[Insta-Gator]

Typo on definition of Pi on previous worksheet...sorry.

 

[Blackwood]

It may be worth noting the GUE suggests a fairly conservative min deco.

Any diver who has an understanding of dive tables also has an understanding of min deco whether or not they realize it.

A dive table is derived using a specific ascent rate. For the PADI/DSAT (gradient model) table, it's 60fpm. Therefore, to comply with the table, for a dive to 60ft, min deco is 1 minute (60ft/60ft/min). GUE's min deco for the same dive would be 4 minutes (30fpm to 30ft and then 10fpm to the surface). Clearly, the former requires less gas.

Personally, I use the OP's min deco concept for my own dives, but for others using a direct table approach, an 80 is far more reasonable in depths approaching 100 ft.

 

[GrumpyOldGuy]

It may be worth noting the GUE suggests a fairly conservative min deco.

Any diver who has an understanding of dive tables also has an understanding of min deco whether or not they realize it.

A dive table is derived using a specific ascent rate. For the PADI/DSAT (gradient model) table, it's 60fpm. Therefore, to comply with the table, for a dive to 60ft, min deco is 1 minute (60ft/60ft/min). GUE's min deco for the same dive would be 4 minutes (30fpm to 30ft and then 10fpm to the surface). Clearly, the former requires less gas.

Personally, I use the OP's min deco concept for my own dives, but for others using a direct table approach, an 80 is far more reasonable in depths approaching 100 ft.

Isn't PADI 30fpm + 3 min s/s = 4 min?