1/3 Rule?

[divealaska]

gas "rules of thumb" that dont take you and your buddies SCR (surface consumption rates) into consideration, should be discarded as useless and and as irrelevant as the "surface with 500 psi" rule.

 

[Steve_C]

A few random thoughts in addition to the good points above:

In terms of gas available, PSI is a relative measure. 500 PSI on an HP 130 is very different from 500 PSI on an AL80 (much less 500 PSI on an AL19 pony)

Gauges can vary a bit. Do you really have 200 PSI when your gauge gets down to 200 PSI?

It is best to keep some pressure in the tank to prevent any possible contamination of the tank.

If it is a bit rough and there are several divers ahead of you, you will want to keep breathing off the tank. One common use of part of the reserve.

You may get up to the hang bar, or be on the anchor line, and discover that something interesting is happening and you want to hang there and watch for a while (still keeping some reserve). Shame to miss the show because you have to keep going.

 

[spectrum]

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

Very preceptive and in all that you can see the danger of dumbed down genralizations.

As an example when the speak of heading up with 1000 PSI............ What size cylinder, what is the divers relative consumption rate the varialble go on and on.

Even "coming up" The whole world does not drop into the water from boats. When " take new buddys shore diving I make it a point to stess that it is a reciprocal course and the need to turn with air to get back with a margin.

Sadly I wouldn't count on getting meaningful information from most AOW courses. Some good information has been presented above and this is a good primer on the topic.

Pete

 

[kelemvor]

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.

I think I've seen you say you carry a pony bottle too. I'm sure that factors into the decision on how much reserve you need, no?

 

[BRT]

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. . .

This is a lot of math in metric that I am not going to try to follow but as a recreational diver I am going to avoid situations where I need to make 1 minute stops every 3 meters from 30 meters. I think it would take someone with a really low SAC to get into that much decompression on an AL80. We make safety stops to be safer, but if something happens and we are low on air we will go to the surface without 10 minutes of stops staged all the way up. Never had to do it myself but have seen it twice where people burned through their AL80 in a few minutes, realized they were REALLY low, and safely ascended from 100 feet.

 

[fnfalman]

A few random thoughts in addition to the good points above:

In terms of gas available, PSI is a relative measure. 500 PSI on an HP 130 is very different from 500 PSI on an AL80 (much less 500 PSI on an AL19 pony)

Gauges can vary a bit. Do you really have 200 PSI when your gauge gets down to 200 PSI?

I want to bold faced the above for novice divers.

Also, mechanical pressure gauges can be wildly inaccurate, especially with use. I've seen old SPGs that were off by 150-psi. A guy ran out of air when his SPG showed 150-psi and I had to feed him my air for a bit. Luckily we were at 30-ft.

 

[Storker]

as a recreational diver I am going to avoid situations where I need to make 1 minute stops every 3 meters from 30 meters.

Sure you will, but what about a nice safe ascent of approximately 3 meters/minute, which translates to about 10 feet/minute? Such an ascent can easily be mathematically approximated by an instantaneous ascent of 3 meters, then a 1 minute stop, then another instantaneous ascent of 3 meters and so on. That is in fact what Kevrumbo is doing in his/her calculations.

When I first learned about gas planning and SCRs, I, as the geek I am, sat down and made myself an excel spreadsheet. In a spreadsheet it's easier to do a proper calculation of a continuously changing air consumption rate instead of Kevrumbo's rough, but workable approximation.

I plugged in a SCR/RMV of 20 liters/minute (based on rules of thumb and my own measured consumption rate), an ascent rate of 10 m/minute and a 3 minute safety stop at 5m depth. I also assumed a 100% increase in breathing rate due to stress, and of course that one tank should provide air for two divers. What I found was that for a 10L tank, the turnaround pressures were:
15m: 97 bar
18m: 106 bar
25m: 129 bar
30m: 148 bar
35m: 169 bar
40m: 192 bar

As you can see, those calculations are a little less conservative than Kevrumbo's, since I've planned with a faster ascent rate.

Assuming I start my dive with 300 bar in my 10L tank, those turnaround pressures can be calculated back into a maximum bottom time at the given depths:
15m: 39 min
18m: 33 min
25m: 23 min
30m: 18 min
35m: 14 min
40m: 10 min


And then we can see something interesting: even with a 10L/300 bar tank (about 106 cf imperial), which is more or less the standard new tank for a rec diver around here and carries significantly more air than the standard issue Al80, those turnaround pressures give me maximum bottom times within the NDLs., at least down to 35m/115 fsw. So If I'm going into deco, I haven't only made a big error in terms of deco/no deco time, I've also done a ****ty job with my gas planning and should have been heading to the surface due to running low on air before I exceeded my NDL. Even with 20% more air in my tank than you guys have in your AL80s...

 

[Kevrumbo]

This is a lot of math in metric that I am not going to try to follow but as a recreational diver I am going to avoid situations where I need to make 1 minute stops every 3 meters from 30 meters. I think it would take someone with a really low SAC to get into that much decompression on an AL80. We make safety stops to be safer, but if something happens and we are low on air we will go to the surface without 10 minutes of stops staged all the way up. Never had to do it myself but have seen it twice where people burned through their AL80 in a few minutes, realized they were REALLY low, and safely ascended from 100 feet.

Since this is the new diver sub-forum, we're not talking about about getting into mandatory deco per se -the gas planning exercise with the one minute "deco stops" every 3 meters is just a very conservative illustrative example and a quick easy calculation method of ensuring a slow ascent from depth - with emphasis of being in control- and not rocketing-up from depth like a sub-launched ballistic missile to your "safety stop" at 6m/20' (or worse, a rapid uncontrolled ascent all the way to the surface), even in the instance of an emergency contingency like air-sharing with an out-of-gas buddy.

Use 30 second stops then per every 3m or 10 feet, and re-allocate that extra gas saved for a slightly longer bottom time or multi-level dive if you want to -the main point is to do the math & the gas planning within NDL and ensure you don't have an ascent rate greater than 60'/18m per minute while having enough gas to get you & your buddy to the surface.

 

[jdg]

Okay, now I have swallowed a bottle of aspirin to remove this headache.

Is there any charts out there that will allow me to just glance at them instead of going through all these calculations. Now don't get me wrong I clearly understand the value of understanding these consumption rates. And allowances for stops and different breathing rates for both divers. But a chart would sure make ones life easier when all is said and done. An again thank you all for the great contributions,

John

 

[chrpai]

Do you have any data in your log books from previous dives? I'd be looking for:

*Average* Dive Depth
Start PSI
End PSI
Tank Type (AL80, LP95...)
Total Dive Time

If you have all of that for a few dives then we can use that to establish an RMV rate for planning.