1/3 Rule?

[jdg]

I did some searching, perhaps I am using the wrong words to search this.


A little confused here.

My instructor told us for Basic Open Water That we should never hit below 500 psi when heading for the surface (60-80 feet dives).

But I am seeing a lot of talk. On 80 Al tanks you generally want to be heading up around 1000psi. This is for basic Open Water diving. The one third rule.

On a recent dive down in the Caribbean, the dive instructors/dive masters asked us to warn when we hit 1000 psi but our hard bottom would be 500 psi. Again never was below 60 feet.

An if we take the depth to 80 feet I imagine the numbers change?

So, just what is the safe margin? Does one really exist? Or is it a generalization? This is for basic open water I am referring not deep water or Tech.

Regards.

John

 

[GrumpyOldGuy]

The rule of 1/3's is 1/3 for your swim out, 1/3 for you swim in and 1/3 for reserve. It's a loose rule of thumb for overhead diving (when you cannot surface at any time you want). Its not a precise rule.
Look up rock bottom. It is much more precise and based on the amount of gas you and your buddy need to make a safe ascent, it varies with depth.
1000PSI is a good rule of thumb for mid level dives (80 feet or less), the 500PSI is often quoted as the min you should have at the surface after you finish, way too little for a reserve unless you are extremely shallow.

 

[chrpai]

You'll want to google the subject "rock bottom". It basically depends on a number of factors such as your air consumption rate, your buddies consumption rate and the risk of not having enough gas to get to the great big tank of air in the sky.

1/3's generally refers to 1/3 out, 1/3 back and 1/3 in reserve. At your farthest point you'll have 2/3's left. 1/3 for you and 1/3 for your buddy. For a basic recreational dive 1000psi is generally enough to get you and your buddy back to the boat.

If I'm in shallow water with no safety stop obligation and easy access to the exit, I'll frequently go all the way down to 200psi. It's not the worst thing in the world if you understand what you are doing. But to be at 100' with 200psi is inexcusable.

 

[Dirty-Dog]

Pretty much every "rule" you're taught in diving is a generalization, and there will be those who think these rules are either too stringent or not stringent enough. What is the safe margin? it depends... and in part it depends on how safe you want that margin to be. As an example, I refer you to this discussion of gas management by our own NWGratefulDiver. It's an excellent example and shows not only how much air may be needed in a crisis, but WHY the "come up with 500PSI" rule may not be smart.

 

[jdg]

I would like to thank you all for the quick and precise response.

That "rock bottom" has opened my eyes, and shockingly so! Clearly one very important factor was not taken into consideration in everything I was told. And that was, 2 of us might be breathing that air. An just how much air we will both be consuming for a safe ascent.

Your answers at the least might of saved a very dangerous situation for me or a buddy, and at worst (case) might of saved a life in the future.

That link is now saved, and will be reread many times (I will become comfortable with the equation).

Thank you.

John

 

[chrpai]

Your next step is to learn how to measure and track your SAC/RMV rate under a variety of conditions. Then you'll be able to include gas management in your dive planning.

 

[jdg]

Funny you should mention that chrpai,

After reading that "rock bottom" I was on a site reading "Air Consumption Rates for Scuba Diving - SAC, RMV Rates, Easy Calculations.
[h=1][/h]John

 

[TMHeimer]

My Deep Course instructor said when you begin ascent you should have 100 psi for every 10 feet. ei. 90'=900psi. If you are at 500 psi when you reach the safety stop at 15' stay the 3-4 mins. anyway rather than skipping the stop. You won't breathe much air down at 15' anyway.

 

[Kevrumbo]

Let's do some gas planning practice starting with a 0.75 cuft/min SAC rate (also known as RMV -Respiratory Minute Volume, or also referred to as Surface Consumption Rate "SCR", a term I like better), a common conservative reference planning figure that even most novice divers can achieve. 0.75 cuft/min in Imperial is approximately 22 litres/min in Metric.


With a 22 litres/min volume SCR and a variety of different tank sizes, your pressure SCR in bar/min obviously varies inversely, depending on the size tank in use :


22 litres per minute -divided-by- 11L/bar tank (AL80): 2 bar/min
22 litres per minute -divided-by- 13L/bar tank (AL100): 1.7 bar/min
22 litres per minute -divided-by- 22L/bar twinset (double AL80's): 1 bar/min
22 litres per minute -divided-by- 32L/bar twinset (double Pressed Steel 104's): 0.7 bar/min

[note: 1bar/min is equivalent to 14.5 psi/min -working with metric system pressure units in bar is so much easiest than US/imperial PSI units]


Your volume SCR, 22 litres/min in the example above, is the "arbitrary" constant across all metric tank ratings ("Arbitrary" in this instance because consumption rate depends on environmental conditions, physical fitness, workload etc). In the example above, the goal IMO is to utilize a pressure SCR that is commonly understood to be predicated on the metric tank rating in use, because your SPG is is in bar pressure units --NOT volumetric litre units.


So for example, going to 30m (100 feet) depth using a single 207 bar full 11L/bar cylinder with a 22 litre/bar volume SCR, which is the same as a pressure SCR of 2 bar/min: 30 meters depth is 4 ATA (divide 30 by 10 and add 1 equals 4); your 2bar/min SCR at depth -or Depth Consumption Rate (DCR)- now becomes 8bar/min. [4 times 2bar/min equals 8bar/min]. So 10 minutes at depth 30m on a 11L/bar tank in nominal conditions, you would expect to consume 80bar of gas (8 bar/min multiplied-by 10min equals 80 bar) and your SPG reading to be down or show a delta of 80bar. . . another 10 min period at 30m would consume another 80 bar for a total consumption of 160 bar of your gas supply --way too close to possibly running out of air on your ascent & safety stop for a 11L tank --so you would in this instance stay no more than 5min or 40 bar consumed maximum to give yourself (and also your buddy in an emergency out-of-gas air sharing contingency) some extra safety margin. . .

----------------
Elementary Gas Planning and The 18m Beginner/Novice OW Limit. . .

A Quick Contingency "Rock Bottom" Calculation and Gas Plan Estimate for Open Water. . . again much easier & understood better with Pressure Bar Units in the metric system.


For a single 11 litre tank (AL80), a total of 11 litres/bar metric tank rating and a volume Surface Consumption Rate (SCR) of 22 litres/min*ATA -same as a pressure SCR of 2 bar/min*ATA (divide 22 litres/min*ATA by 11 litres/bar equals 2 bar/min*ATA)- using an example NDL air dive to 30m (4 ATA) depth in Open Water.


Emergency Reserve/Rock Bottom pressure calculation, from 30 meters with one minute stops every 3 meters to the surface,
-->Just "tally the depth ATA's":


4.0
3.7
3.4
3.1
2.8
2.5
2.2
1.9
1.6
1.3


Sum Total: 26.5


Multiplied by 2 bar/min*ATA equals 53 bar Rock Bottom absolute reading remaining on your SPG. --this also happens to be the pressure in bar needed for one person in an emergency contingency to reach the surface with the above minimum decompression ascent profile.


So ideally for a two person buddy team, multiply 53 by 2 which is 106 bar for both to reach the surface (sharing in a buddy Out-ot-Gas contingency).


But realistically, for two experienced divers stressed: 106 bar plus 30% of 106 bar equals 138 bar Rock Bottom SPG reading.


For two novice divers stressed: 106 bar plus 100% of 106 bar equals 212 bar (!!!) A single full 11L/bar (AL80) Cylinder is 200 bar!
--->obviously then, two novice divers on single 11L tanks should not be diving to 30m for any significant length of time. . .

 

[eponym]

Good stuff above.

I'll add two rules of thumb I think can be useful to newly-minted divers who haven't yet figured out their consumption rates:

1. Never go deeper (in feet) than the number of cubic feet of gas you're taking with you.

2. When your backgas pressure (in psi) is ten times your depth plus 300, it's time to signal your buddy and agree to start your ascent.