Air management?

[Dantastic]

I know this is definetly a newbie question, but as an old teacher once said, " There are no stupid questions, just stupid people." Anyways..

After getting certified about a year ago, i have done a couple of dives but always with a buddy of mine who is a divemaster. This was great both for my confidence and learning to be a better diver. The downside was that i never did much of the dive planning , i just kinda went along for the ride, and followed my buddy.

My question is what is the rule for air management as far as how much air to have left in your tank when you head for the surface, and how , if at all, do you adjust for shore diving or boat diving.

As i said earlier i know this is a newbie question, and i have tough skin and dont mind a little berating for it, but i havent been able to find a good answer in either my dive book or the search feature here on the board.

Sorry for the length and thanks again for your help.

 

[Web Monkey]

My question is what is the rule for air management as far as how much air to have left in your tank when you head for the surface, and how , if at all, do you adjust for shore diving or boat diving.

It's not a dumb question and most new divers never ask it, but should.

There is a very detailed explanation here: http://www.scubaboard.com/showpost.php?p=401869&postcount=65

Terry

 

[jbd]

I know this is definetly a newbie question, but as an old teacher once said, " There are no stupid questions, just stupid people." Anyways..

After getting certified about a year ago, i have done a couple of dives but always with a buddy of mine who is a divemaster. This was great both for my confidence and learning to be a better diver. The downside was that i never did much of the dive planning , i just kinda went along for the ride, and followed my buddy.

My question is what is the rule for air management as far as how much air to have left in your tank when you head for the surface, and how , if at all, do you adjust for shore diving or boat diving.

As i said earlier i know this is a newbie question, and i have tough skin and dont mind a little berating for it, but i havent been able to find a good answer in either my dive book or the search feature here on the board.

Sorry for the length and thanks again for your help.

There seems to be a standard phrase on most dive boats--Be back on the boat with no less than 500 psi in the tank.

Other divers, primarily those following DIR will calculate "rock bottom" to determine how much air to use.

In some places I will run a tank down to 150 to 200 psi. Other places I wouldn't even dream of getting that low.

The rock bottom calculation is a good one to work with. You will find plenty on it by doing a search for rock bottom.

And your teacher was right---There are no stupid questions.

 

[H2Andy]

Dantastic, it's one of the "key" questions to ask... good job

there are a few rules of thumb. the one i like to use is "rock bottom"

rock bottom is the minimum amount of air to get you and your buddy safely to the
surface in case you or the buddy have an out of air (OOA) situation.

rock bottom includes:

--1 minute for emergency
--1 minute for each 30 feet of ascent
--1 minute pause at 80% of ATA's
--1 minute stop at every 10 feet thereafter (3 minute minimum)

Standard tank rock bottoms are as follows:

Depth--------------AL80----------ST95/104------HP80--------HP100

0-60 feet----------700psi----------500psi--------800psi-------600psi
60-100 feet-------1000psi----------700psi--------1100psi----- 900 psi

(this is from GUE sources)

rock bottom pressure is substracted from your starting pressure so you
know what your "usable gas" is. the rock bottom is kept for emergencies only.
for example, if you start with 3500 psi and your rock bottom is 1000, your usable
gas is 2500 psi. you can plan your dive with all or part of this gas, depending on
your circumstances.

assuming an OW dive, with no strong currents (i.e. getting back to the
departure point is desirable but not oligatory), you should plan on being able
to use only 1/2 of your usable gas for the trip out, and save 1/2 of the usable gas
for your trip in.

for example, on an AL80 dive to 80 feet, your rock bottom will be 1000 psi.

your usable air is whatever psi you started with minus 1000 psi (your rock bottom).
let's assume you started with 3200 psi, so

3200 psi - 1000 psi (rock bottom) = 2200 psi usable air

now, you're going to cut that in half to arrive at your "turn around gas,"
in this case 1100.

so substract 1100 from 3200 psi and you get 2100 psi as your "turn around
point"

your dive then calls for the outward leg to end at 2100 psi. you turn
around. you arrive at the boat at 1000 psi, and you have plenty of gas to
ascend, do your shallow stops, etc., and plenty left over for any emergency.

this is a conservative calculation, but i like it that way.

 

[theskull]

. . . as an old teacher once said, " There are no stupid questions, just stupid people." . . .

A brave quote to start a question with!

H2Andy gave an excellent answer. I'd like to add some tips to his rules:

1. Start the dive by swimming into the current, so you know you can make it back (with the current) with less air than it took to go out.
2. Start the dive at your deepest planned depth, and work your way shallower as the dive progresses. (Assumes a multi-level dive. If a single-depth dive, start away from the descent line and end near it.)
3. KNOW where the ascent line (or exit point) is at all times. Use compass, UW landmarks, time, air pressure, and/or kick cycles to accomplish this.
4. Most air management rules assume you want to use the maximum amount of bottom-time (and we usually do!) There is nothing at all wrong with having a beautiful and relaxing dive and ending with 2000 psi, or terminating a difficult dive early. And do keep an eye on your available NDL time--very important as well.

Great question. Dive safe and have fun,

theskull

 

[Diver0001]

I know this is definetly a newbie question, but as an old teacher once said, " There are no stupid questions, just stupid people." Anyways..

After getting certified about a year ago, i have done a couple of dives but always with a buddy of mine who is a divemaster. This was great both for my confidence and learning to be a better diver. The downside was that i never did much of the dive planning , i just kinda went along for the ride, and followed my buddy.

My question is what is the rule for air management as far as how much air to have left in your tank when you head for the surface, and how , if at all, do you adjust for shore diving or boat diving.

As i said earlier i know this is a newbie question, and i have tough skin and dont mind a little berating for it, but i havent been able to find a good answer in either my dive book or the search feature here on the board.

Sorry for the length and thanks again for your help.

Let met get down a few basic ideas to get you started.

The most simple answer is to surface with a reserve. If your reserve is 500psi or 750psi or even 300psi isn't directly relevant as long as you're not out of air. The size of the reserve you want to keep depends to a great exent on what you'll be doing and what kinds of possible problems you think you might encounter. A good way to calculate the *absolute minimum* reserve you'll need is to use a rock-bottom calculation. You'll hear 500psi reserve mentioned a lot but that's a sort of catch-all and although it works for a lot of dives it can leave you a little short at a bad moment depending on which "doom scenario" you're planning for. Too much in reserve isn't a problem (safety-wise). Too little can be.

The second thing you need do consider is navigation. Suppose you're going to swim in a straight line out-and-back at a constant depth from your starting point (or left/right along a shore line). It stands to reason that if you swim out until you have 1/2 tank and turn around that you'll have about zero when you get back. That's probably not enough. So say your rock-bottom calculation was for 500psi reserve. If you're planning to get back with 500psi you would want to turn at 1750psi (1/2tank+250psi) so you end up back at your starting point with about 500psi. In fact, if you break your dive up into "legs" based on your navigation then you should be able to run fairly complex navigation and still end up where you want to be with the amount of air you want to have at the end. To get started the easiest thing to do is navigate based on constant depth out-and-back with 1/2 tank + 1/2 of your desired reserve as a rule of thumb for the pressure when you want to turn around and go back.

If your dive is multi-level then things get even more complex because you'll typically use more air on one part of the dive than the other. For example, if you swim out at 100ft and back at 30ft then you'll use more air on the way out than the way back and you'll end up back at your starting point with more air than you wanted in reserve. The more complex your profile gets (more legs, more depths, current etc) the more calculating you'll need to do come to some kind of plan. There are many possibly situations whereby you can't really plan your gas effectively on the fly any more. That's where all those calculations come in that Web Monkey cited.

At it's most complex you'll find divers plan their dives entirely on paper before they get in the water and they'll calculate run-times (number of minutes to different events in the dive - especially changes in depth or turns for navigation) pretty much down to the minute based on knowing their typical air usage. This kind of planning is kind of overboard for a lot of dives but there is something to be said for learning the skills involved in this kind of planning.

This is kind of the tip of the gas-management iceberg but I hope you can get something out of it.

R..

 

[Dantastic]

A brave quote to start a question with!

theskull

Not so much brave as for the humor value. I myself am quite cynical when it comes to "stupid" questions.

I greatly appreciate all the info, it is exactly what i was looking for. I am surprised that they don't teach the rock bottom method or something similar in the OW class.

As jbd said, i knew the standard "be on the boat with XXX psi." but i had no idea how to get there. Thanks again for the info.

 

[MASS-Diver]

I know this is definetly a newbie question, but as an old teacher once said, " There are no stupid questions, just stupid people." Anyways..

After getting certified about a year ago, i have done a couple of dives but always with a buddy of mine who is a divemaster. This was great both for my confidence and learning to be a better diver. The downside was that i never did much of the dive planning , i just kinda went along for the ride, and followed my buddy.

My question is what is the rule for air management as far as how much air to have left in your tank when you head for the surface, and how , if at all, do you adjust for shore diving or boat diving.

As i said earlier i know this is a newbie question, and i have tough skin and dont mind a little berating for it, but i havent been able to find a good answer in either my dive book or the search feature here on the board.

Sorry for the length and thanks again for your help.

Great Question - I had about 100 dives an AOW card, and deep speciality and I didn't know how to plan my gas! PADI just plain does not teach it. Picking random numbers (500psi, 1000psi, etc) may either not leave you with enough gas or may be too conservative. Now, I always use rock bottom - it's very simple to use (you can just memorize the numbers - although it's goood to understand the math).

 

[theskull]

Not so much brave as for the humor value. . .

I know that.

I was going for the humor, too.

Sometimes my humor is too subtle or sarcastic for everyone to get. Sorrry.

theskull

 

[Zeeman]

There are 2 things stopping me from understanding the equations outlined in the linked post, and also the info presented here...

they are these 2 acronyms

ATA
SAC

Can someone let me know what they mean please? Thanks

Z...