Rule of Thirds & Shallow Rec diving

[supergaijin]

You do this because you are teaching outside of the standard definition of ow and aow.

Advanced Open Water Diver Course from PADI Professional Scuba Divers' Training Organization

What You LearnThe knowledge and skills you get in the Advanced Open Water Diver course vary with your interest and the adventures you have, but include

• Practical aspects of deep diving
• Physiological effects of deeper scuba diving.

Practical aspects of deeper diving = being able to come up from the "deeper dive' when the SHTF in my mind. Therefore I try to train people for it.

For OW students I introduce the concept- not because they need it for diving shallower than 3ata (where the surface is sometimes a very good option), but for the eventuality where divers will continue their education with AOW and deeper diving.

I live in a world where many customers should not be diving independently below... 30 feet yet hold certificates which give them the notion that they are good to 120 feet. I don't want to be part of that anymore.

 

[MXGratefulDiver]

Your comment really sums it up dale

General practice says do not dive beyond your training. ow and aow are limited depth and time certifications. As such gas management is easy, You hit xxx psi and you head up. If you or your buddy has a gas supply issue your xxx will cover it. Once you are past the aow profiles the rule of thumb is out the window and you have to calculate a real functional number. There is no need to teach ow's and aow's the ins and outs of the rock bottom other than it exists and this is what it covers,..bla bla bla..... A preview of what is to come with further training, while at the same time a subliminal warning, if you will, about pushing your training limits. That became very clear to me when i got into the cave related and trimix training pipelines. Air need to get to the surface was changed for Next availiable air. ect. Deep deco confined environments are all places the rock bottom process is vital to survival. Untill then the simple depth = xxx psi scale will work just fine as far as the curriculum goes.

It depends on where you dive ... I can think of a few OOA incidents here, some that resulted in fatalities, that were completely within the parameters of OW and AOW training. One of those was the very dive profile I used as an example earlier in this thread, which never gets deeper than the 100 feet you're supposedly qualified for once you receive an AOW card. Newer divers in particular tend to be gas limited due to poor technique, smallish rental cylinders, and high consumption rates ... so it's relatively easy to find yourself low on or out of gas while staying completely within the depth limits of your training ...

... Bob (Grateful Diver)

 

[Sqaksums]

Since there is no ocean at 8000' I am one of those vacation divers. I take a vacation to fl about every four months. I've become comfortable with sedh in pompano, out the Hillsboro inlet. Last time down there I buddied up on the boat w/ a nitrox diver from n/c. I was breathing air. Our diving profiles were different, so I followed my profile, not the buddy. At the end of a dive,I like having at least 300 ON THE SURFACE, so, if I'm waiting at the marker(for the boat or the buddy) I'm not out of air. That's my personal cardinal rule. I'll be down there on the 22, if anyone cares to buddy up.

 

[Foxfish]

It depends on where you dive ... I can think of a few OOA incidents here, some that resulted in fatalities, that were completely within the parameters of OW and AOW training. One of those was the very dive profile I used as an example earlier in this thread, which never gets deeper than the 100 feet you're supposedly qualified for once you receive an AOW card. Newer divers in particular tend to be gas limited due to poor technique, smallish rental cylinders, and high consumption rates ... so it's relatively easy to find yourself low on or out of gas while staying completely within the depth limits of your training ...... Bob (Grateful Diver)

The question is what caused the fatality and would changing the ascent procedure from 'make sure you surface with 50 b' to 'begin your ascent at a predetermined rock bottom pressure' have changed the outcome?

Prior to this thread I've never heard the term 'rock bottom' or seen anyone use it. The approach 'surface with 50 b' is evidently taught by the major agencies and is exclusively used in my experience for OW/AOW diving. Not sure about the deep diving specialty. I'd have to conclude that the major agencies consider it an unnecessary complication to their procedures. Maybe they just don't like it because it is recommended by DIR. The feuding between DIR and the major agencies is well known.

The rock bottom approach seems sensible to me. Some interpretations seem conservative. I'd probably just calculate a couple rock bottom depth pressures for a 12 L tank (only ever filled to 232 b), remember those, and then estimate other pressures and then vary them mentally for depth and tank size.

So the numbers to remember may be:

12 L tank ascent from 15 m - 60 b
12 L tank ascent from 30 m - 80 b

And mental estimates derived from those numbers would be:

15 L tank ascent from 10 m - 55 b
15 L tank ascent from 30 m - 70 b
12 L tank ascent from 23 m - 70 b

Rounded to the nearest 5 b or maybe that should be rounded up to the nearest 10 b. It would be interesting to run the numbers based on a normal ascent to approximate the ascent pressure needed to get to the surface with 50 b and then compare the two. Is it really a big deal?

I was speaking to a friend recently. He shared the following story. See if you can spot the problem. He took a freediving buddy without any scuba experience or certification for a dive. First dive they went to 12 m without incident. Second dive they went to 25 m and the buddy went off chasing fish with a spear gun. After about ten minutes my friend felt a tap on the shoulder. He turned to see his buddy pointing to his regulator. He'd run out of air. My friend doesn't have an octopus and they did the ascent on one regulator.

I suspect that most problems that occur for experienced divers are not because the diving procedures taught by the main agencies are inadequate but that they are either forgotten or ignored. Adding more complicated procedures will only compound the problem.

 

[Doppler]

... Maybe they just don't like it because it is recommended by DIR. The feuding between DIR and the major agencies is well known...

Have no idea what you mean since DIR is not an agency!

However that may be, you should understand that gas volume management is not proprietary to any one particular agency... if it were, then that agency would likely be the PSA, NSS-CDS or the NACD. These and several others (IANTD, TDI, and NAUI to my certain knowledge) were teaching and promoting the technique many years before the term DIR was first coined by George Irvine (whose birthday was yesterday).

 

[NAM001]

The diver does not have to plan the min gas. They would use a simple chart 80' is x psi. Someone else would have done the calculations for them. My point is that when the dive becomes other than the simple mnimum skills aow 80' dive then youhave to leave the simple chart behind and calculate the min gas for the dive you are doing. Once again this is really a never done thing in rec diving. If you want to make a 160 ft dive long enough to go into deco , then you are no longer diving in the restraints that the ow/aow training encompasses and hense the simple chart is no longer functional for that type of diving. The details on min gas has as much need as functional deco theory in an aow or ow class.

So how do you determine what XXX psi for that dive is?



Isn't that more or less the definition of rock bottom?

 

[Foxfish]

Have no idea what you mean since DIR is not an agency!

However that may be, you should understand that gas volume management is not proprietary to any one particular agency... if it were, then that agency would likely be the PSA, NSS-CDS or the NACD. These and several others (IANTD, TDI, and NAUI to my certain knowledge) were teaching and promoting the technique many years before the term DIR was first coined by George Irvine (whose birthday was yesterday).

In my experience and area, PADI and SSI are the major OW and AOW diving instuction agencies. The others you mention are either unheard of or at best uncommon in this area of instruction. I've noted the difference in approach specifically to gas planning for an ascent between these major instruction diving agencies and the approach by DIR and apparently the others you mentioned.

 

[MXGratefulDiver]

Have no idea what you mean since DIR is not an agency!

However that may be, you should understand that gas volume management is not proprietary to any one particular agency... if it were, then that agency would likely be the PSA, NSS-CDS or the NACD. These and several others (IANTD, TDI, and NAUI to my certain knowledge) were teaching and promoting the technique many years before the term DIR was first coined by George Irvine (whose birthday was yesterday).

... I learned it from a YMCA instructor ...

... Bob (Grateful Diver)

 

[TMHeimer]

Have been away a while so I'll admit not reading it all. Reserve should be based on what is practical--the site, current, surf, what you're planning, etc. My normal 20--30+ fsw shore dives means make sure I don't wander out too far based on the slight ebbing tidal current, beware of possible severe cramping, and aim to exit with 200-400 psi left from staring with 3000. A little under 200 is Ok as well--just don't want and water/corosion entering the tank. For boat diving, I generally use rule of thirds around 80' or deeper--shallower than that I just keep track of my gas, bottom time, where I am going, where the anchor line was, and of course my buddy (if applicable). For an 80' dive I usually find that I go about 10 mins. out, 10 mins. back, 5 minutes around the anchor line then up. I guess that's a good rule of thirds for me.

 

[DaleC]

I learned it from the internet
(Lamont's page)

I think it depends on what one is calling a simple AOW dive. I do a lot of lake diving and 99% of it would fall under the 1/2 rule. But most local divers, after getting their feet wet, consider this to be very boring stuff, like quarry diving when you live next to the ocean. I don't of course, because what I want to see is in the lake.

A much more common local ocean AOW dive though is the HMCS Saskatchewan, an artificial reef wreck.

• Main deck is at 100' and most divers do this using EAN 32% (to maximize BT) so pushing NDL's means ascent rate and SS's are mandatory, not optional.
• Wreck length is 360' and charters usually tie up to a buoy. Regaining the line for ascent is preferred to avoid retrieval so bottom travel needs to be considered.
• Current may make travel more strenuous than anticipated.
• Boats just deliver divers and do not, as a rule, supply in water DM's.

When I do this dive I consider the following and I have only taken AOW training (right after OW to boot):

I know from calculations and experience that from 100' I need 15cuft to surface at 30'/min + 3min SS (no buffer). Choosing 20cuft adds 1 minute buffer at depth. X 2 (for air sharing) = 40cuft needed for safe team ascent.

Ok, that's a static reserve volume but what does it mean as far as the dive goes. How much gas do I really need?

I know 30 minutes at depth with a .50SAC = 60 cuft bottom gas (.50sacX4atm'sX30min). But I also know that a .50sac is best case and does not adjust for effort or stress. More likely, a .75sac should be used to add buffer thus making it 90cuft bottom gas needed.

So:
Best case bottom volume + reserve volume = 100cuft.
With moderate effort: 130cuft.

But I don't own a 130cuft tank?

I could use my twin independent 72's which would allow 100cuft bottom gas + 20cuft reserve in each tank (140cuft total) or use a single tank + pony.

By taking a pony I know I can reduce my back gas reserve volume in half (by splitting it between both tanks), thus reducing the size of main tank needed. Remembering the reserve volume needed, I can take either a 20, 30 or 40cuft pony.

With a 20cuft pony I still need 80-110cuft of back gas.

By using a 40cuft pony I know I can use 20cuft from it as bottom gas and keep 20cuft for reserve. This reduces my back gas needs to 60-90cuft. So I can use my 72 or borrow my buddies LP95.

At this point, all these calculations are done pre-dive without knowing more than the basics of the dive (intended depth and BT based on mix).

Now I'm ready to dive. The only adjustments I am going to make at depth is an assessment of my gas consumption (if using a small single tank + pony). If it is good (around .50cuft/min) I will aim for max BT. If higher (.75) I will cut the BT short. I don't calculate numbers at depth because I am a math dope.

If I don't want to cut my BT short I know I will need a bigger tank. I also know that, even with a larger single tank/pony, there is very little room for penetrations, deco or going deeper so I will avoid doing those things.

Before learning about rock bottom calculations I could only guess about these things or do what someone else told me, assuming it was safe. Now I can demonstrate to myself needs vs resources and flex my understanding to do the same dives with different tanks, depths, working effort etc...

Calculating RB causes you to do two things:

Think about your gas consumption
Think about your dive parameters.