1/3 Rule?

[fnfalman]

Not "calibration" but I do have 2 SPGs and 2 Computers that I will use to cross check each over.

None of which are properly calibrated so you really don't know which ones are accurate and which ones aren't. Hence the recommendation to end the dive at the surface with 500-psi to cover the "oops" factor.

 

[lamont]

I guess you would not had dived in the early 70's then.... First time I had a SPG was like 1979-1980.... J valves were your SPG....

Jim...

And in the 70's you would have needed to have considerably higher fitness and swimming and freediving skills to become certified.

The OP has an average SAC rate of 0.95 which means he's just not very comfortable in the water. He should not be getting advice from 'old timers' that its cool to drain his tanks below 500 psi.

You can derail the thread and down a "get these PADI whippersnappers off my lawn" tangent, but dealing with the reality of training today its horrible advice.

 

[Peter Guy]

Is there any charts out there that will allow me to just glance at them instead of going through all these calculations.

Yes. Here it is:

Gas Management Cheat Sheet

I created this several years ago as a result of a similar thread. I provide it to my OW students. I hope this helps.

 

[oldschoolto]

And in the 70's you would have needed to have considerably higher fitness and swimming and freediving skills to become certified.

The OP has an average SAC rate of 0.95 which means he's just not very comfortable in the water. He should not be getting advice from 'old timers' that its cool to drain his tanks below 500 psi.

You can derail the thread and down a "get these PADI whippersnappers off my lawn" tangent, but dealing with the reality of training today its horrible advice.

I didn't mean for him to suck a tank dry at 50' and then have to do a OOA free assent ..... I was pointing out the fact that "WE" dove without SPG's and knew how to do it.... I'm also not about dumping on the "NEW" diver, I am all for pointing out the things they really "NEED" to know that they were never taught.... I like to say, There are no bad students... Just bad instructors ....

JIM....

 

[chrpai]

60' for 18 minutes? So that's where the cause of 30 minute dives is.

Maybe a better table would be:

Columns: Max Depth For Buddy with Lowest Pressure for a given tank size ( AL80 edition )
Rows: Highest RMV rate for all buddies


So at PSI 3000 you should be no deeper then 80... at XXXX no deeper then 70 and so on. Perhaps this would result in a nice multilevel profile that has a decent total dive time.

 

[eponym]

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

Okay, I like your post above enough to submit for consideration (and possible ridicule) a slice out of a multivariate spreadsheet I once concocted. In this small excerpt, we are assuming two divers, each consuming one cubic foot per minute surface equivalent, using one eighty-cubic-foot aluminum tank (sharing air), making a thirty-foot-per-minute ascent with a three-minute safety stop at fifteen feet, and draining the tank upon surfacing. A nearly-worst-case analysis.

beginning depth 30	rock bottom pressure 600
40	680
50	760
60	860
70	960
80	1060
90	1180
100	1300
110	1430
120	1570
130	1720

A little thought will show how the "ten times your depth plus 300" rubric came about.

And remember, this is for two divers with 1 cu.ft. consumption rates. New divers sharing air could consume several times that. As would a freeze-up free-flow. And many other things.

 

[jdg]

I noticed how little opinions differ on this subject. For better or worse, I do appreciate everyone's input. As you can all see by my SAC rate I am new at this, 53 years, old diabetic, I will be retiring soon and have decided to look at this as a hobby (5 years ago they would not even allow diabetics, type 2 so no danger of passing out), and am looking for advice, some I will take with a grain of salt others much more serious. Like all forums I have visited some people are quick to criticize or offer flippant remarks. But that is to be expected. An even in those, words of wisdom may be found. What I have noticed, a genuine interest exists. And a true love for this sport. And a great willingness to help.

I have received some very valuable information and great links. The link to "Rock Bottom" has opened my eyes to some very great deficiency in some of what I was taught. I have purchased "The Six Skills And Other Discussions". Learning something and never having to need it is nothing compared to not learning and regretting it.

I am looking at the idea of a pony bottle in the future. At the very least respect the limits more.

So Thank you everyone. For all the links, charts, and information.

A special thanks to Chrpai for taking the time do that chart.

Oh and most of all happy safe diving.

 

[chrpai]

No problem. Btw, although I own a pony, there are many, myself included, that would say to be careful now to use gear to solve a skills problem. A good buddy and a good dive plan is your "pony".

I got it because 1) any dive on lake travis can turn into a solo dive in a moments notice. 2) I sometimes like to solo dive intentionally. Like tonight.

 

[fnfalman]

Just keep in mind that all these gas calculations are based on your SAC/RMV, which can be measured and calculated out. However, does your buddy that you may share air with know what his or her own SAC/RMV? That buddy may know if he/she is your regular dive buddy and you're both in tune with each other.

But what about the insta-buddy that you're mated up with?

So, err on the side of caution. Also, as a recreational diver, you don't need to worry about safety stop and a bunch of other hoola when one of you is running out of air. Make a safe and direct descend to the surface. Don't go faster than your smallest bubbles.

---------- Post added April 30th, 2013 at 03:35 PM ----------

Even assuming that its calculations are correct, how does it know about the buddy you're sharing gas with?

When both of you start sucking on the regulators, the computer will automatically calculate the new gas consumption rate and give you a bottom time remaining based on your new air consumption rate.

 

[Tigerman]

...
When both of you start sucking on the regulators, the computer will automatically calculate the new gas consumption rate and give you a bottom time remaining based on your new air consumption rate.

Well, its a bit late to find out that you've got gas for 3 minutes for both of you after your buddy has ran out and youre 4 minutes from the surface?