How is that different from the rule of thirds used in cave diving?
Must I abort if computer fails?
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How is that different from the rule of thirds used in cave diving?
Reg Braithwaite
Contributor
I'm not a caver, so I don't know what the rule of thirds really is. In my OW class I remember being told it was to return when you have used 1/3 of your gas so you have 2/3 to handle yourself and your buddy.
If that's what you mean, it feels incomplete to me. If gas management was my only consideration when diving, do I have an objective? Do I know that 1/3 of my tank will get me to whatever I want to reach and do what I want to do? Do I know that I can do all of that and stay within my NDL for a recreational dive? If not, will 2/3 handle myself, my buddy, and my deco obligation?
So what I'm asking is really whether you would consider a dive plan a dive plan if it doesn't involve having some idea of your depth and time underwater and a reasonable guess as to whether this is within your comfort level.
I'm not asking if you would do such a thing, but rather whether you would say, "Yup, that's a plan alright!" or whether you would say that's diving without a plan.
Same disclaimer as Reg above, but my understanding is that a simple turnaround pressure based on gas volume is not an apples-to-apples comparison of determining NDLs in a pre-dive plan versus simply flying a computer real-time.
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So if your computer failed and you didn't bring "more stuff" ... how do you know when you're at 15 feet?
... Bob (Grateful Diver)
Different plans work for different situations. You can't say a system will never work because you can name a situation in which it is not appropriate.
The following plans are followed by thousands and thousands of divers all over the world. I guess they aren't adequate. You would be doing a great service to these people if you can tell what they should be doing instead so that these directions can be given to them.
1. Diving a shallow off shore reef with a mild current, the divers agree to follow the outer edge of the reef against the current until one of them reaches a specified gas level, at which point they turn and follow the inner edge of the reef until they reach their starting point. They then turn and exit.
2. Diving a simple, open wreck, the divers agree to circle the lower level once and then explore the upper area until they either reach a specified gas level or NDL level. They will then ascend.
3. Divers agree to explore a small wreck at 80 feet for 15 minutes and then ascend to the shallow section of the reef at 20-25 feet and explore it until they reach a specified gas level or NDL level. They will then ascend.
4. On a drift dive, the divers follow the reef until someone reaches a specified gas or NDL level. They then begin their ascent where the boat picks them up, either following their float or spotting their SMB.
5. Diving a wall in a mild current, divers note their starting point and descend to an agreed upon maximum depth. They begin the dive into the current, with a designated leader gradually ascending according to a multi-level design. They have not predetermined the time at each multi-level step but rather intend to let the situation dictate it. If there is something interesting to see, they may overstay a depth a little and then make it up later. They turn when one of them reaches a specified gas level. They continue the multi-level ascent, reaching their starting point after having been at the 15-20 foot range for about 10 minutes.
The following plans are followed by thousands and thousands of divers all over the world. I guess they aren't adequate. You would be doing a great service to these people if you can tell what they should be doing instead so that these directions can be given to them.
1. Diving a shallow off shore reef with a mild current, the divers agree to follow the outer edge of the reef against the current until one of them reaches a specified gas level, at which point they turn and follow the inner edge of the reef until they reach their starting point. They then turn and exit.
2. Diving a simple, open wreck, the divers agree to circle the lower level once and then explore the upper area until they either reach a specified gas level or NDL level. They will then ascend.
3. Divers agree to explore a small wreck at 80 feet for 15 minutes and then ascend to the shallow section of the reef at 20-25 feet and explore it until they reach a specified gas level or NDL level. They will then ascend.
4. On a drift dive, the divers follow the reef until someone reaches a specified gas or NDL level. They then begin their ascent where the boat picks them up, either following their float or spotting their SMB.
5. Diving a wall in a mild current, divers note their starting point and descend to an agreed upon maximum depth. They begin the dive into the current, with a designated leader gradually ascending according to a multi-level design. They have not predetermined the time at each multi-level step but rather intend to let the situation dictate it. If there is something interesting to see, they may overstay a depth a little and then make it up later. They turn when one of them reaches a specified gas level. They continue the multi-level ascent, reaching their starting point after having been at the 15-20 foot range for about 10 minutes.
SkimFisher
Contributor
This is why I dive with a backup computer. Too much to take into consideration otherwise and I don't want to learn this the hard way or on the fly.
Just a few observations as I see them:
A computer isn't relevant in this situation, just an SPG.
Just relying on a computer, they won't know what their NDL is until they hit it. If a computer dies, all they can do is abort this and any subsequent dives. Planning based on the gas and wreck depth before the dive, everyone will know before the dive what time to call the dive.
First, without planning ahead or having some knowledge of basic table profiles, how do you know 15 minutes at 80ft is NDL? Second, once you're at 20ft, are you likely to hit an NDL?
Same as 2 & 3.
If relying on a computer's real-time output, how will this plan account for sufficient reserves to cover the overstay? If two divers have overstayed and one goes OOA?
In all of these situations, my main comment with the "dive the computer" plan is the same: the divers are reacting to what is displayed on the screen, without knowing beforehand what their hard turnarounds should be based on minimum gas, NDLs and gas profiles, or whether any of these mesh. You can turn at point X, but is that enough gas to get you and a buddy to the surface? You can ascend when your computer tells you to, but does that get you back to the boat? Also, if a computer goes kaput, they're completely in the dark and must ascend, even if there's a surface current that recommends getting back to an anchor line or a deco obligation from an overstay.
I think Matt's point is that relying on a computer to call NDL doesn't give you a lot of information on which to make a good plan. If the captain asks you how long you'll be underwater for, your answer ostensibly is, "I'll know when my computer hits 00:00."
Sure, the vast majority of people get away with this kind of planning every day. In reality, whether or not they're adequate depends on if anything goes wrong. If nothing ever goes wrong, why do you need a dive plan to begin with? Just jump in without looking. The ABS, airbags, seat belts and reinforced bars in my car aren't there for when everything goes right.
A computer isn't relevant in this situation, just an SPG.
Just relying on a computer, they won't know what their NDL is until they hit it. If a computer dies, all they can do is abort this and any subsequent dives. Planning based on the gas and wreck depth before the dive, everyone will know before the dive what time to call the dive.
First, without planning ahead or having some knowledge of basic table profiles, how do you know 15 minutes at 80ft is NDL? Second, once you're at 20ft, are you likely to hit an NDL?
Same as 2 & 3.
If relying on a computer's real-time output, how will this plan account for sufficient reserves to cover the overstay? If two divers have overstayed and one goes OOA?
In all of these situations, my main comment with the "dive the computer" plan is the same: the divers are reacting to what is displayed on the screen, without knowing beforehand what their hard turnarounds should be based on minimum gas, NDLs and gas profiles, or whether any of these mesh. You can turn at point X, but is that enough gas to get you and a buddy to the surface? You can ascend when your computer tells you to, but does that get you back to the boat? Also, if a computer goes kaput, they're completely in the dark and must ascend, even if there's a surface current that recommends getting back to an anchor line or a deco obligation from an overstay.
I think Matt's point is that relying on a computer to call NDL doesn't give you a lot of information on which to make a good plan. If the captain asks you how long you'll be underwater for, your answer ostensibly is, "I'll know when my computer hits 00:00."
Sure, the vast majority of people get away with this kind of planning every day. In reality, whether or not they're adequate depends on if anything goes wrong. If nothing ever goes wrong, why do you need a dive plan to begin with? Just jump in without looking. The ABS, airbags, seat belts and reinforced bars in my car aren't there for when everything goes right.
Blackwood
Contributor
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