Rule of Thirds & Shallow Rec diving

[victorzamora]

OTOH, people around here have ten-threehundreds, or twelve-two-three-twos, or fifteen-twohundreds. I've hardly met anyone who just has a ten, twelve or fifteen. At least when we talk about single tanks.

I can honestly say I've never seen that written or heard that said before your post. Every other reference I've seen to metric tank sizes was just: 12L, 11L, 15L, etc.

With Imperial, I dive HP100s because they didn't have LP85s when I bought my tanks and they've got a smaller diameter than LP95s. Immediately, everbody (imperial) reading this knows pressure and nominal gas capacity.

 

[Foxfish]

I asked:

I'd be interested to know:

1. Where the various requirements for 'rock bottom' gas planning originated;
2. Which of the dive instruction agencies are now teaching this method; and
3. On which courses they are teaching the method.

One response was:

4. You need to have enough to get you both to the next breathable gas(surface or deco) while allowing enough gas to do any required stops.

I was questioning the origin of the specific requirements like the one minute stops at certain depths. Is is a part of the published course notes of any of the major OW/AOW diving instruction agencies.

So far I've got that it is taught by DIR. Did they make up the rules for determining the amount of air required for the 'rock bottom' calculation.

Any DIR agency teaches this style gas planning. It has many names but the most common are "min gas" and "rock bottom" it is taught from the very beginning. I know some NAUI instructors teaching this style gas planning, especially in their more advanced classes.​

​

 

[rongoodman]

PADI talks about 3 min. safety stops, GUE and UTD use 'minimum deco', or you might have incurred some required deco. Minimum gas/Rock Bottom just implies you'll have enough gas for the two of you to finish whatever stops your dive plan calls for. There's nothing particularly agency specific about it, and if you want to change the parameters, stressed SAC rates, or whatever, that's up to you. If you don't know that you have enough gas to get you and your buddy out of trouble, though, you're going way out on a limb. Rock Bottom is a methodology to do the analysis, rather than guessing, or quoting some rule of thumb like 'be back with 500 psi'.

(There is no dive agency called DIR. There are several which use the philosophy.)

 

[DaleC]

Rock bottom was probably derived from an earlier concept known as Bingo fuel which was popularized by pilots during WWII needing to determine when to turn flights with enough fuel to successfully return to the nearest friendly base.

The similarities to gas needed to reach exits during diving appear to be obvious. How much gas do you need, when considering:

Consumption rates adjusted for depth
Ascent rates
Stops
Overheads (deco, kelp, boat traffic, specific exits)
Current
Air sharing
Safety buffer considerations

Many dives are different so there is no one rock bottom calculation. Some dives follow similar profiles so "rules of thumb" or preset values) may be used.

Those interested in considering themselves "divers" in the sense of being able to move from one dive profile to another (without guidance) should be familiar with some realistic method of calculating rock bottom values. The degree of complexity varying with the dive profiles.

If you only have rudimentary skill in this area then you are restricted to rudimentary dive profiles, using guides or carrying ridiculous amounts of gas to compensate for having no clue as to actual anticipated consumption. For tourist divers, some charters use all three: simple dives, guides and large reserve values.

Learning to calculate rock bottom values is not an ego stroke as there is nothing wrong with doing simple dives if that's your thing. Lot's to see at shallow sheltered depths. It is a matter of safety though, when a diver begins to expand their experience to include unguided trips to more complex (than rudimentary) sites. At deeper depths, with other sensible divers, it becomes problematic to solely rely on retaining ridiculous amounts of reserve volume for safety and some may balk at this glaring deficit of knowledge.

 

[Storker]

Many dives are different so there is no one rock bottom calculation. Some dives follow similar profiles so "rules of thumb" or preset values) may be used.

Those interested in considering themselves "divers" in the sense of being able to move from one dive profile to another (without guidance) should be familiar with some realistic method of calculating rock bottom values. The degree of complexity varying with the dive profiles.

Rock bottom calculations while underwater is too much for me. I've got enough task loading as it is, so I've made a small table giving depth and minimum pressure at that depth and put it permanently on my slate. If my tank pressure goes below my predetermined minimum pressure, it's time to either get to the surface or, according to my SOP, continue the dive at a shallower depth.

I check that table when planning the dive, and I check it during diving if we for some reason choose to change the planned depth profile. Almost none of my dives are square profile, virtually all of them follow some kind of multilevel profile, so a high number for minimum pressure at some depth isn't limiting to my diving. For dives deeper than 18-20m starting with a full tank and with a normal SAC rate, I reach the NDL before I reach the minimum pressure anyway, and for shallower dives I can comfortably get 45-60 minutes' worth of diving before reaching minimum pressure. After 45-60 minutes, either my fingers are getting cold(ish) and/or my bladder starts nagging me, so it's time to go topside anyway, to take a pee and have a cup of hot coffee. :cool2:

This is the table, based on a breathing rate which is slightly on the high side¹ for me (16 liters/min - 0.57 cu.ft/min - at surface pressure). It's pretty simple: depth (in meters) and minimum pressure (in bar) for the two tank sizes I'm using:

[TABLE="width: 500"]
[TR]
[TD]Depth[/TD]
[TD]10L[/TD]
[TD]15L[/TD]
[/TR]
[TR]
[TD]10[/TD]
[TD]50[/TD]
[TD]35[/TD]
[/TR]
[TR]
[TD]12[/TD]
[TD]75[/TD]
[TD]55[/TD]
[/TR]
[TR]
[TD]15[/TD]
[TD]80[/TD]
[TD]60[/TD]
[/TR]
[TR]
[TD]18[/TD]
[TD]90[/TD]
[TD]65[/TD]
[/TR]
[TR]
[TD]20[/TD]
[TD]95[/TD]
[TD]70[/TD]
[/TR]
[TR]
[TD]22[/TD]
[TD]100[/TD]
[TD]75[/TD]
[/TR]
[TR]
[TD]25[/TD]
[TD]110[/TD]
[TD]80[/TD]
[/TR]
[TR]
[TD]30[/TD]
[TD]120[/TD]
[TD]90[/TD]
[/TR]
[/TABLE]


¹ These days I'm down to about 14-15 SLM (0.49-0.53 cu.ft), but hey, there's no harm in being conservative, right?

 

[rivers]

Fox fish, here is a very good article explaining minimum gas/rock bottom, and how to do the calculations.
Minimum Gas Explained | Theory Articles | Articles | DiveDIR

And to answer your other questions:
both GUE and UTD, as well as a few other lesser known agencies, teach minimum gas/rock bottom.
GUE teach it from rec 1 (their OW course) or fundies if you did your OW with another agency. UTD do the same (except its essentials instead if fundies).

 

[NAM001]

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.

Rock bottom was probably derived from an earlier concept known as Bingo fuel which was popularized by pilots during WWII needing to determine when to turn flights with enough fuel to successfully return to the nearest friendly base.

The similarities to gas needed to reach exits during diving appear to be obvious. How much gas do you need, when considering:

Consumption rates adjusted for depth
Ascent rates
Stops
Overheads (deco, kelp, boat traffic, specific exits)
Current
Air sharing
Safety buffer considerations

Many dives are different so there is no one rock bottom calculation. Some dives follow similar profiles so "rules of thumb" or preset values) may be used.

Those interested in considering themselves "divers" in the sense of being able to move from one dive profile to another (without guidance) should be familiar with some realistic method of calculating rock bottom values. The degree of complexity varying with the dive profiles.

If you only have rudimentary skill in this area then you are restricted to rudimentary dive profiles, using guides or carrying ridiculous amounts of gas to compensate for having no clue as to actual anticipated consumption. For tourist divers, some charters use all three: simple dives, guides and large reserve values.

Learning to calculate rock bottom values is not an ego stroke as there is nothing wrong with doing simple dives if that's your thing. Lot's to see at shallow sheltered depths. It is a matter of safety though, when a diver begins to expand their experience to include unguided trips to more complex (than rudimentary) sites. At deeper depths, with other sensible divers, it becomes problematic to solely rely on retaining ridiculous amounts of reserve volume for safety and some may balk at this glaring deficit of knowledge.

 

[supergaijin]

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.....

I disagree. In my neck of the woods a typical AOW depth is 4ata. This is deep enough to go beyond 'Rock Bottom' gas reserves. I am training people to dive to those depths without a 'pro' looking out for them. 5ata is not unheard of in SE Asia and this is really too deep for many 'divers' who have thus far survived simply because they have not dived enough times for Murphy to get around to paying them a visit.

I include a 'Rock Bottom' briefing in most of my OW classes. It depends on the individuals and language abilities but all understand the concepts and simple calculations involved. I have them write their RMV and Rock Bottom equation (calculated during training) in to their logbooks and ask them to consider this if/when they start diving deeper than 3ata. Diving shallower tha 3ata means simply swimming towards the surface will probably be OK depending on previous dive profiles.

In my AOW Deep dive 'Rock Bottom' is part of the course but according to my agency (PADI), I will not fail a student for not understanding it. I have never had to as it is so simple to grasp why 'having enough gas to surface in the (eventual) emergency' is the most important safety-aspect of scuba diving below OW depths.

 

[NAM001]

You do this because you are teaching outside of the standard definition of ow and aow. Ie no overhead ow to 60' no diving beyond ndl.
Your neck of the woods aparently is teaching deep and beyond skills, of which the most basic gas planning is inadaquate. The terms ow and aow is a term to reference to a set of minimal standards. Those minimal standards and the dives associated with them do not need complex gas planning. I know that agencies and instructors vary in thier courses. It is a double edged sword. no ow needs to know rockbottom calculations to do a 40 ft dive. same goes with a aow going to 90 '. If you are training your students to competantly finction at 5ata and you are issueing a ow card for it, then they are not ow divers they are deep divers with an ow card. As such a deep diver needs a much more indepth knowledge of minimum gas than the true ow diver that has minimum skills.

I disagree. In my neck of the woods a typical AOW depth is 4ata. This is deep enough to go beyond 'Rock Bottom' gas reserves. I am training people to dive to those depths without a 'pro' looking out for them. 5ata is not unheard of in SE Asia and this is really too deep for many 'divers' who have thus far survived simply because they have not dived enough times for Murphy to get around to paying them a visit.

I include a 'Rock Bottom' briefing in most of my OW classes. It depends on the individuals and language abilities but all understand the concepts and simple calculations involved. I have them write their RMV and Rock Bottom equation (calculated during training) in to their logbooks and ask them to consider this if/when they start diving deeper than 3ata. Diving shallower tha 3ata means simply swimming towards the surface will probably be OK depending on previous dive profiles.

In my AOW Deep dive 'Rock Bottom' is part of the course but according to my agency (PADI), I will not fail a student for not understanding it. I have never had to as it is so simple to grasp why 'having enough gas to surface in the (eventual) emergency' is the most important safety-aspect of scuba diving below OW depths.

 

[boulderjohn]

As such gas management is easy, You hit xxx psi and you head up.

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

If you or your buddy has a gas supply issue your xxx will cover it.

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