Rule of Thirds & Shallow Rec diving

[DaleC]

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?

(Bolding is mine)

The OP holds all the data for the answer. If charters are using this "rule" it has little to do with dive planning and much to do with unskilled tourist protection. For businesses that deal with an unknown element (individual skill level) the risk needs to be mitigated by a blanket policy that anticipated lowest common denominator behavior.

What some are describing is becoming familiar with gas consumption planning. Not all dives are conducted with a high degree of it but having a working knowledge is what separates "divers" from "underwater tourists". We probably don't need to teach dive planning to tourists but I think we should not promote tourist planning to divers.

Many people go diving; not all of them are divers.

 

[NAM001]

Very well said

(Bolding is mine)

The OP holds all the data for the answer. If charters are using this "rule" it has little to do with dive planning and much to do with unskilled tourist protection. For businesses that deal with an unknown element (individual skill level) the risk needs to be mitigated by a blanket policy that anticipated lowest common denominator behavior.

What some are describing is becoming familiar with gas consumption planning. Not all dives are conducted with a high degree of it but having a working knowledge is what separates "divers" from "underwater tourists". We probably don't need to teach dive planning to tourists but I think we should not promote tourist planning to divers.

Many people go diving; not all of them are divers.

 

[Foxfish]

Good to see you coming around Bob. In the past you were adamant that to dive safely you needed a predefined route and depth profile. The previous comments were clearly made with reference to the way we dive over here. Have a look back over your posts if you have forgotten.

Are you now prepared to concede that you can dive safely in our setting without meeting your restrictive requirements.

 

[MXGratefulDiver]

Good to see you coming around Bob. In the past you were adamant that to dive safely you needed a predefined route and depth profile. The previous comments were clearly made with reference to the way we dive over here. Have a look back over your posts if you have forgotten.

Are you now prepared to concede that you can dive safely in our setting without meeting your restrictive requirements.

Actually, I've been consistent from the very beginning of the thread. You insist on misrepresenting what I've tried to tell you. I'm not sure if you're trolling or just having comprehension issues. Either way, I'm done wasting my time talking to you.

... Bob

 

[victorzamora]

Many people go diving; not all of them are divers.

It's the first time I've ever felt close to inclined to doing so....so I'm stealing this quote for personal use and my sig, if that's cool

---------- Post added December 1st, 2013 at 07:43 AM ----------

Good to see you coming around Bob. In the past you were adamant that to dive safely you needed a predefined route and depth profile. The previous comments were clearly made with reference to the way we dive over here. Have a look back over your posts if you have forgotten.

Are you now prepared to concede that you can dive safely in our setting without meeting your restrictive requirements.

The last several posts you've made have made we wonder if I've missed something important to the thread. Each time you post, I go back and reread large sections of the thread to find that I'm wrong. I have NO idea where you're getting this notion from. Bob has had no restrictive requirements, just a little logical thought.

 

[supergaijin]

I believe Foxfish is trying his darndest to win one of these:

 

[Foxfish]

Those that can join the dots will. The OP started off highlighting the misapplication of certain rules to recreational diving. Have a good read of this thread if you want to gain some insight into how this happens.

 

[GCullen94]

The rule of thirds is so butchered by so many people and instructors. Yes thirds makes gas planning easy but it is vary rarely right. I'm going to post three profiles with three typical ascent patterns comparing thirds and true rock bottom gas. All profiles will be on an AL80. Rock bottom gas will have you on the surface with nothing in your tanks if you are sharing gas and have an increased SCR.

Dive 1: 40 ft max reef on AL 80.

Profile 1. Ascend to 15 feet with 3 minute stop.
Gas needed: (SCR of 1 to simulate stressed situation, 30FPM ascent, sharing gas) 18 cubic feet.
Rock bottom: PSI: 720 round up to 800
Rule of thirds: 1000.

Profile 2. Ascend to 20 for 1 minute (30 seconds at 20, 30 seconds moving to 10) 10 for a minute (30 seconds at 10, 30 seconds moving to surface)
Gas needed: 14 CF
Rock bottom: PSI: 560 round up to 600
Rule of thirds: 1000.


Profile 3. 40 ft straight to the surface at 30FPM skipping safety stop.
Gas needed: 7 CF
Rock Bottom: Less than 500 (start ascent when hitting 500)
Rule of thirds: 1000.


DIVE 2: 70 ft max

Profile 1. Ascend to 15 feet with 3 minute stop
Gas needed: 25 CF
Rock bottom PSI: 1000
Rule of thirds: 1000

Profile 2. Ascend to 40 ft, 40 for 1, 30 for 1 20 for 1 and 10 for 1
Gas needed: 30 CF
Rock bottom PSI 1200 PSI
Rule of thirds, 1000... not enough to get you home

Profile 3. Ascend to surface, skip any safety stop.
Gas needed: 6CF
Rock bottom: Less than 500 PSI
Rule of thirds: 1000


DIVE 3: 100 ft max

Profile 1. Ascend to 50 for 1 minute and 15 for 3
Gas needed: 40 CF
Rock Bottom: 1600 PSI
Rule of thirds: 1000... not enough to get you home

Profile 2. Ascend to 50 for 1, 40 for 1, 30 for 1, 20 for 1, 10 for 1. (spend 30 seconds at stop depth, 30 seconds moving)
Gas needed: 40 CF
Rock Bottom: 1600 PSI
Rule of thirds: 1000 PSI... not enough to get you home

Profile 3. Ascend to 15 for 3
Gas needed: 35 CF
Rock Bottom: 1400 PSI
Rule of thirds: 1000 PSI... not enough to get you home

Profile 4. Ascend straight to surface.
Gas needed: 23 CF
Rock Bottom: 800 PSI
Rule of thirds: 1000 PSI... Will get you home, but not much of a stop


The rule of thirds works in caves, however in open water rec diving it doesn't translate that well. Rock bottom gas planning is a much safer way of making sure you have the right amount of gas to make it home if you are sharing gas with a stressed diver. The rule of thirds is way conservative on shallow dives and not conservative enough on deeper dives. Yes safety stops can be skipped but how wise of an idea is that if you are already approaching NDL times? I'd prefer to leave the bottom with the amount of gas that I know will get me and my buddy home. There is no magic number of what gas to come up with (50 bar, rule of thirds, be on the boat with 500) It's the amount in cubic feet that will get your buddy home when things go wrong.

 

[supergaijin]

The rule of thirds is so butchered by so many people and instructors.

A third diving away from the entry point, (turn at 2000psi)
A third returning to the exit point, (ascent at 1000psi)
A third for emergencies.

You would never have only 1000psi at your deepest point of the dive using 'thirds'. And I doubt you'd be doing 'deep stops' either.

 

[GCullen94]

What I am trying to say is it doesn't always work like that. a "third for emergencies" isn't the best idea. It's way too conservative for shallow dives and not enough for deeper dives. I was listing off 4 profiles that are commonly used. Um yeah it's possible to be at the deepest point with 1000 PSI. I can think of many dives out here where we start the ascent at a deep point.