Rule of Thirds & Shallow Rec diving

[Foxfish]

I do know that after our debate, PADI conducted a joint study on dive fatalities with DAN. That study showed that OOA situations are the primary triggering event in the overwhelming majority of dive incidents not related to health issues.

Sounds credible. It would be interesting to know how many OOA situations occured for divers who ascended with adequate air to reach the surface with 50 b on a normal ascent.

What do you see as being the main advantages of the rock bottom approach to planning for an ascent as opposed to say surfacing with 50 b.

This discussion sounds a lot like the one on the solo forum regarding the air needed in a pony bottle for a safe emergency ascent. Double the volume and you've got how much you need for two divers. Again it's a compromise between convenience and risk.

 

[victorzamora]

Sounds credible. It would be interesting to know how many OOA situations occured for divers who ascended with adequate air to reach the surface with 50 b on a normal ascent.

How do you determine what that is?

What do you see as being the main advantages of the rock bottom approach to planning for an ascent as opposed to say surfacing with 50 b.

You always know exactly when to start your ascent so that you and your buddy can surface safely.

This discussion sounds a lot like the one on the solo forum regarding the air needed in a pony bottle for a safe emergency ascent. Double the volume and you've got how much you need for two divers. Again it's a compromise between convenience and risk.

It's exactly the same calculation, essentially. They may be using different ascent rates or safety stop considerations, but it's the exact same calculation.


Foxfish, I'm honestly and sincerely asking a question. Please, use detail in your answer:
You are diving your current, standard configuration. You are diving to 30m with a buddy (obviously) using your current, standard, gas-planning methodology. Everything on this dive has gone right so far. RIGHT before you start to ascend, your buddy signals OOA due to an unforseeable error, not caused by your buddy. You had a good SAC rate, so you really pushed your NDL. What do you do?

 

[nimoh]

For a lot of simple dives such as dropping down an anchor line, swimming around at 30m and starting the ascent at 70 bar, and getting to the surface with 50 bar does seem fairly reasonable. Furthermore, it is a gas plan albeit a simple one.

The biggest problem I see with it is that under stress, it is common for a diver's breathing rate to double (or more). So, under stress, this plan puts the diver on the surface with 30 bar. If the stress is due to an OOG situation with their buddy, there is not enough gas to make a safe ascent. Basically the problem is that when using experience to formulate a gas plan, the vast majority of dives never make use of the reserve, so it is difficult to determine what the reserve should be. Putting some thought into it, identifying variables such as stressed breathing rate help to make the estimate of reserves needed more accurate.

The other problem I see is that it doesn't help the diver prepare for more advanced dives such as those requiring a diver to get back to the starting point, or even just dives where the diver would prefer to get back to the starting point.

 

[MXGratefulDiver]

Sounds credible. It would be interesting to know how many OOA situations occured for divers who ascended with adequate air to reach the surface with 50 b on a normal ascent.

What do you see as being the main advantages of the rock bottom approach to planning for an ascent as opposed to say surfacing with 50 b.

... it gives you the tools to determine how much air you need if your ascent isn't normal ... in other words, if you and your buddy both need to come up on a single tank. Without it, you're just assuming that the 50b you're reserving is going to be enough. That may or may not be the case.

That's the question victorzamora keeps asking you ... how do you know how much gas you would need to surface with your buddy from 30 meters? At what point would you need to go shallower because you've reached a gas level in your tank that would be insufficient to get the two of you safely to the surface? What is your methodology for determining how much gas that would be?

You haven't attempted to respond to his question because, frankly, you don't know how to answer it ... which is going to be very bad for your buddy if you should ever find yourself in that situation ...

... Bob (Grateful Diver)

 

[boulderjohn]

What do you see as being the main advantages of the rock bottom approach to planning for an ascent as opposed to say surfacing with 50 b.

First of all, I concur with those who haven't written before me in response to this. I will add something else to the discussion. Here is the shocker--rock bottom can give you a longer dive under certain circumstances than getting back on the boat with 50b of 750 PSI.

Let's say you are using a shorthand system for rock bottom calculations, one that works for the size tank you normally use. In such a system, you use a simple formula to remember what your rock bottom levels are for each depth. One such formula is to take the depth in feet (X10) and add 300 PSI. That means that rock bottom for 100 feet is 1300 PSI--simple math. You and your buddy are diving a reef system at 100 feet, and one of you hits 1300 PSI. You begin an ascent on the reef at that point, because that is your worst possible situation. Tht does not mean, however, that your dive is over. You reach 70 feet in a minute, and you are just under 1300 PSI. Now your rock bottom pressure is 700 + 300 = 1,000, so you can hang around at 70 feet for a while. When one of you hits 1,000 PSI, you take another minute to ascend to 40 feet. Now your rock bottom calculation is 400 + 300 = 700, and you can continue to explore until one of you reaches that pressure.

The you can ascend to the top of the reef and complete your dive, completing it with much less than your 50b/750 PSI system.

If you use rock bottom calculations to figure out how much air you use to get to the surface with 750 PSI, you are actually combining two different systems. you can do that, but it sin't necessary.

 

[GCullen94]

BoulderJohn that is the first time I've ever heard that method referred to as rock bottom or min gas. It would be interesting to hear the ways other people have been taught what rock bottom or min gas means. The example below is how I calculate it on the surface. I also have an in water "on the fly tool" that is simpler and more conservative.

Lets do a 3 minute stop at 15 feet and skip the deep stop this time..
.
2 minutes of gas at 4 ATA = 8 CF
6 minutes of gas at 2.76 ATA = 16.5 CF
6 minutes of gas at 1.5 ATA 9 CF
1 minute of gas at 1.2 ATA 1 CF

34.5 Round up to 35 CF needed for ascent.

Took about 45 seconds that time to calculate. We can also use the shortcut way again because 15 minutes X 2.5 ATA = 37 CF.

So now that we know how many cubic feet we need let's take a look at what pressure that is in different tanks. Different tanks have different tank factors. A tank factor is how many cubic feet there are in 100 PSI. Here is a basic chart


AL 80- 2.5
LP 85- 3
LP 95- 3.5
LP 104- 4
LP 120- 4.5
HP 100- 3
HP 120- 3.5
HP 130- 4

To calculate min gas pressure take CF/TF= min gas pressure x 100

So for an AL 80, our TF=2.5, We need 40CF for our chosen ascent profile. Therefore 40/2.5= 16 (x100) = 1600 PSI needed to ascend safely if SHTF.

If we were using HP130's for this same dive we'd use the following formula: 40/4= 10 (x100) = 1000 PSI needed to safely ascend.

Take the following scenario:

You and your buddy are on a dive and there is a reef at 30 feet that turns into a sheer wall and dropping off down to 400. The dive plan for the first dive is to check out the wall at 100 and come up at min gas or 2 minutes before NDL, whichever comes first, you are using a HP100 and your buddy a HP120. You determine that you need 1200 PSI as min gas because 35/3 x 100 = 1166, round up to 1200 PSI. Your buddy needs 1000PSI as his min gas.

You get down to the wall and it's actually pretty boring. Your buddy suggest checking out the reef for the rest of the dive. The "on the fly" tool allows you to do this easily. Total ascent time from 30= 5 minutes (3 min safety stop, 1 minute ascend, 1 minute to deal with problems). Gas time needed is 10 minutes and instead of taking the average ATA (1.9) take the nearest whole (more conservative whole) ATA (2). 10*2=20 CF needed. 20/3 =70 (x100) = 700 PSI= your new min gas pressure. Your buddy's new pressure is 20/3.5= 600PSI (easiest way to do this is make 20/7 and use the nearest divisible number that is more conservative i.e. 21/7= 300*2 = 600) This calculation can be done on the ascent and should take about 20 seconds. Yes it's more conservative but allows depth changes and better than surface with 50B for ever situation.


20Ft=2 ata
30ft=2 ata
40ft=3ata
50ft=3ata
60ft=3ata
70ft=4ata

 

[NAM001]

Nice post. May i say that most divers learn the 500 rule in ow but neglect to accept that it is for the ow max depth of 60 ft. The rule of thumb i use for ows and aows is depth *10 + 300 for an al 80 (the most common tank) I think tha numbers fall pretty well in line with your figures.

Always have to remember that rules of thumb are based on a certain set of prereques like dive limits and standard equipment config's.

Onece again good post.

Foxfish it is not about the pressure to get you home but rather the cubic footage. If I was doing this dive on double 100s, my min gas pressure would be 700 PSI, this was planed using the following criteria:

-Ascent rate of 30FPM
-A stressed SCR rate of 1 CFM for each diver
-Sharing gas on ascent
-One minute at depth to get on buddy's gas, solve immediate issues.

So now to walk you though the 3 of the profiles I posted for the deep 100' dive on an AL80

Profile one: stop at 50 for one minute and 15 for 3

So we need (for each diver)
1 minute of gas at 100
2 minutes of gas to go to 50
1 minute at 50
1 minute to get to 15
3 minutes at 15
.5 minute to go to the surface

Because we are sharing gas we need double the amount of breathable gas time we have. Because we are using a SCR of 1 for each diver just multiply gas time needed x ATA = CF needed to reach surface. Following?

2 minutes of gas at 4 ATA = 8 CF
4 minutes of gas at 3.27 ATA = 13 CF
2 minutes of gas at 2.51 ATA = 5 CF
2 minutes of gas at 1 ATA = 2 CF
6 minutes of gas at 1.5 ATA = 9 CF
1 minute of gas at 1.2 ATA = 1 CF

total= 38 CF, rounded up to 40 to make things easy.

That took about 60 seconds to do. If you wanted to make it even faster, take gas time needed X 1/2 max ATA... 17 minutes of gas *2.5 ATA = 42.5 CF... easy enough?



Now let's look at a GUE/UTD/DIR ascent. They use an ascent rate of 30FPM until half max depth, then 30 second stops every 10 feet from 1/2 max to the surface and spend 30 seconds moving in between stops.

2 minutes of gas at 4 ATA = 8 CF
4 minutes of gas at 3.27 ATA = 13 CF
2 minutes of gas at 2.51 ATA = 5 CF
2 minute of gas at 2.21 ATA 4.4 CF
2 minutes of gas at 1.9 ATA = 3.8 CF
2 minutes of gas at 1.6 ATA = 3.2 CF
2 minutes of gas at 1.3 ATA = 2.6 CF

total gas needed= 40 CF

Again took about 60 seconds to do. To make it even faster do the same as above, half max depth in ATA X gas time needed to surface = 40 CF
16 minutes * 2.5 ATA= 40 CF



Lets do a 3 minute stop at 15 feet and skip the deep stop this time..
.
2 minutes of gas at 4 ATA = 8 CF
6 minutes of gas at 2.76 ATA = 16.5 CF
6 minutes of gas at 1.5 ATA 9 CF
1 minute of gas at 1.2 ATA 1 CF

34.5 Round up to 35 CF needed for ascent.

Took about 45 seconds that time to calculate. We can also use the shortcut way again because 15 minutes X 2.5 ATA = 37 CF.

So now that we know how many cubic feet we need let's take a look at what pressure that is in different tanks. Different tanks have different tank factors. A tank factor is how many cubic feet there are in 100 PSI. Here is a basic chart


AL 80- 2.5
LP 85- 3
LP 95- 3.5
LP 104- 4
LP 120- 4.5
HP 100- 3
HP 120- 3.5
HP 130- 4

To calculate min gas pressure take CF/TF= min gas pressure x 100

So for an AL 80, our TF=2.5, We need 40CF for our chosen ascent profile. Therefore 40/2.5= 16 (x100) = 1600 PSI needed to ascend safely if SHTF.

If we were using HP130's for this same dive we'd use the following formula: 40/4= 10 (x100) = 1000 PSI needed to safely ascend.

Calculating min gas is very simple and really doesn't take that much time at all. I'd rather take the minute it takes to do and dive with a solid gas plan.

 

[boulderjohn]

BoulderJohn that is the first time I've ever heard that method referred to as rock bottom or min gas.

What I described is a short cut derived after you have done all those calculations. Notice that I specifically said, " one that works for the size tank you normally use." If you do the complete rock bottom process and learn that your ascent from 100 feet with the size tank your are using should begin with 1300 PSI remaining (etc.), you can remember that for each time you use that size tank. (Note that the numbers I used were an example--not what I think works with all tanks in all situations.) Most people will almost always use the same tanks. For those people, they don't have to sit down and do a math exercise every time they dive. They just need to remember the PSI numbers for a handful of depths.

Using rock bottom calculations to determine the starting point of an ascent does not commit you to doing a square profile. It does not require you to use the min deco procedure for the ascent. You can calculate it first using min deco as your ascent profile if you wish. You can calculate it using a traditional safety stop if you wish. It is nothing more than a process for determining that you and your buddy can safely reach the surface in a worst case air sharing scenario.

 

[Foxfish]

... it gives you the tools to determine how much air you need if your ascent isn't normal ... in other words, if you and your buddy both need to come up on a single tank. Without it, you're just assuming that the 50b you're reserving is going to be enough. That may or may not be the case.

That's the question victorzamora keeps asking you ... how do you know how much gas you would need to surface with your buddy from 30 meters? At what point would you need to go shallower because you've reached a gas level in your tank that would be insufficient to get the two of you safely to the surface? What is your methodology for determining how much gas that would be?... Bob (Grateful Diver)

Here are some depth profile numbers for an emergency ascent someone gave in a previous post and which I referred to in the past. The first example was for an emergency ascent from a thirty metre dive. It is based on DIR methodology

Minimum Gas Explained | Theory Articles | Articles | DiveDIR

What do you think?

 

[DaleC]

Foxfish, I'm honestly and sincerely asking a question.

I asked as well:

You are at the far end of a wreck HMCS Saskatchewan, 360' from the upline. How do you know what pressure you should turn at to get to the upline with enough gas to be at the surface with 50b.

He wants to know what you think because he can't answer those kinds of questions.