Air management for beginner.

[fisherdvm]

This is what I have been gleaned from my research, and mind you this is completely based on research and questions I have asked in my training class. I am NOT yet certified (but should be on the 17th). Do your own research and do not consider my information as absolute fact, I may be incorrect on some points! During your check out dives, note the amount of air you use. Use this as a starting point for calculating your SAC rate. Then after every dive do the same calculation until you get a steady rate. Scuba Toys has a great page explaining how to calculate this. (http://www.scubatoys.com/education/sac.asp)

The plan your dive backwards, 1 atm for the air above you plus 1 atm for every 33' fresh water, rounding up if you are close.

1. 15' safety stop to surface @ 33'/min .5 min @ 15' = .5 * 2 atm = 1 min
2. Safety stop 4 min @ 15' = 4 * 2 atm = 8 min
3. Ascent from planned depth @33'/min 2 min @ 60' = 2 * 3 atm = 6 min
4 Descent 60'/min 1 min @ 60' = 1 * 3 atm = 3 min

Using the example on Scuba Toys of 27#/min

You will use 27*18 = 486 psi going up and down.
Now add the 500 for your buddy

So before you get in the water you have used 986 psi.
Now for that 60' dive on a 3000# tank. You are down to 2000 psi.
2000/(27*3) = 24.69 min.

So you can figure on being roughly safe to dive to 60' for about 20 min and have plenty of air to surface and take your safety stop and have enough air for your buddy.

If I have any mistakes here, someone please correct them.

Thanks for being very objective. Appreciated the calculation. Unfortunately, on Scubaboard, folks like to be dogmatic. "Back to the boat with 500 psi"....

 

[TSandM]

Lamont has an excellent essay on minimum gas here.

NWGratefulDiver has a beautiful syllabus on gas management which he freely shares.

In my opinion, all new divers should read both.

 

[fisherdvm]

Thanks TSM, why didn't they teach this before??

 

[fmw625]

I love drift diving where the boat picks you up where you send up marker. When I drift dive I go to safety stop between 700 and 500 and send up surface marker on an AL 80. When on return trip dive I use rule of thirds when over 50 feet. On return dives less than 50 feet I fudge the rule of thirds a little. On dives of 30 feet or less I usually turn at a little over half since safety stop is usually unnecessary since these are usually from shore and you are gradually ascending all the way back. On these dives I try to surface exactly where I entered using my compass and navigation skills with out using landmarks or other visual aids.

 

[TSandM]

Thanks TSM, why didn't they teach this before??

My husband had a couple of conversations about this with the PADI instructors we went to Indonesia with. His question: "When in the PADI sequence is gas management taught?" And the answer was that it's not. It's felt to be overkill for recreational diving.

I happen to disagree.

BTW, just after reading a number of above posts: A very quick description of the gas management ideas is that you have dives where you can surface whenever and wherever you want. (An example is a drift dive with a live boat pickup.) Those dives allow you to use all the gas you have except for your rock bottom reserve, which is what you need to get you and a buddy to the surface safely.

Dives where you can surface if you need to, but it would be better to get back to the starting point, can be managed by dividing your usable gas (amount above rock bottom) in two, and figuring an outbound leg and an inbound leg.

Only dives where you cannot surface or end the dive until you return to the starting point need to be managed by the rule of thirds. And in open water, that rule of thirds needs to be applied to usable gas, not total volume -- In other words, you must still reserve your rock bottom before you break the gas into three parts.

 

[ArcticDiver]

The OP asked for some guidelines for a Newbie. Perhaps my definition of that person is different than others. Under my definition that person doesn't yet have the consistent numbers necessary to do a "proper" job of gas management.

Once the new diver has the experience and data to do that proper job then use of any of the methods recommended in this thread will do the job.

It would be preferable for a new student to get enough dives to get the numbers before certification. But, business being what it is I doubt that is going to happen any time soon.

 

[Nudgeroni]

My husband had a couple of conversations about this with the PADI instructors we went to Indonesia with. His question: "When in the PADI sequence is gas management taught?" And the answer was that it's not. It's felt to be overkill for recreational diving.

I happen to disagree.

As a soon to get the OW cert diver ala PADI, I am more than a little grateful that I bumped into this thread.

I can't believe they didn't go into this in my classes. I will be diving with a dm that I don't know (the ds has a good rep.), and I would feel blind not being able to calculate this stuff for myself.

Thank you!

 

[neil]

My husband had a couple of conversations about this with the PADI instructors we went to Indonesia with. His question: "When in the PADI sequence is gas management taught?" And the answer was that it's not. It's felt to be overkill for recreational diving.

I happen to disagree.

I agree with your disagreement. It takes ten, maybe fifteen minutes in class to explain the principle of rock bottom gas management. I don't need the students to be able to do the math, just understand the why's and then use simple, easily memorized values like in Lamont's tables.

 

[Soggy]

Thanks TSM, why didn't they teach this before??

Most open water classes are crap, that's why.

Having read this post by you, I can understand why you are supportive of CESAs (the other thread). Hopefully the information in this thread will lead you down a much more productive path.

 

[neil]

The OP asked for some guidelines for a Newbie. Perhaps my definition of that person is different than others. Under my definition that person doesn't yet have the consistent numbers necessary to do a "proper" job of gas management.

Once the new diver has the experience and data to do that proper job then use of any of the methods recommended in this thread will do the job.

It would be preferable for a new student to get enough dives to get the numbers before certification. But, business being what it is I doubt that is going to happen any time soon.

A diver has to know how to dive without running out of air. Period. To send a student out into the world without teaching them that is irresponsible. I don't want part of their experience to be running OOA. The numbers are easy to learn without having to master gas management from first principles.