The rule of thirds works with any size tanks dissimular or not and with any sac rates.
it is the math you need to know before appling skills that are beyond your training
Rock Bottom Gas Management - A Hypothetical Accident Study
- Thread starter MikeFerrara
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The rule of thirds works with any size tanks dissimular or not and with any sac rates.
it is the math you need to know before appling skills that are beyond your training
I'm talking about the way the rule is expressed in OW courses and such.
I still maintain that the problem is that the instructors and courses are teaching in terms of PSI instead of cu/ft of gas.
Thinking in terms of PSI doesn't work for disparate gas consumers; it only works for YOU, when YOUR consumption rate is matched to a given cylinder.
I still maintain that the problem is that the instructors and courses are teaching in terms of PSI instead of cu/ft of gas.
Thinking in terms of PSI doesn't work for disparate gas consumers; it only works for YOU, when YOUR consumption rate is matched to a given cylinder.
you must think in PSI because that is the only gauge you have to monitor under water.
yes you need to folow the calculations in cubic feet but eventualy you need to convert it to PSI so that it can be monitored
yes you need to folow the calculations in cubic feet but eventualy you need to convert it to PSI so that it can be monitored
jeffsterinsf
Contributor
Errrr....
May be the case that "rule of thirds" is close, but let's take and extreme case.
Dick - 300 cuft tank, burns air 10x faster than Jane
Jane - 30 cuft tank
Dick and Jane go to their respective 1/3 point, both showing 2000 psi (of 3000) on thier gauges. Dick has burned 100 cuft, has 200 cuft left, needs 100 cuft to exit. Jane has burned 10 cuft, has 20 cuft left, needs 10 cuft to exit.
Dick's reg blows - even if he doesn't panic or exert himself any more, where does the 100 + 10 cuft come from? Jane only has 20 cuft in her tank!
My math says that they are 90 cuft short of their needs.
I'd be willing to submit that the "rule of thirds" only works if the divers have the same (or lower) air consumption needs during an air-sharing exit as they did during the entrance and the same capacity tanks.
That does not rule out its usefulness as a "rule of thumb" for situations where either or both:
a) "Emergency" exit to surface can be significantly faster than time to turn point
b) Air consumption during emergencies is comparable to that during normal diving
are significant factors, and the air supplies of the team are of comparable volumes.
Not intending to thrash a dead camel, but this is where we started, isn't it?
From the Pug-bot...
your mistake lays right here...2000 psi is not the turn pressure
yes you can work it out with the example you have given, however you would need to know both divers SAC rate and the depth of the dive.
look through a couple of the threads above or read up on it a little and it will come to you
for those who do want to do the math and figure the turn pressure lets a ssum a dive to 60 feet [because of the small tank] and assume a SAC rate of .75 for the larger tank diver and .50 for the smaller tank diver.
who can find the true turn pressure of the example. a hint it is not 2000psi.
now all of you with tech training wait, lets see where the recreation divers goe with it. we know that with tec training you can do it.
jeffsterinsf
Contributor
The point is that the "rule of thirds," as taught in most non-rigorous courses, is, at best, a guideline. As soon as you include differing air-consumption rates, cylinder sizes, etc., you get into a situation closer to the "rule of halves."
...as for the calculation, "proof left as an exercise"
...as for the calculation, "proof left as an exercise"
The Iceni
Medical Moderator
You are right, Genesis, in that the rule of thirds is not really appropiate for basic open water diving and appears to be being mistaught.
Let's start at the beginning again.
A possible use in a Basic NCD open water application; The rule of thirds can be used by the inumerate and those who want to get into the water without spending an an hour with a calculator.
a) Take any combination of reasonably sized cyllinders/sets but do not expect to have a very long dive if one of them is smallish and either you or your buddy is a gas guzzler.
b) Calculate;-
1) The number of bar (or psi) left after deducting your own emergency reserve (i.e. the absolute minum required with which to surface. This is usually 50 bar for a no decompression dive).
2) Divide this residue by three.
3) One third out, one third back and one third for the buddy's ascent. (i.e the end of the dive in open water, which is not the same as turning a dive in the overhead environment.)
4) Use this with the the reserve to get the "waypoints".
When returning to the shot, or anchor line turn at reserve plus two "thirds".
When ascending on a DSMB, deploy this when you have the reserve plus one "third".
As Uncle Pug said in respect of the overhead environment, this third is for you buddy to use, if necessary, on the ascent.
In my opinion, anything more involved than this not BASIC open water diving, in which case we are talking technical, so as you say Genesis
As little gas is needed to make a direct ascent in open water, as I see it, the rule of thirds is inappropriate for open water diving.
It is most certainly not a requirement but might be used, as I suggested in my last post, for planning excursion-and-return dives.
Editing added:- Bear in mind folks, I am not even an instructor now. I just wish to dive safely with the minimum of mental arithmatic as it just cannot be done in a rush and brain cells stop working properly underwater!
landlocked
Contributor
... I think, they don't teach the rule of thirds in an OW course. It was in applying the rule of thirds as learned from the Internet, rather than what they had been taught, that got the fictictious couple in trouble.
Most divers with one month experience don't have any idea what their SAC (or SAR) is and may not even know how to calculate it. This kind of makes gas management, as being discussed here, a moot point in reference to this thread. It requires more skills than the divers have.
From the SSI Deep Diving Specialty Manual. "There is a formula that you can use to help ensure that you will have an adequate air supply for your ascent: for every 10 feet you must ascend, reserve 100 pounds of air. ....you add your 500 psi reserve .... Because this formula is conservative, it should work for even the fastest breathers." (Additionally, the book teaches how to figure your SAC, mentions that you should dive with a buddy with a similar SAC and tank size, etc. Thats about it.)
That is the formula given for new divers who want to venture to around 100 feet.
landlocked
Contributor
Should new rec divers use rock bottom, rule of thirds, rule of halves, etc. No. They don't have the foundation.
Should new rec divers be making horizontal ascents from depth. No. They don't have the bouyancy skills. (They are taught to use the anchor line etc. as a safety net until they have developed the skills to do otherwise.)
Should new rec divers be making multiple safety stops during an OOA situation. No. Again new divers don't have the skills.
Should new rec divers be making OOA ascents horizontally? No. They don't have the skills. (See O-rings report on DIRf.) (They are taught to lock arms, etc. until they gain the skills.)
Should new rec divers be criticized for prefering poodle jackets. No. They may still feel more comfortable in a life jacket.
Should new divers have their backup reg and gauges secured and for the most part stay of off the bottom. Absolutely! Once these skills are learned then they can begin on the ones above. ONE at a time.
Just my opinion based on my first half year of diving. A half year which I can still remember.
So tell me Mike, is this the whole point of the your little ficticious post?
Should new rec divers be making horizontal ascents from depth. No. They don't have the bouyancy skills. (They are taught to use the anchor line etc. as a safety net until they have developed the skills to do otherwise.)
Should new rec divers be making multiple safety stops during an OOA situation. No. Again new divers don't have the skills.
Should new rec divers be making OOA ascents horizontally? No. They don't have the skills. (See O-rings report on DIRf.) (They are taught to lock arms, etc. until they gain the skills.)
Should new rec divers be criticized for prefering poodle jackets. No. They may still feel more comfortable in a life jacket.
Should new divers have their backup reg and gauges secured and for the most part stay of off the bottom. Absolutely! Once these skills are learned then they can begin on the ones above. ONE at a time.
Just my opinion based on my first half year of diving. A half year which I can still remember.
So tell me Mike, is this the whole point of the your little ficticious post?
rakjar
Guest
I am new to diving and have only taken the open water cert class. Although I read all the information on these posts, I can not remember or better yet, I don't think I was ever taught SAC.
How do you calculate the SAC?
Second question.
How do you know the cu of air in a cylinder if your computer only shows psi?
I have the suunto cobra computer....
How do you calculate the SAC?
Second question.
How do you know the cu of air in a cylinder if your computer only shows psi?
I have the suunto cobra computer....
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