Rule of Thirds & Shallow Rec diving

[mala]

For vacation diving I have a rule of halves, when half my air is gone I ascend to half the depth and turn around 180 degrees to head back to the boat.

that's the most sensible plan I have seen on this thread so far.

 

[TSandM]

Foxfish, you really are trying to make this into more than it is.

If I get on a boat and the boat takes me out to a reef, what I want to know is what the topography of the area looks like, and what the likely range of depths I might see are going to be. Any boat with a depth sounder can tell you that. Even when we have dived sites we've never been to before and about which nobody seems to know anything, we can run the boat back and forth and see whether the bottom slopes, and if so, how much!

So, assume the max depth I could reach (or want to reach) on the dive is 30m. I know the reserve for 30m is 40 cf, so I figure out what 40 cf is in my tank (in mine, 1100 psi) and I take that out of what I'm willing to use at depth. I now have roughly 2300 psi in my tank, that I can apportion in a variety of ways. I can use it all and surface from 30m (live boat). I can use half of it and then turn back to the boat. Or, if I absolutely MUST get back to the boat before I surface, I'd better use only a third of what I have. Then I go diving. Now, it may be that we wander around and never get very far from the boat, so going back to the anchor line when we hit half of our usable gas doesn't make sense. But you have to KNOW that you aren't far, to change the dive plan. (Sometimes one does know that!) It may also be that we find so much cool stuff in 15 meters that the dive is much shallower. Then we adjust the safety reserve on the fly, to 20 cf or 700 psi.

I realize that looking at a post like Kevrumbo's makes this seem like rocket science, but it isn't. I've used these tools for guided dives; for dives on unknown sites; for drift dives and anchored dives. It takes very little time to put this kind of gas planning framework in place for a dive.

And btw, when I did the dive Bob is describing for the first time, I did the gas planning and concluded that using an HP130 would give me plenty of gas. My buddy decided to bring along an Al40 pony for gas reserve. I finished the dive having used almost EXACTLY the gas we calculated we needed. It was actually a fun exercise!

 

[Nemrod]

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?

Whilst in Cozumel a week ago, our group of divers upon arriving were observed apparently by some of the DMs as being "old" and some apparently "fat". I do not think however, that they could be possibly talking about me? I am not old and I am not fat and I was highly perturbed. So, this was a mistake upon the DM. I told my wife not to breath, apparently some of the other of the group must have had similar ideas. We made the DMs stay in the water for 1.5 hour dives several times and most of the group was still at or above 1,000 psi. At a dive briefing the next day the poor DM stated that he would call dives at 1 hour or no more than 1.5 hours since he had to obey nature's call and that was his (physical) limit.

Now, truly I was not wanting to harm the fellows, by pushing them to deco. They have to dive everyday so they need to be conservative, but pretty sure they were no longer concerned about being old and, er, fat. Fat, WTH. I told my wife I was going to make them suck their tanks flat. Well, might not can do that with a Coz DM, but can make them want to get out to pee. Just kidding, they were all super nice and at least one of them has no SAC rate, lol. They are fish. But so are we.

DM expertly putting the kill on a lion fish:

The point is, I do not consider having 1,000 psi in the bank at surface to be a bad thing. On one dive, at least three of us, including me, went into deco, even with Nitrox. On that dive 90 feet was the max depth and I still had nearly one thousand psi on the gauge coming up the steps, 1 hour, 29 minutes. Of course, by the time I had worked my way back to 20 feet, the computer was happy again. Even for long dives in moderate depths (50 to 100 feet) it may well be your nitrogen partial pressure, not your tank pressure that calls your dive.


For recreational diving, at moderate to shallow depths, at one thousand psi notify your buddy, begin working upwards (or sooner actually), if possible so that at 700 psi (500 minimum) you do your 3 minute stop. This assuming the boat will pick you up on a drift. If, however, your dive will require a swim or an up current leg to return to an anchor line or entry/exit point, I would be more in favor of sticking with the thirds so that you arrive back to the ascent/exit point with 1,000 psi, assuming a no deco profile.

N

 

[Hatul]

DiveNav has an iPad app DivePal, that allows you to draw a dive profile and it will work out the gas consumption for you for the profile. It's very useful for gas planning without having to do all the arithmetic. I've used it a couple of times on deep solo shore dives.

Mostly when I dive I just follow the Remaining Bottom Time of the Galileo, which is quite accurate and does the math for me in real time. To account for Rock Bottom for my buddy I just add a required few minutes by a simple rule: the time in minutes to ascend at 30 ft/minute +1. So say I'm at 90 feet, it takes 90/30=3 minutes to ascend, so require 4 minutes of RBT at minimum for Rock Bottom. If you set a reserve of 550 psi it allows for safety stop for both divers. I have the math to back up this simplified rule.

Note that as I ascend my gas requirements decrease dramatically and I can immediately see what they are in real time. I can improvise the dive and still meet the safety requirements. This is one reason I like air integration at least for recreational open water diving. One thing to mention is that this rule applies only to the Uwatec, Oceanic and Atomic implementation of "air time". A different rule applies for Suunto's and Lynx air time definition.

If I need to get back to the boat's anchor line before ascending I just add a requirement to my remaining bottom time RBT. If I'm solo diving and need a couple of minutes to get to the anchor line I add 2 minutes to RBT. If buddy diving I just double that and add 4, in case I need to share my air.

 

[KeithG]

that's the most sensible plan I have seen on this thread so far.

that rule is what i posted a while ago and seems to be universally followed by Caribbean vacation dvier operations. i just labelled it as "turn at 1500 psi".

and yes: foxfish seems to be over complicating the issue.

it appears that foxfish is mixing a "navigation failure" with a "gas management failure" situation.

if you can't navigate you are already in trouble if you are leading your own dive...

 

[boulderjohn]

that rule is what i posted a while ago and seems to be universally followed by Caribbean vacation dvier operations. i just labelled it as "turn at 1500 psi".

I have only dived in about a dozen Caribbean islands, so maybe I don't have enough experience to judge. I have only encountered two islands where that plan ("when half my air is gone I ascend to half the depth and turn around 180 degrees to head back to the boat") would be common. With my limited experience, I can't call it "universal."

As has been stated several times in this thread, you have to make a dive plan that makes sense for the site you are diving. If you only have one plan in your repertoire, you will have limited success when you come to a site where that plan really doesn't work.

 

[victorzamora]

Agreed, I've never seen this before.

 

[Foxfish]

So it's now obvious that when some tech divers claim you need to know the route of the dive, the depth profile along that routes and distances and directions along the various legs of the dive and that they can do the dive plan using mental mathematics in less than the time it takes to 'read a post out loud' it was too good to be true. But we all knew that.

 

[tracydr]

"What is the problem with surfacing part way through the dive. If it was unsafe due to say shipping in the area I'd probably avoid the dive. It's not my idea of a recreational dive. AND..


If you've never done the dive before how do you know how long it will take to swim the legs? If you have done the dive before then why do you need to do a gas management plan?"
Somebody posted recently that they got bent, probably due to the fact that they surfaced a couple of times during the dive to navigate to their intended spot and also on the way back to shore. This was in a lake, if I remember right. I've come to realize that the surfacing that I've done in the past to check my location is in fact a dangerous practice. When you surface during the dive without safety stops, you're running the risk of DCS. I guess that's the problem with surfacing part way through the dive.

 

[MXGratefulDiver]

So it's now obvious that when some tech divers claim you need to know the route of the dive, the depth profile along that routes and distances and directions along the various legs of the dive and that they can do the dive plan using mental mathematics in less than the time it takes to 'read a post out loud' it was too good to be true. But we all knew that.

You're continuing to argue with yourself. All any of us have said is that for certain dive profiles, winging it is fine. For other dive profiles, it's not enough and it's good to have more complete dive planning tools available. You keep tossing out strawman arguments and making wild-ass claims about tech divers who miss the dive because they take too long to plan it.

As I said in I believe it was the second post in this thread, it depends on the profile, the conditions and the circumstances. The gas management technique that's most appropriate depends on the type of dive you're doing. Everything else I've posted in this thread was in response to specific scenarios that you put out there ... responses that you specifically asked for. Nobody ever said or implied they were necessary for all types of diving.

Seriously, if you want to jump in the water, paddle about till you get low on gas, then surface to see where you are, have at it. Just recognize the limitations of your planning methods and apply it to dive profiles where you can't get yourself in trouble doing that and you'll be fine. When or if you ever get to the point where you want to do more aggressive types of diving ... as with the examples I've provided ... that method can get you in trouble. If you have such a hard time believing that, spend some time down in the Incidents and Accidents forum.

And FWIW - neither I nor anyone else posting in this thread has claimed to be a "tech diver" ... you're the only one using that term. Most of my dives are no-deco dives within recreational depth limits. Terms like "tech diver" are an artificial distinction, due to the fact that we're all diving for recreational purposes. It is, in this case, clearly being used by you as an epithet ... for reasons I'm not clear on.

... Bob (Grateful Diver)