Rule of Thirds & Shallow Rec diving

[MXGratefulDiver]

Agreed that it is good to plan ahead. My plan was listed in the last response in case you missed it. That is more detailed than most people I've dived with.

There are some very skilful divers on this forum, but sometimes the advice given is out of whack with what you'd expect on a recreational dive. The OP gives one example. If it puts your mind at ease I wasn't thinking of anyone in particular on this thread. Relax. :kiss2:

It's not rocket surgery ... I teach gas management in AOW ... and sometimes do seminars on the topic for recreational divers from instructor all the way to OW student ... and I've yet to have anyone indicate that they had any difficulty comprehending the subject ...

... Bob (Grateful Diver)

 

[Foxfish]

It's not rocket surgery ... I teach gas management in AOW ... and sometimes do seminars on the topic for recreational divers from instructor all the way to OW student ... and I've yet to have anyone indicate that they had any difficulty comprehending the subject ...

... Bob (Grateful Diver)

I looked at your notes Bob. What's wrong with working off the numbers I gave and simply adjusting them if you find you consistently are high or low when your hit the surface? As long as you're monitoring your air on the dive and head for the surface when you hit those pressures you won't have problem.

 

[MXGratefulDiver]

I looked at your notes Bob. What's wrong with working off the numbers I gave and simply adjusting them if you find you consistently are high or low when your hit the surface? As long as you're monitoring your air on the dive and head for the surface when you hit those pressures you won't have problem.

Nothing wrong with it at all ... I applaud you for putting some thought into it. Most folks don't, beyond looking at their gauge and aborting the dive with it gets into the red zone. And even that works somewhat ... until the day they forget to look ...

... Bob (Grateful Diver)

 

[Foxfish]

Foxfish, have you read NW Grateful Diver's article on gas management? Or Lamont Grandquist's? Or my husband's? All of them will give a little more specific tools for figuring out how to manage your gas than, "If I'm a long way from the boat, I may turn the dive on 100 bar."

None of this is "tech diving". It's rational gas management, with a strategy that's appropriate for the kind of dive one is doing.

Like I said, our dive plan is - jump in and swim around. We only occasionally visit the same location but even then it can be approached from a different location. At best we get vague directions to the best sites and the range of likely depths from the dive master. After about twenty minutes of diving we've usually got only a vague idea of where the boat is. Rigorous gas planning and navigation in these circumstances is pointless. It's these vagaries that some technical divers seem to find hard to understand and accept.

I suspect that some of this relates to the kind of diving you are used to doing. If you are diving the same areas regularly then rigorous navigating and gas planning becomes more feasible. Again I'd question its usefulness.

 

[TSandM]

If you're just swimming around, and you don't have to get back to the boat, then that's an "All available gas" dive, and the only reserve you need to maintain is enough to get you and your buddy to the surface in the event of a gas emergency. That's totally dependent on depth (and your baseline gas consumption) and is pretty easy to calculate and memorize. I don't try to know what my reserve for 73 feet is; I have memorized 100 feet and 60 feet, and just use those. Obviously, if you start your dive at 100 feet but move up into the shallows, the necessary reserve changes through the dive. I've chased a gorgeous jellyfish in 6 feet of water until my tank was quite a bit into the red . . . But of course, I had no surface swim to look forward to and the water was typical Puget Sound, and glassy flat.

I think all responsible, regular divers develop a "sense" for how to manage their gas -- one rarely hears of a well experienced diver simply running out of gas. But especially for novices, and for people who do not have the pleasure of getting to dive their own tanks, having a conceptual structure to organize different types of diving, and to understand that reserves loom significantly larger when you go deeper, or when the constraints on what you can do become greater, is an awfully useful tool. I'm not objecting to your strategy for yourself, Foxfish; I just wanted to refute the allegation that gas management, as presented in the articles to which I linked, is a "techie" thing. It's not.

 

[Foxfish]

If you're just swimming around, and you don't have to get back to the boat, then that's an "All available gas" dive, and the only reserve you need to maintain is enough to get you and your buddy to the surface in the event of a gas emergency. That's totally dependent on depth (and your baseline gas consumption) and is pretty easy to calculate and memorize. I don't try to know what my reserve for 73 feet is; I have memorized 100 feet and 60 feet, and just use those. Obviously, if you start your dive at 100 feet but move up into the shallows, the necessary reserve changes through the dive. I've chased a gorgeous jellyfish in 6 feet of water until my tank was quite a bit into the red . . . But of course, I had no surface swim to look forward to and the water was typical Puget Sound, and glassy flat.

Which was pretty much what I said.

I think all responsible, regular divers develop a "sense" for how to manage their gas -- one rarely hears of a well experienced diver simply running out of gas.

I heard we had three divers who ran out of air in one weekend in our city. All ended up in the chamber and at least one died. I heard in one case it was simply ignoring the need to surface with 50 b. Learning gas management doesn't stop stupidity.

But especially for novices, and for people who do not have the pleasure of getting to dive their own tanks, having a conceptual structure to organize different types of diving, and to understand that reserves loom significantly larger when you go deeper, or when the constraints on what you can do become greater, is an awfully useful tool.

A few weeks back I asked one diver how much air she had in the tank. She looked at the compass on her console and read of 190 degrees. Seriously, how many divers do you think remember how to calculate their gas consumption let alone use it to plan a dive. They don't need to.

I'm not objecting to your strategy for yourself, Foxfish; I just wanted to refute the allegation that gas management, as presented in the articles to which I linked, is a "techie" thing. It's not.

You previously stated that if I knew how to do gas planning I'd be able to do a better assessment of the pressure at which I needed to turn the dive. It would be interesting to hear how you'd actually do this calculation given the variables like how fast you fin, how well you can navigate and of course there is the problem of knowing how far you are from the boat in the first place. Are you going to do this calculation under water? Doing it on the surface presupposes you know how far you will end up from the boat. We don't!

I read the article by NWGratefulDiver. It tells you how to calculate your air consumption. I learned how to do that on my OW. Where did I say that was a "techie" thing?

Seems to me that you sometimes read things into what I'm saying that were not intended. I'm normally fairly careful in what I say but at the same time I don't try to think of every possible angle that people may take on the comments. For example, in this case I'm not saying that learning how to determine your air consumption rate and using that to manage your gas should be omitted from the course. It gives you a taste of what can be useful in certain circumstances. Divers should however understand that this level of planning except maybe in its crudest form has limited application on most recreational dives. If nothing else, it helps you understand this to be the case.

I'd contend that for most recreational diving, knowing how long we have on a dive, when to turn the dive and at what pressure we should ascend is best assessed from a few simple rules of thumb tweeked with experience rather than by calculation or rigorous gas management plans. I don't see any convincing argument against this.

 

[supergaijin]

Circles within circles....

It comes as no surprise that posters here are talking from personal experiences and strategies. However not all dive sites or profile are alike.

As mentioned, Rock Bottom is a tool to know when to leave your maximum depth on 'deep' dives.
'Thirds' is used for penetrations and/or when you must return to a specific point to end the dive with ample reserve.

When buddy divers make a plan and dive the plan, air consumption and turn pressures should be agreed upon. Turn pressures can be calculated or guessed at. It's safer to calculate it, but some believe guessing is also OK. Experienced divers can 'guess' based on experience. Novices cannot as they have no experience.

Guess vs. Calculate... hmmm

 

[DevonDiver]

I heard we had three divers who ran out of air in one weekend in our city. All ended up in the chamber and at least one died. I heard in one case it was simply ignoring the need to surface with 50 b.....how many divers do you think remember how to calculate their gas consumption let alone use it to plan a dive. They don't need to......I'd contend that for most recreational diving, knowing how long we have on a dive, when to turn the dive and at what pressure we should ascend is best assessed from a few simple rules of thumb tweeked with experience rather than by calculation or rigorous gas management plans. I don't see any convincing argument against this.

Orly?

 

[TSandM]

I'd contend that for most recreational diving, knowing how long we have on a dive, when to turn the dive and at what pressure we should ascend is best assessed from a few simple rules of thumb tweeked with experience rather than by calculation or rigorous gas management plans. I don't see any convincing argument against this.

Well, to begin with, a novice diver doesn't have any experience with which to tweek the rules . . .

I guess we will just have to agree to disagree. I think these tools are very useful. I don't quite understand your description of the diving you do . . . I wouldn't be very happy with a dive plan where I didn't know how deep I was going to go or where I was going to go, and where I didn't know where I was underwater (unless it didn't matter). We often don't know where the boat is, but that's because the boat is following us and will pick us up here we surface. But as I said, that falls under the "all available gas" rule. Planning shore dives as halves (or anchored boat dives, where it doesn't matter if you get back to the boat) is a USEFUL way of apportioning gas and understanding at what pressure you ought to be heading back. Realizing that dives where you MUST get back to your point of origin put some significant constraints on your gas plan is also important.

I like giving people tools, and I think if we gave more novice divers these tools, we'd see fewer people out of gas.

 

[Foxfish]

Jumping off the boat without a predetermined and pre-calculated dive plan is nothing to worry about. A little story to show how its done.

You board the boat with your buddy. It's a big double deck charter boat about 30 m long. You and your buddy have done about 20 dives but this is the first one in Perth, Australia. You've heard that the dives are pretty unstructured but recall the suggestions of someone on Scubaboard.

You discuss the dive with you buddy. You're a little nervous and don't want to venture too deep or too far from the boat. Your buddy feels the same way and you agree to a maximum depth of 12 m. In the site briefing the dive master points out roughly where the main attractions are located. He tells you reef depths range from 6 to 25 m.

You jump in and start poking around. Visibility is 15 to 20 m. You follow the anchor line for a while and then start doing little circles first away from the boat and then back again. After a while the circles get bigger. You see pretty fish, little caves with colorful corals and sponges that beckon you to explore. You monitor your computer regularly and in what seems like no time your tank is down to 70 bar. You're at 12 m near the boat and signal to your buddy to begin the ascent. You surface near the boat with 60 b and make a mental note that on the next dive you'll start the ascent at 60 b if it is not too deep. Everyone comes back raving about the things they saw. Your one regret was those things you saw off in the distance that you didn't explore. You agree with your buddy to look move away from the boat on the next dive. You notice a couple on the boat arguing with the dive master. They're in full kit but missed the first dive.

The second dive site is similar but shallower. Depths range from about 6 to 15 m. You descend and your buddy signals a direction to follow. You check the compass to establish where north is and make a mental note of where the boat is located. You follow your buddies direction for a while and then signal an alternative direction. Again you check for north and make a mental note of the direction to the boat. You're going at a leisurely pace and there is so much to explore so progress is slow. Soon you reach the agreed limit of 100 b to turn the dive. At this stage you are only 100 m from the boat. You head straight back to where you think the boat is located. In a few minutes you see the anchor line. You've still got 80 b and do some circles around the boat. You're pleasantly surprised to find that one of the best spots on the dive is almost directly under the boat. You make for the surface with 60 b and finish the dive a few metres from the back of the boat.

It's been a great day. You again notice that couple talking to the dive master. They look annoyed. They're in dry suits and have twin tanks. The temperature is about 36 degrees C and they're bright red. Obviously they're tourist tech divers. You overhear them asking about the terrain of the area. They've got a big map spread out over the deck, a ruler and calculator. You hear words like air consumption rate and gas management plan. You've had a hot shower, got changed and finish off the last bit of apple crumble and custard when all of a sudden the tech divers give a shout and hold up a piece of paper with a triumphant look on their face. The dive master mutters something about an idiot gas management plan. There is a clunk as the anchor is pulled into position. The boat heads back to port.

The end.