Sharing air & continuing dive???

[dbulmer]

Ron,
I'm 100% certain that Peter and Lynne will have calculated a reserve based on the depth and a couple of minutes to sort out a problem or two.

This reserve will be a fixed amount based on anticipated needs deco depth duration of the dive etc- what Peter will do is use the rest of the gas that they have as a shared resource. What they as a team will not do is violate the reserve they have agreed upon.

Peter has already stated why it would not be a good idea for some divers to do this but you have to bear in mind that Peter and Lynne will have practised 00A procedures ad nauseum to the point where stress is at a minimum. A better plan is indeed to carry more gas but sometimes it is also good practice to share air and if it is agreed upon before the dive starts and there has been sufficient practice and discipline in executing the plan , why not?

 

[Dan Weidert]

Also, the suggestion that the additional effort may negate the benefit: If we are at 85' and I have 800psi and my buddy has 2400psi. We can end the dive or we can share air and extend the dive until my buddy is down to 1000psi and then end the dive. Benfit? We extended the dive beyond where it would have ended otherwise.

I'll ask again though, any safety concerns if I am down to 1000psi and my buddy has 2400psi, we share air until my budy is down to 1000psi then end the dive??

Steve,
Let's look at the question from a gas management standpoint.
Let's assume a Respiratory Minute Volume of 1 cubic foot per minute per diver which equals 2 cuft/min at the surface (1 Atmosphere Absolute) for both divers.
At 85 ft you are using air at 3.58 ATA which equals 7.16 cuft/min.
Ascending at 30 ft/min to 15 ft takes 2.33 minutes and gives you an average depth of 50 ft. At 50 ft you are using air at 2.52 ATA which equals 5.04 cuft/min
AT 15 ft you are using air at 1.45 ATA which equals 2.9 cuft/min

So 1 min at depth to problem solve = 7.16 cuft
2.33 min ascending to safety stop uses 2.33 X 5.04 = 11.74 cuft
3 min safety stop X 2.9 cuft = 8.7 cuft

Now my tank has 95cuft at 2400 psi which equals 25.26 psi per cuft
7.16+11.74+8.7 = 27.6cuft X 25.26 cuft/psi = 700psi; add in 500 psi (get you from 15 to surface and coverage for gauge error) and my rock bottom pressure is 1200 at 85 feet. If my buddy is diving LP 95s we have the same rock bottom pressure (because we assumed the same RMV). If my buddy had a different configuration we would have to calculate his/her rock bottom as well.

The point is, both divers have to be above their rock bottom to air share and not thumb the dive. Otherwise, both of you are diving without a redundant air supply.

If you search SB for gas management you find some great information.

Dan

 

[Peter Guy]

A little rebuttal:

a. For those opposed to "underwater gas equalization" as I have proposed, are you also opposed to regularly practicing air sharing with one's regular buddy? For example, every "X" dives, do an air share drill during the safety stop? If so, why are you opposed?

b. For those of you who have argued against this, I'd really like to understand your rationale for the opposition? Most have merely written, "It is not a good idea" (or words to that effect) but without setting forth the rationale therefore. ASSUMING both divers ALWAYS have the needed minimum gas reserves [aka Rock Bottom], what is the problem of treating the remaining gas as a "team resource" to be used by both? So, having taken away the argument regarding breathing down one or both tanks past Rock Bottom, what is the problem y'all fear? Why is it, to use Ron's phrase, a "dicey proposition?" Or as written by Zen "Sharing air as part of your dive plan is a bad idea. Losing your equipment redundancy in case of emergency or diver error.." but how are you losing redundancy AS LONG AS BOTH ALWAYS HAVE ROCK BOTTOM?

Please help me understand the problem from your POV. The instructor I decided NOT to do my DM with would only say -- "Air sharing is only to be done in an emergency and then you must go directly to the surface." But he was unwilling (unable?) to come up with any rationale as to why. So what is yours?

 

[The Kraken]

Because that is the way in which I elect to conduct my class and to educate my students. Upon the completion of their courses, they may elect to take their respective diving careers in any direction in which they so desire and make their decisions based upon their own collective experiences and education. You may accept or you may decline. It is a free market society.

It suits me to do so. I teach within the parameters and standards of my certifying agency. That is all the rationale I need to provide.

the K

 

[cudachaser]

The topic is this: Right or Wrong, these scenarios do occur...probably a lot more common than an actual emergency and they should taught to be conducted safely

 

[Peter Guy]

I teach within the parameters and standards of my certifying agency. That is all the rationale I need to provide.

But do YOU understand the rationale for the "standard" from your certifying agency? Don't all agencies try to teach their students to be "thinking divers?"

 

[eponym]

What about begining sharing while you still have 800-1000psi and then returning to your own air when your buddy hits 1000psi and ending the dive? Something tells me it would still be a bad idea but I haven't come up with a good reason why. Any thoughts?

Rock bottom, as already raised in this thread (PeterGuy et.al.), is an important consideration and one I'd want to have discussed before starting the dive. So that includes distance from boat/shore, depth, lots of variables. By the way, good question.

As Cuda said, these things do happen. But in Open Water Diver courses this is one of several topics where I only give the party line. The more complicated truth about what's possible should wait until skills are developed enough to allow mid-dive plan changes to be concocted, communicated, and consummated safely.

The instructor I decided NOT to do my DM with would only say -- "Air sharing is only to be done in an emergency and then you must go directly to the surface." But he was unwilling (unable?) to come up with any rationale as to why. So what is yours?

Succinctly, CYA. I spend a lot of time talking liability considerations with divemaster/assistant instructor candidates. They can do what they please but they should know that post-accident the lawyers will look for any "non-compliance" chinks in the armor, extending back to when you first began diving. In the USA that's a fact of life.

In my case, diving with my wife would take quads for me or her on a small pony.

Good one. I dove with a petite shark researcher (years ago) in the Indian Ocean. She joked once that she didn't like diving with her new husband because he limited her dive time too much.

 

[The Kraken]

Peter,
"Don't all agencies try to teach their students to be "thinking divers?"

As you ask . . .

I'm a graduate of the United States Army Aviation School. I served two tours in Viet Nam as a helicopter pilot. My first tour I flew Medevac helicopters, my second tour I flew assault helicopters.

My instructors taught me the basics of what to do in emergency situations. Those tools and skills were drilled into our heads, our minds, our hearts, out feet, our souls and any other part of the human physiology or physiognomy which can be imagined. But they did not teach us all that we "needed" to know. . . such things as flying through artillery strikes, B-52 strikes and other little things which could ruin your whole day, not to mention the later ability to procreate.

I have lived a long, successful and happy life by learing the basics from those who know better than I. By learning my abilities and capabilities, how to expand upon them and when to stop.

Now, did these people impart some knowledge along the way that was not in the cirriculum, yes. But it wasn't written in stone. It was observations upon their part.

You start the aircraft by the check list. You take off by the check list. You fly your mission as best as current circumstances allow, you do a pre-landing check, you land, you do a shut down check list.

You may not live by this, but I did, and I have.

After 3800 hours of combat flying and over 500 logged dives and still being alive, I stand by the method by I learned to live, fly, dive and by which I elect to instruct.

As a fledging pilot, or a new diver, it is my considered opinion the student, for the most part, does not have the necessary mental/physical/practical resources to "think" through a problem . . . they react. Why? Because they have yet to be presented with the situations which might befall them.

But, once again, you take your course, and I shall take mine.

I'm sure you have a more extensive history in instruction than have I, and I defer to your experience.

"There are old pilots, but there are bold pilots; but there are no old, bold pilots." (Anonymous)

the K

 

[Web Monkey]

A little rebuttal:

a. For those opposed to "underwater gas equalization" as I have proposed, are you also opposed to regularly practicing air sharing with one's regular buddy? For example, every "X" dives, do an air share drill during the safety stop? If so, why are you opposed?

Air sharing is great for training, practice or in case something goes wrong, however I don't think air sharing is a good replacement for proper gas planning and diving with whatever amount of gas you require.

Starting the dive knowing that you're unprepared before you even hit the water is a bad dive plan.

Terry

 

[RonFrank]

The problem with the share air gas management plan is that you assume you will have 180CF of air to do the dive for two individuals.

The reality is that if the sipper's first stage goes, you are down to one tank of air, that is likely mostly drained to get one air hog, and one sipper to the surface.

This could happen even if you don't share air, but the fact that one individual PLANS on sucking down the tank of a second is just not a great plan, especially on deeper dives. On a shallow dive... NP!