diving thirds vs rock bottom

[Centrals]

How about both divers have significant different SAC?
You will need a conversion table.
It does get complicated in Basic Scuba discussion.

 

[CamG]

Greetings Smokey and good thread with great responses.
Personally I use both to mull over dive plans and usually use a more conservative gas plan for advanced dives.
Any Tech or Cave dive is always more conservative or padded with more gas allowing for the buddy bail out.
This is deco gas as well which is beyond true Basic but how I roll.

On normal Rec dives I still do quick calculations and try to roll with far more gas than I think I will need.
Since I started diving two tanks it has been a mute issue.
People give me strange looks at times but it is my choice I dive like I train, they do not have to like it!
I SM the shallow stuff where it is tougher entries.

Either tool you use you are demonstrating a desire to form a gas plan. Kudos!
To many just use the back on boat with 500-700psi, which is ok for shallow easy dives but toss depth and current in and it can get dicy.
It really is up to you but I always lean on the conservative when it comes to gas.
I want to be able to account for the worst failures, get my buddy out form max depth and deco obligation.
To conservative to many yes but to me NO!
If the dive looks insane then I alter or bag the plan all together.
It only takes one dive when things go crazy and your gas is the determining factor in whether you get out alive and I promise you will never forget IF YOU SURVIVE IT!

CamG Keep Diving....Keep Training....Keep Learning!

 

[lv2dive]

Uh Lynne . . .your SPG reads in pressure units, does it not?

For a buddy team with dissimilar tank sizes the point is to derive a usable turn pressure for your own tank, AFTER calculating a normalized volume of Rock Bottom and/or Thirds for the teammate with the smallest tank. The arithmetic is fairly easy and straight forward (in metric of course):

Example:
12 litre/bar tank multiplied by 180 bar equals 2160 litres
15 litre/bar tank multlied by 150 bar equals 2250 litres

Therefore the 12 litre tank has the smaller volume at 2160 litres.

Calculate thirds of smaller volume tank's fill pressure of 180 bar, and you get 60 bar delta turn pressure. 12 litres/bar multiplied by 60 bar equals 720 litres volume (the normalized value with respect to the smaller tank).

Hence , the calculated turn pressure for the larger volume tank is:
720 litres divided-by 15 litre/bar equals 48 bar.

(How objectively simple was that?)

Can you go through the same "objectively simple" exercise with 104's filled to 3600 and 85's filled to 3800 without converting to volume?

 

[boulderjohn]

First of all, this is way too complex for the Basic Scuba section.

I'm not sure if it's too advanced for a basic scuba discussion.....rock bottom is included in gas planning/ dive planning in basic open courses. I'm glad that it's being discussed as its an important part of diving safely.

I think the rest of the thread shows why it is too complex for recreational diving. If you are talking about basic rock bottom process alone that Lynne (TSandM) describes in post #16, then, yes, it is basic enough for Basic Scuba. Comparing it to the rule of thirds brings in the unnecessary complexity. Lynne again does a good job of giving a basic overview of that in that post and in subsequent posts, and she is correct in saying that it will normally have no true use in recreational diving. Unfortunately, it will take a whole lot of discussing to iron out all the other misinformation that has arisen in this thread.

And Rock Bottom is NOT included in gas planning/dive planning in basic open water courses unless the instructor puts it there in addition to standard instruction. I include planning for ascent when I teach OW, and I do not teach the rule of thirds in that class.

I created a PADI-approved Distinctive Specialty class in dive planning that includes an extensive section on gas planning. In negotiating the outline with PADI, the discussion about that section centered upon what was appropriate for recreational diving and what was appropriate for technical diving. What I ended up with is completely designed for recreational diving, and it does include an overview of the use of the rule of thirds, which I contend should be a rare event in such diving. It includes Rock Bottom. That chapter goes into why you use different plans in different situations, and how to decide which is correct. It is the complexity and length of that chapter that makes me think it is a bit much for this forum.

---------- Post Merged at 09:01 AM ---------- Previous Post was at 08:55 AM ----------

You plan a dive, calculate how much gas that you'll need, then take twice as much. Pony up. Hang bottles. Stage bottles. This isn't tennis, you know.

I am not sure I understand your point.

Let's say I want to do a simple recreational dive to 60 feet, with the ability to ascend at any point in case of emergency. I determine that for the length of time I want to be at that depth, 60 cubic feet, the amount of gas frequently used by an OW diver on a recreational reef dive with an AL 80, will be sufficient. Are you saying that I need to bring 120 cubic feet with me to do that dive? Or are you saying that if you think you will need an AL 80 to complete a dive, you should bring 2 AL 80s?

 

[PfcAJ]

You plan a dive, calculate how much gas that you'll need, then take twice as much. Pony up. Hang bottles. Stage bottles. This isn't tennis, you know.

Ponies and hang bottles? Come on.

Its not tennis, but it isn't a moon landing either. Reserve enough gas to get you and your buddy home. Dragging extra nonsense for no reason is miserable and without point.

 

[windapp]

How about both divers have significant different SAC?
You will need a conversion table.
It does get complicated in Basic Scuba discussion.

Thirds takes into account sac rates. Example (assuming same size cylinders).

Diver A starts with 1800 psi. Diver B 3000. Use 1800 for rule of thirds. Diver B breaths twice as fast as Diver A. Diver B has 2400 PSI at the turnaround, and Diver A has 1500. Diver B has a total air supply failure at the turnaround. Diver B requires 600 PSI. Diver A requires 300. That totals 900, leaving an extra 300.

Different sac rates always result in a more conservative plan when using rule of thirds.

My understanding that cave trainers usually advocate using the same size cylinders to eliminate needed conversions, and the mistakes that can be made by having to convert.

 

[lowviz]

...//... What I ended up with is completely designed for recreational diving, and it does include an overview of the use of the rule of thirds, which I contend should be a rare event in such diving. It includes Rock Bottom. That chapter goes into why you use different plans in different situations, and how to decide which is correct. It is the complexity and length of that chapter that makes me think it is a bit much for this forum....//...

Kudos,

I'll stand down.

 

[decompression]

I think the rest of the thread shows why it is too complex for recreational diving. If you are talking about basic rock bottom process alone that Lynne (TSandM) describes in post #16, then, yes, it is basic enough for Basic Scuba. Comparing it to the rule of thirds brings in the unnecessary complexity. Lynne again does a good job of giving a basic overview of that in that post and in subsequent posts, and she is correct in saying that it will normally have no true use in recreational diving. Unfortunately, it will take a whole lot of discussing to iron out all the other misinformation that has arisen in this thread.

And Rock Bottom is NOT included in gas planning/dive planning in basic open water courses unless the instructor puts it there in addition to standard instruction. I include planning for ascent when I teach OW, and I do not teach the rule of thirds in that class.

Just to clarify, rock bottom is part of recreational 1 UTD course (basic open water), apology accepted.

 

[Peter Guy]

OP -- I think a big part of the problem with your questions, and many of the responses, is because "Rock Bottom" and "Thirds" (or "halves" or "all available gas") are really about two different concepts.

"Rock Bottom" is merely (!) about the concept that a diver should always have enough gas to get him and his buddy back to the surface safely while making all planned stops. It is a dynamic number (that is, RB for 100 ft is different from RB for 30 ft) and is based on certain assumptions as to how much you and your buddy will consume (stressed SAC rate). The "Rock Bottom" number is a safety margin number.

On the other hand, "Thirds" (or "halves" or "all available gas") are concepts relating to how a diver plans to use (and uses) his gas (in particular, the "available gas") during the dive. IN RECREATIONAL DIVING (caves are a special case and NOT part of this) a risk averse (or to me, a properly trained!) diver will always use Rock Bottom AND Thirds/Halves/All Available together for each dive because they complement each other.

So, sorry to say, I can't answer your questions because you are asking the wrong questions.

For what it's worth, I have posted my slides from my standard "Air Supply Management" talk and a perusal of them might help you understand the differences.

Air Supply Management Slides

 

[lowviz]

...//... a risk averse (or to me, a properly trained!) diver will always use Rock Bottom AND Thirds/Halves/All Available together for each dive because they complement each other. ...//...

Indeed!