Rule of Thirds & Shallow Rec diving

[MXGratefulDiver]

... in the real world it's usually those cautious divers in doubles who have to go retrieve the bodies when the experts end up running out of air. The whole reason I wrote that article was I got tired of seeing that happen. Some of those folks used to be members of ScubaBoard ... one in particular used to deride gas management in almost exactly the same language you're using. Too bad he never took it seriously ...

... Bob (Grateful Diver)

 

[D_B]

There's a similar rule of thumb mentioned in the RB discussions: (depth x 10) + 300 which pretty much agrees with the above and probably would suit the OP's needs at this point. Also, max depth not to exceed the volume of the tank, i.e. 80 cft tank equals 80 ft max.

I believe the Rule Of Thumb you are referring to is from Lamont's Rock Bottom Gas Management (thank you Lynne for the link) http://blog.scriptkiddie.org/2010/0...ational-divers/#sthash.wfthYVRw.cOGYGNM9.dpbs

It is what I use and I can recalculate it on the fly for whatever depth we are currently at ... Depth +0 + 300 psi is what I use for my HP100 tank (example 80ft = 800psi + 300psi = 1100psi to start ascent ..
So simple even I can do it , and works for recreational depths and more importantly, it can give new divers something other than " I guess I should start up soon"

 

[DevonDiver]

... in the real world it's usually those cautious divers in doubles...

Yep, and they're cautious because they dive for the worst case scenario, in anticipation of the rare occasions when Murphy joins the dive. That contrasts greatly with the "we'll be fine, mate" divers who throw caution to the wind because their experience doesn't encompass the reality that things can go wrong, or mistakes can be made.

Ignorance is bliss. Until it bites you in the rump.

Whenever I hear so-called 'experienced' divers expressing disdain for robust risk mitigation and proper dive planning, I just know that the quality of their experience is severely limited.

 

[Diver0001]

Hey guys,
I'd love to hear you opine about this rule in regards to "vacation divers." If you're under 50 feet do you really want to be getting back on the boat with 1000psi left in your tank?
I understand in tech diving it makes sense as it is an overhead environment, but this seems overly cautious when the surface is a cesa away. I'm not advocating running it empty, but when you dive tropical beginner level dives what psi do you want to return with?

I tell my students to plan to arrive at the safety stop with 50bar (750psi). Provided you're within the NDL's and there are no other factors that would deny you access to the surface from your SS depth then 50 bar is plenty of reserve.

Were you told to be on the surface with 1000psi?

R..

---------- Post added November 27th, 2013 at 03:07 PM ----------

Having a target psi to shoot for is a great idea for divers who haven't yet learned how to manage their gas supply.

Maybe I'm missing the point here, Bob, but it seems to me that all gas planning from the most rudimentary to the most complex involves having 'target' pressures. The difference is possibly that an OW diver with limited gas management training might call it (or think of it) as a "target psi" and a technical diver who works it all out ahead of time is going to call it a "reserve".... Tomato, ToMAto. It's gas in a tank either way.

R..

 

[boulderjohn]

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 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. ...

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.

That's quite a bit of planning for a no planning dive.

I wrote a PADI Distinctive Specialty on Dive Planning. It teaches that different plans are appropriate for different sites. What you described is not far off from what is described as an appropriate plan for a dive in which you are going to explore an area surrounding a boat and then return to it. It is a bit different, though. It includes more sophisticated navigation that would have allowed you a more leeway to explore a wider area and still know precisely where the boat can be found at all times.

 

[MXGratefulDiver]

Maybe I'm missing the point here, Bob, but it seems to me that all gas planning from the most rudimentary to the most complex involves having 'target' pressures. The difference is possibly that an OW diver with limited gas management training might call it (or think of it) as a "target psi" and a technical diver who works it all out ahead of time is going to call it a "reserve".... Tomato, ToMAto. It's gas in a tank either way.

R..

What you're missing, Rob, is the purpose of the target. A "reserve" is how much gas you want to have in your tank when you end the dive ... but once you're on the surface, who cares? The "target" I'm referring to is how much gas you have when you turn your dive ... or when you decide to head up from a deeper depth to a shallower one. Those are very different things.

Telling someone to "end the dive with 500 psi" is all well and good ... but in the real world it's about as useful as giving someone directions to your house by telling them to turn left a half mile before they see the fire station ... a better approach is to tell them how much distance there is between where they start and when they need to turn ...

... Bob (Grateful Diver)

 

[Foxfish]

... in the real world it's usually those cautious divers in doubles who have to go retrieve the bodies when the experts end up running out of air. The whole reason I wrote that article was I got tired of seeing that happen. Some of those folks used to be members of ScubaBoard ... one in particular used to deride gas management in almost exactly the same language you're using. Too bad he never took it seriously ...

... Bob (Grateful Diver)

Hmm. Think I hit a raw nerve.

The only thing not real about this dive were the two tech divers I tele-ported into the story that gets played out each weekend I go diving. Oh and the novices were probably a lot more organized and cautious than most I've seen.

I'm interested to hear from some of the technical divers posting here suggesting that the way in which the novice divers conducted their dive in the story was unacceptable. Please explain. Why was it unacceptable? Provide a description of how diving in this way could result in harm to the divers.


---------- Post added November 27th, 2013 at 10:10 AM ----------

That's quite a bit of planning for a no planning dive.

I wrote a PADI Distinctive Specialty on Dive Planning. It teaches that different plans are appropriate for different sites. What you described is not far off from what is described as an appropriate plan for a dive in which you are going to explore an area surrounding a boat and then return to it. It is a bit different, though. It includes more sophisticated navigation that would have allowed you a more leeway to explore a wider area and still know precisely where the boat can be found at all times.

And all that without the need for a calculator or having a predefined route.

 

[rivers]

Whether or not you are a recreational or technical diver, your gas planning should be more than 'be back on the boat with 50 bar/750 psi'. I will agree the rule of thirds isn't the best plan for most recreational profiles. It certainly isn't what I used to plan my dive. Minimum gas is a much better plan, and it's something that is easily taught. It doesn't require calculators or anything of the sort. But it does require a diver knowing how much gas they need to get themselves and their buddy from the bottom to the surface, whilst making a safe ascent and completing any stops. Generally speaking, if I'm on a 30m, minimum gas is 100 bar if you're in a single 12. Why, because that is how much gas you need to get 2 divers to the surface (well technically it's 80 bar, but I'm not one to cut things too close), assuming a 30 l/minute SAC rate (x2). Assuming a 230 bar fill, that's 130 bar useable. If you don't need to return to the shot or anchor line, and can just ascend, you now have all useable (130 bar). If it would be nice to return to the shot line, but not necessary, you have can use halves. Once you've gone through 65 bar, time to turn around. If you hit 100 bar, but not quite at the shot, pop a blob and up you go. If you need to return the shot, now you dive thirds. You use 40 on the way out, 40 on the way back. If you reach the shot and you still have more than 100 bar, explore that area a little until you hit the 100 bar and up you head.
Obviously, if you are shallower, your minimum gas needs may be lower (ie for a 12m minimum gas may be 50 bar).
Gas planning made simple

Note:
I am not a technical diver.

 

[DivemasterDennis]

If you are on a dive charter, the boat will usually state it's policy- "tell the DM when you are at 1000 psi or 700 psi," "you must have 500 psi at the time you break the surface," or some other standard. My personla rule is to have more than zero upon surfacing. But in the normal course of events I rarely break the surface with less than 500 psi. Debbie uses little air, and regularly completes dives of close to an hour duration with more than 100 psi, depending of course upon depth, current and other factors. We do a lot of diving on our own, or our own profiles off a boat, and the dive plan we uise states the amount of air available at the time we head to the surface. Than amout is 750 psi, a little more if surfacing from deeper than 75 feet. We vary that with the dive profile. Neither of us has ever run out of air or close to it. Follow the rules of the charter or other boat you are on. Dive a safe profile, and head to the surface with ample air for a slow and safe ascent, adequate safety stop(s) and you will be fine.
DivemasterDennis

 

[MXGratefulDiver]

Hmm. Think I hit a raw nerve.

Not really. On the one hand, I don't know you, and if you should end up dead on the bottom you're nothing more to me than another screen name on the internet.

On the other hand, some of the bodies that have been pulled out of Puget Sound with empty tanks over the past 12 years were people I did know ... some were people I cared about. And those casualties did hit a raw nerve. It's why I try to give people information to help them prevent what is completely preventable.

What I've learned over the years is that you can't really teach anybody anything ... you can only put information out there for those who choose to pay attention to it. That's what I've been doing with the gas management info. A lot of people have decided it's useful. Some of those have gone on to tech diving ... but most haven't. They apply it to their recreational diving quite comfortably, and without excessive effort. If it doesn't work for you, great ... there's lots of approaches to diving that don't work for me, so I understand. But there's never a need to be disdainful of those who make different choices than you do ... particularly when it's so clear that you don't have any real knowledge about why they make the choices they do. That shows a level of cockiness that often comes back and bites you in the arse ... sometimes with tragic results, as it recently did with one particularly prominent member of this board who had a great deal more experience than you do.

There is no one "right" way to dive. If you're comfortable doing it your way, well and good ... I only hope you're as "right" as you believe yourself to be ...

... Bob (Grateful Diver)

---------- Post added November 27th, 2013 at 07:44 AM ----------

If you are on a dive charter, the boat will usually state it's policy- "tell the DM when you are at 1000 psi or 700 psi," "you must have 500 psi at the time you break the surface," or some other standard.

Where I dive, the charters don't have DM's ... you get in the water with a buddy and conduct your own dive. The only person you're expected to communicate with during the dive is your buddy.

Now here's something else to think about ... if you should for any reason find yourself low on air, or out of air, because you weren't paying attention to your gauge ... how likely is it going to be that your buddy will have enough for the both of you to surface with?

... Bob (Grateful Diver)