Rule of Thirds & Shallow Rec diving

[MXGratefulDiver]

As a novice diver, how do you determine reasonable numbers for a rock bottom calculation?

Here's how I do it in my AOW class ...

Descend to 33 fsw. Take note of your tank pressure. Do a hard swim, maintaining a depth of 33 fsw for 5 minutes. Take note of your tank pressure again.

Wait a few minutes for your breathing rate to return to normal.

Take note of your tank pressure again. Swim normally at 33 fsw for 10 minutes. Take note of your tank pressure again.

For each of those two swims ...

- Determine your actual gas usage (subtract end pressure from starting pressure).
- Divide by 2 (you're swimming at 2 ATA's and want to "normalize" gas usage to 1 ATA.
- Divide by the number of minutes of the swim (you want a "usage per minute" SCR)
- Multiply by the baseline (cubic feet per psi) of your cylinder

This will give you an SCR based on cubic feet per minute for each of the two swims. The "kicking hard" swim is your WORKING SCR ... the "swimming normally" swim is your RESTING SCR.

Actual consumption is somewhere between those two figures, based on conditions ... but the two numbers will give you a rough idea of what you can expect to use ... analogous to "city mileage" and "highway mileage" for your car.

Once you know working and resting SCR, you plug them into the calculations for ascent depths and times as previously described ... it's simple grade-school level arithmetic.

My students have been doing this in AOW class for nearly 10 years now ... I've yet to encounter a student who had any problems at all comprehending either the math or the reasons why it's worth the effort to get this information ...

... Bob (Grateful Diver)

 

[GCullen94]

So when you calculated the numbers in imperial you got around 1166 psi (80.3 b you rounded that up to 1200 psi (82.7 b).

Then you did the numbers in metric and got 86.9 b (1260 psi) and rounded up to 90 b (1305 psi).

That means the variation in your numbers due to conversions and rounding off was 90-82.7=7.3b.

Incidentally in previous posts your calculated range varied from 83 b to 88 b.

Personally I don't have a big problem with that though if I really did do the calculation and came up with 90 b I'd stick with that rather than say 80 b.

However you were adamant that variations of this magnitude could kill a diver. So if the correct number was 88 bar (the one you calculated) and I said I'd stick with 80 b you said I'd die.

I said:



and you replied:



If this really was critical you'd need more signifiant figures in your calculation and on your computers and air gauges. The reason they don't is because it isn't necessary *to work out calculations to a greater level of accuracy.*

What you are failing to understand is your 50B rule happens to work with a 12L tank. Change tank sizes and your rule never works. Also my answers have always remained constant just saying. There are slight differences in the imperial calculation and the metric calculation that can cause a slight discrepancy if you use conversion factors. What you also don't understand is that we ROUND UP TO 90B in a 12L because you can't read 88B on a gauge. The discrepancies are not caused by rounding but rather different numbers used in the calculation as I have already said. 1CFM DOES NOT EQUEL 30L/M EXACTLY. 10M/M DOES NOT EQUEL 30FPM. This is what causes the discrepancies. NOT ROUNDING. I'd rather not run out of gas than run out at my safety stop, causing me to BOLT for the surface in the last 5M/15 feet where the pressure change is THE GREATEST AND BUBBLES WILL EXPAND THE MOST.

Good luck diving to you, hope we never get on the same boat.

 

[NAM001]

This has turned into a list of endless what ifs. prehaps we should reserve 90% of the tank to RB. Then we wouldnt get to 30M and the problem for this isolated depth of 30m will be resolved.

What i think this discusssion has shown is that everyone is approaching this problem from diferent angles. I think more important is that No planning is bad. No question... We have 30 scenerio's with 50 answers. So hypothetically a NEWBY can have a chart that says using a S80 tank 1500 is RB for 30 M for dives that are valid dives for OW'S ansd AOW's to do. If we were to do this in true olympic fashion and toss out the high and low numbers and average ther rest Just how far off would it be. Probably not much. and then compare it to my simple process of depth *10 + 300. Over preciceness makes over complication, ocer comlexity fosters anxioty. What is simple computations to the well seasoned of many is perhaps a major fear for the many of the new born diver generation. I think too much motivation for extensive planning comes from the non recreational communities. The one method fits all crouds. Face it 150 ft and 50 ft can be treated the same but is it necessary.

For example when I was on the ARK noah taught me that sac was 1 cuft on the surface for planning. that meant i had (with a (st72 at 2250) one hour on the surface 1/2 at 33 ft 1/3 at 66 and 1/4 at 100 ft. Was that accurate, precice or anything other than a rule of thumb. NO was it functional. YES. Did we use these times for bottom time. NO we used it for Total Dive time. So if we were making a dive to you guy's 30m we had 1/4 hour to do it and get back to the surface. The errors in the rule of thumb were all in the conservative side of things. And we had J valves. We learned on our own that when we asscended to 33 ft that we couldstay longer till we got to less than 1/2 full presure. It was easy for us because we treated our st72's as if they were 60's. that made the presure and time allign. Once again an error on the safety side. After a few dives we learned to treat the tank as a 72 or an 80 as needed to account for our real SAC. so if it took 45 min at 33 ft to get to 1/2 psi on a st72 then we treated like a 140/120 cuft tank. And then still applied the 1 cuft/min sac. Funny that that was the same results as using a 72 at 1/2 sac. We used those methods because we, (the majority) did not have SPG's, we had J valves. Oh yes that lost 12 cuft (72 treated like a 60) comped for the J valve unknowing being turned. We dove based on time and not psi. Of course we had a co2 horse collar when things went south as a backup. All of todays advances souould be making this process simpler not more difficult. We have progressed from using a watch to having computers that sence +/- 5psi and feel the need to be more accurate because we can. And then use every last bit of it and then argue about how someones calcs at 50psi different than someone elses. For most the final numbers are not that far off % wise. I know i will not push the planing for a simple AOW dive plan, to the extent that would be used in a 150 ft dive or a overhead dive. Will it work YES is it needed NO. I do believe that its a good idea to know what yor sac is. You meet up with an insta buddy and he has a .75 and you have a.5 then you use your RB and add a couple hundred psi to the number and jump in before the sun goes down.

 

[Storker]

Do you mean to say you've never heard of double 18Lx232bar rigs? One of those should take you at least to 25m, maybe even 30 if you're willing to shave a little bit off of your margins. At least if you sling a couple of 11Lx232bar bailout tanks as well.

---------- Post added December 10th, 2013 at 09:24 PM ----------

On a more serious note: With my personal numbers for RB calculations (16SLM, 20bar left after surfacing, 10m/min ascent speed, 3min safety stop at 5m), I still have enough gas in my 15Lx200bar tank to take me to 30m and keep me there until I reach PADI's NDL for that depth. Quite another thing is that at 30m, I'm so stupid that I prefer somewhat shallower depths...

 

[shoredivr]

For example when I was on the ARK noah taught me that sac was 1 cuft on the surface for planning. that meant i had (with a (st72 at 2250) one hour on the surface 1/2 at 33 ft 1/3 at 66 and 1/4 at 100 ft. Was that accurate, precice or anything other than a rule of thumb. NO was it functional. YES. Did we use these times for bottom time. NO [......]
we did not have SPG's, we had J valves. Oh yes that lost 12 cuft (72 treated like a 60) comped for the J valve unknowing being turned. We dove based on time and not psi.[.....]

I learned to dive in 1998, and at that time PADI was not teaching anything other than "watch your SPG and be back on the surface with 500PSI" IIRC. So back on the Ark, Noah was teaching better gas management planning to deal with the fact that SPGs weren't available IMO.

The people who learned to dive through PADI in the intervening years since Noah and the Ark are likely stuck with an SPG and a lack of gas planning skills. I recognize that there are of course more agencies than PADI, however the majority of divers worldwide seem to be trained by PADI, so I feel safe saying the majority of OW/AOW divers don't have good gas planning skills. PADI may be changing that, someone else can enlighten me on that. All I know is that I had to wait till I took a deco course before I learned real gas planning.
Those on the Ark had much better knowledge it seems ;-)

 

[Foxfish]

We now agree that 80 b (1160 psi) is adequate air for an emergency ascent from 30 m with a 12 L tank. It's the number you get for a typical 'minimum gas' calculation and using the 50 b rule.

If you are using a 10 L or 11 L tank, which rarely happens over here, the numbers do change. My rounded calculations indicate that for a 11 L tank you need 90 b and for a 10 L tank 100 b. Keep in mind that people with a smaller physique often use 10 L tank because they have a very low air consumption rate. In that case their breathing rate in an emergency could be 20 L/min rather than 30 L/min and turning the dive at 80 b works.

For an 11 L tank the 50 b rule works up to about 27 m. You need about 10 b extra in emergency air going from 27 to 30 m based on the 'minimum gas' methodology we've typically used. As you decrease depth the 50 b rule becomes increasingly conservative.

I'd expect most recreational diving takes place at depths less than 27 m and that most of the OOA emergencies occur in that depth range. That being the case, teaching a method that is more confusing, harder to remember and use and less conservative for all but the outer limits of OW/AOW diving is more likely to increase the OOA incidents.

---------- Post added December 10th, 2013 at 08:27 PM ----------

Here's how I do it in my AOW class ...
... Bob (Grateful Diver)

Except that this is a forum for basic diving not advanced diving. The OP refers to depths of less than 50 ft. As you may have picked up, my problem is with people having more advanced training trying to force an approach on OW divers that will only serve to introduce a whole lot of unnecessary complexity and confusion.

At present it is not a requirement of any of the major recreational courses. Apparently there are some changes afoot in the courses. Till then, I think the sentiments of my SSI OW books says it well. If you require more advanced gas planning, do a specialty course.

 

[GCullen94]

NO WE DO NOT AGREE!!!!!!!!!!!!!! using the METRIC VERSION WE GET 86B and you will RUN OUT OF GAS AS YOU BREAK THE SURFACE. we ROUND UP TO 90B. STOP CONVERTING FROM METRIC TO IMPERIAL USING CONVERSION FACTORS. Really Foxfish? you started this whole debate on surface with 50B. Thats it. Just surface with 50B. You never once explained yourself and then you start throwing different numbers around once you realized you are wrong. 30L/M in an emergency is not that high of a breathing rate. So what one person breathing 20L/M and the other person still breathing 30L or 35L and you still have 50L-55L/M draining out of the tank. I have shown you that time... and time again. THAT AN 11L NEEDS 100B. YOUR 50 BAR RULE DOESNT WORK.

It started out as rule of thirds as well and how rule of thirds is too conservative for dives of less than 50 feet. The calculations are stupid easy. anyone can learn them if you put 10 minutes into learning and only take 1 minute to do. Whats funny is changes are coming to OW courses with this methodology. And to your post about NWGratefulDiver, basic scuba includes AOW. Heck a 30M dive is an AOW dive. There was a discussion on how much deco theory should be taught? AOW is a basic scuba class in the realm of things.

In my opinion, basic scuba, is anything that is deemed in the recreational realm. IE non overhead (soft or hard), non trimix diving, dives only on OC, however can be in SM or BM configuration. The present requirement for SSI and PADI is taught to WRSTC standards which are HIGHLY criticized as being too dumbed down. There is a reason NAUI, GUE, UTD, BSAC, and CMAS are NOT part of WRSTC because they felt as if the standards were too soft. You do realize that some specialties are taught to make more money right? such as boat diver...

 

[ShakaZulu]

My computer tells me when to ascend...

 

[shoredivr]

I don't comprehend the apparent disdain of "more advanced training." Good and safe ideas have come from the so called technical world. The use of the alternate air source is an example of this, if my dive history books are correct, the alternate regulator came out of the cave diving community. Now an alternate air source is seen as standard safety equipment. Wouldn't be surprised if there was initial resistance to it too.

Perhaps foxfish also sees the dive world through the lens of previously being a free diver with a lot of comfort in the water. However, with the decreasing importance placed on "watermanship"....swimming and snorkeling skills....fewer divers are in this enviable state of being. Here gas planning that is tailored to tank size, depth and ascent rate takes increasing importance.

Except that this is a forum for basic diving not advanced diving. The OP refers to depths of less than 50 ft. As you may have picked up, my problem is with people having more advanced training trying to force an approach on OW divers that will only serve to introduce a whole lot of unnecessary complexity and confusion.
At present it is not a requirement of any of the major recreational courses. Apparently there are some changes afoot in the courses. Till then, I think the sentiments of my SSI OW books says it well. If you require more advanced gas planning, do a specialty course.

 

[Scott]

I learned to dive in 1998, and at that time [-]PADI[/-] my instructor was not teaching anything other than "watch your SPG and be back on the surface with 500PSI" IIRC. So back on the Ark, Noah was teaching better gas management planning to deal with the fact that SPGs weren't available IMO.

There fixed it for you.

While the recent changes in the course lays it out for the instructor, many will be learning for the first time, it was taught in my IDC in the early 90's. Not RB and the calculations that are being discussed here but a 'simpler' version but still more complex than be back with 500 psi.