Redundant airsource question.

[fisherdvm]

I find this formula in Scuba Diving magazine interesting:

[ { (Max depth of dive / 33) + 1 } x 1 cfm ] x number of minutes needed to surface.

Whereas max depth divided by 33 plus 1 give you the compression factor (2 atm = 33 ft, 3 atm = 66 ft, 4 atm = 99 ft).

1 cfm is the assumed air consumption at the surface (at depth, will be multiplied by the ATM factor).

And minute to surface is your safety ascend rate (most agencies recommend 30 ft per second, or 1 minute for 30 ft, 2 minute for 60 ft).

Based on this assumption, some people consider in an out of air situation, you should ascend as slowly as possible, but it would likely be faster than 30 ft per second.

An example would be a diver at 60 ft, to ascend at the safe 30 ft per minute, he would need at least 5.6 cu ft pony tank. But if he ascend at 60 ft per second, a 3 cu ft spare air would suffice, and if he ascend at 120 ft per second, he would need only a 1.7 cu ft spare air.

 

[fisherdvm]

Why do I post polls? Because I want to hear honest opinions from fellow divers.

If you don't like these polls, make your own, or don't participate... Actually, it keeps my wife happy that I am not bugging her.

 

[spectrum]

http://www.scubaboard.com/showpost.php?p=1590880&postcount=4

 

[fisherdvm]

Excellent point, I'll post your thoughts:






Per Spectrum:


Let me take a stab at this, I'll make some assumptions and you can do you own math based on you and your diving.
*You want redundancy to 99 feet
*You want 2 minutes of air on the bottom to collect yourself, solve a problem or move to an ascent location which may or may not be a wise thing to do.
*I'll assume a SAC of 1 which is common for a stressed diver. Your actual may easilly be 1/2 of this but the situation may change that.
*An ascent rate of 30 FPM
*3 minutes at 15 Feet for a safety stop

So.....
2 minutes on the bottom X (3+1) Atmospheres X SAC of 1 = 8 cubic feet will be consumed on the bottom

During ascent to 15 feet your average depth is 2.75 ATM X ~3 minutes X SAC of 1 = 8.25 cubic Feet

At your safety stop your depth is 1.5 ATM X 3 minutes X SAC of 1 = 4.5 cubic feet

A leusurely minute to surface in a minute 1.25 ATM X 1 minute x SAC of 1= 1.25 cubic feet

All told the 4 increments add up to 22 cubic feet of air. The SAC of 1 is agressive but you need to bounce that against how perfect the fill was, how accurate your gauge is, you don't want to be sucking the cylinder dead empty and a host of other safety factors. You may choose to consider the safety stop as expendable, you may prefer to add a deep stop 1/2 way up.

As Awap stated 19 can be nice and many diver go for more than that. Don't guess, consider your needs and decide what is appropriate for you. Knowing the math behind it can make a big difference in your comfort and safety if the day comes when you are relying on this as your air source.

Pete

PS Almost everyting I know about this I learned here so if I messed up correct me.
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[spectrum]

Why do I post polls? Because I want to hear honest opinions from fellow divers.

If you don't like these polls, make your own, or don't participate... Actually, it keeps my wife happy that I am not bugging her.

Running a poll is an OK way of putting a question out there. Keep in mind that the poll results lack context. A focused question of what you are trying to sort out (where, when, why stuff) will generate more relevant responses.

I have contributed in both forms.

Is your wife your dive buddy?

Pete

 

[fisherdvm]

No, 90 % of my assigned buddies are not very good. Wife will need a double hip replacement this summer. But she likely will not dive even after that is done.

 

[rmannix]

Enough polls.

 

[fisherdvm]

Enough polls.

Why don't you make a poll about it?? He, he, he....

I'll bet you can't resist the urge to push one of the buttons, right??

Kinda like a drug addict, who says, "don't do drugs??"

 

[spectrum]

No, 90 % of my assigned buddies are not very good. Wife will need a double hip replacement this summer. But she likely will not dive even after that is done.

I'm sorry to hear that. Wish her luck and find better buddies. A pony can't solve everything that can go wrong.

 

[meesier42]

OK, lets go back to basics. In OW they taught you, or should have taught you, that at recreational depths within recreations limits you should always be able to safely perform an emergency free swimming ascent. Therefore the largest you NEED is 0.

Now it would be irresponsible of me not to mention(or remind) that those rules obivously don't apply to everyones body, or even all the time.

If you are recreational diving, and you use it as a bail out bottle, go with your gut, something will probably be better than nothing. Worthless buddies are more dangerous than diving solo.