Rule of Thirds & Shallow Rec diving

[tomfcrist]

You would be correct , but the key is to start your ascent at your planned exit location....vise randomly as is being suggested by someone here.

 

[TSandM]

I'm a technically trained diver, and I can tell you absolutely for certain that I have a) never delayed the entry of our team into the water because we were arguing about the gas plan; b) never created a problem for anyone because I was low on gas before I should have been, and c) never felt the least need for a calculator or a pen and paper to do my gas plan.

I think the novice divers in your scenario were prudent. I would have done it in a little bit more structured fashion . . . I would have said, "Okay, max depth 15m, rock bottom's 500 psi in these tanks. We need to get back to the boat, but we can swim on the surface if we have to, so this is a halves dive. So we can head away from the boat at a given compass heading for x minutes (x being figured on depth and estimated gas consumption) and then we ought to turn around. If we don't want to go that far, we can do a star pattern out from the anchor line, and make sure we get back to the bottom of it with 500 psi." It's similar to your strategy, but just a bit more rigorous, and it takes very little time and delays no one.

Your example story, with the fictional tech divers, shows that you have the same misconception about this kind of planning that some of the people in the audience have when folks like Bob or my husband do gas planning talks. They envision people with a long checklist and a calculator, fussing about details until they don't even get to dive. Really, nothing could be further from the truth. I do a lot of dives with our GUE Seattle group on our local charter boats. They LOVE us, because we are so efficient and quick into the water -- and we respect the time limits they ask us to respect, and nobody ever runs low on gas. You can learn to do this gas planning thing in less time than it would take me to read my average post aloud

 

[Foxfish]

Bob gave a little example a while back of a scenario where he considered gas management was important. It involved a diver swimming around a fishing boat pier. He didn't answer my questions so I'm a bit hazy on some details. But as a matter of interest, can you talk us through how would you go about planning this dive. You have no prior knowledge of the distances and depths or the time to traverse the various legs of the dive. Is that important? How would you research that information if it was needed to plan the dive?

 

[supergaijin]

It is simple to find approximate depths for most areas if you do a little research. With applications for i-phones etc. you can get hold of some pretty detailed nautical charts for quite cheap.

For an anchored dive and depending on topography, you can plan a square route or triangle using compass headings. With a triangle, decide on your reserve eg. a quarter of your gas. Then swim your first heading for a quarter, and repeat to make the triangle.

(PADI) OW teaches you to make a reciprocal heading- so you can 'wagon wheel' from the anchor, using kick cycles or time.
(PADI) AOW gives you an experience towards making more complicated navigation routes such as squares and triangles.

 

[D_B]

"... all that without the need for a calculator ..."

LOL .. you have no idea
.. if even I can do simplified gas planning in my head .. than ANYONE can

 

[Rich Keller]

If you are going to use a rule of thirds for shallow rec diving with a 3000psi tank try using 1000psi on your way out, 1000psi for your way back to the boat and 500psi to reconnoiter the area near the boat or for a safety stop if one is called for.

 

[mala]

under 50ft beginner level dives.

I would expect to be in sight of the exit point with 50bar out of a 230bar fill.

rule of 1/4.5?

 

[MXGratefulDiver]

Bob gave a little example a while back of a scenario where he considered gas management was important. It involved a diver swimming around a fishing boat pier. He didn't answer my questions so I'm a bit hazy on some details. But as a matter of interest, can you talk us through how would you go about planning this dive. You have no prior knowledge of the distances and depths or the time to traverse the various legs of the dive. Is that important? How would you research that information if it was needed to plan the dive?

You don't need to do a dive to have a pretty good idea about the dive profile. It's simple enough to ask someone who has done it before ... or to go to an online forum and pull it out of a dive site review ... or look it up in a book ... or go to a local dive shop and see if one of their DM candidates has ever mapped the site before ... or ask the fellow on the boat where you're going before you get there (they're not going to dump you in a place they're not familiar with). There are any number of ways to obtain information about a site without actually diving it.

One of the fundamental tenets of diving is "plan your dive and dive your plan". Part of the planning process involves having some idea of where you're going, the approximate depth of the dive, and the approximate time you plan to spend underwater. These things are all discussed in the dive planning section of your basic OW training class ... I'm surprised you're not familiar with at least the concepts.

Once you've got that information, the only other thing you need to know is what your own consumption rate is. Using that, and approximating average depths for each part of the dive (i.e. "time to depth", "time at depth", and "time to ascend from depth") you can easily figure out the answer to the most fundamental question of all ... do I have enough gas to do that dive?

It's no different, really, than the driving example I gave you earlier ... it's fourth-grade level arithmetic.

Sure, you can fly your computer and watch your gauge ... and for most dives that will work just fine. For some it won't ... so what do you do on those dives? So far all you've suggested is that you "come up and check to see where you are". That's not always going to be a good option ... particularly on deeper dives, or dives where there's kelp or an adverse current that would then prevent you from surface swimming back to your boat or entry point.

It's way better to know ... because you planned for it ... that you have adequate gas for the dive you're planning to do. That is ... assuming you've bothered to make a plan. And if you haven't, then you're not following your training ... you're relying instead on luck and instinct. And that's how people get in trouble underwater ... eventually you'll hit a profile where that simply isn't enough ...

... Bob (Grateful Diver)

 

[Foxfish]

Once you've got that information, the only other thing you need to know is what your own consumption rate is. Using that, and approximating average depths for each part of the dive (i.e. "time to depth", "time at depth", and "time to ascend from depth") you can easily figure out the answer to the most fundamental question of all ... do I have enough gas to do that dive?

It's no different, really, than the driving example I gave you earlier ... it's fourth-grade level arithmetic.

... Bob (Grateful Diver)

So far to do the gas planning you consider necessary for a safe dive you need to know your air consumption rate for the given activity, the finning rate, the direction you will travel for each leg of the dive and the depth profile for each leg of the journey. You would also need to know your tank size or at least be able to make adjustment for you tank size. It implies that you must adhere to the plan.

To make it a little more realistic and applicable to the way most recreational dives that I've witnessed are conducted, assume you do not know the time required to travel each leg of the dive.

Let's assume for the moment that you have the information I listed above. With reference to the previous example, outline your gas planning procedure in detail. From what you have explained to date it would go something like this:

1. Calculate the average depth of the leg in question. This can be done by adding the maximum and minimum depth and dividing by two or approximating the average based on a detailed visualisation of the profile.

2. Calculate your air consumption rate. To be accurate it needs to be adjusted for your activity level and be based on a relatively recent assessment of your rate. Mine can vary significantly over a period of some months.

3. Determine your average finning rate. This assumes you know from previous dives how long it takes to cover a fixed distance. Again this is highly dependent on what you're doing. It also implies that you must maintain that finning rate for the dive plan to work. Assume you have calculated and remembered a range of finning rates from a previous dive. Otherwise you'll need to jump in the water and time your finning rate doing various activities.

4. Knowing your finning rate, determine the time it will take to cover a given segment of the dive plan. For example, if the first leg was to be 50 m and your finning rate was estimated to be 10 m per minute then the time to travel that distance is 5 minutes. You'll need to assume you know all the points of interest on the dive and make allowance for the time you will spend at each. Note that on more complicated dives you may get lost and you would also need to factor in an allowance for time needed to backtrack.

5. Determine the air consumed for the individual leg of the dive by multiplying your air consumption rate by the time at that depth. Then correct the volume calculated to a reference depth. I'll let people like Bob and TS&M explain in more detail how they will do this using simple primary school mental arithmetic.

6. Do 4-5 for each leg of the dive. In the case of this example there are three legs with two being the same.

7. Sum the air consumed for each leg of the dive.

8. Calculate the amount of air your cylinder holds at the required reference pressure and compare that with your estimated air requirement.

9. If the available air is less than your required air adjust change your dive plan to minimise the air needed. Iterate until the available and required air is approximately equal.

So who is going first? Bob or TS&M?

 

[KeithG]

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?

The standard routine we hear a lot as vacation divers is "turn back at 1500 psi".
the dive master will lead the group into the current at the maximum depth (50 feet in this example).
once the first person gets to 1500psi the group ascends to half the depth (25 feet) and swims back towards the boat with the current. Anyone buddy teams with extra air can then puddle around unde the boat until the 60 minute time limit is up.
this is using single al80 with 3000+ psi.