Inexperience resulted in OOA at 66 feet (long)

[GeekDiver]

Chris,

I just checked again, Are you the original poster? If not were talking two different problems.

The thought the orginal poster stated he was certified, so should have known better. OW training recomends limiting your diving to less than 60' without additional training which would require AOW or deep. They componded their problem by placing the burden of safety on someone else. The org poster should have stayed shallow with out the proper training and shouldn't have waited for someone to hand hold them to the surface.

If the over all group dive plan is that all divers will surface when the first diver reaches 500 PSI then I would agree with them to an extent. If it was a group dive plan and the DM didn't follow it and caused a safety concern they they could have chose to create there own plan or not dive. From the posting it didn't sound like they were conducting the dive as one large escorted group with the DM controling the dive but more along the lines of individule buddy teams and they expected the DM to hold their hands.

On a training dive, yep the DM is there to hold your hand and keep you out of trouble, on a normal non-training dive it's the diver who should be responsible to plan the dive and be safe. The DM is there to provide info, help if you need it, supervise divers and manage an emergency, not to be a baby sitter. A good example of this is If you go out to the gulf on a flower garden trip, the boat DM's don't dive with the guests, they supervise from the boat. People use DM's as a crutch and never develop futher training and skills.

My point is to be self suficient, don't blindly follow anyone, practice what you learned in OW, and dive conservatively until you get more training and experiance. It ok to say no when a dive excedes your traing and experiance. It's also ok to take safety in your own hands which is where it belongs anyways. It's my life, I'm responsible for it, not some one else. Most of us on the board are of voting age so we shouldn't need someone to make us be safe and end a dive when it's time. If your in a fire, are you going to wait till the firman comes to your rescue and carries you out of a fire? Why as divers, when under water, we wait for a DM to act on our behalf concerning our safety?

Geek

 

[Sideband]

If my buddy or I need to surface from ANY dive for ANY reason we are going to do it on OUR schedule. I'm not waiting around for anyone else to tell me it's OK. I'm new but I learn fast. The thumb, in my book, is not a request. It is an instruction. You wait for the sign to be returned NOT to see if they agree but to make sure they understand.

Joe

 

[lamont]

Boyle's Law and Gas Consumption

An understanding of Boyle's Law is critical to being able to understand
gas management in scuba diving. Boyle's Law can be stated as "if the
temperature remains constant, the volume of a given mass of gas is
inversely proportional to the absolute pressure." This is critical in
scuba diving because the deeper the diver goes, the more pressure they are
under. Because of the way a scuba regulator operates the pressure of gas
in a scuba diver's lungs will be the same as the pressure of the surrounding
water.

If the scuba diver is at 33 fsw (10 msw/2 ata) of depth, this means that
for a constant mass of air, the volume will be half. Turning this on its
head, for a constant volume of air, the mass of air will be doubled. When
the scuba diver breathes off of a scuba regulator, the volume of air they
draw into their lungs is a constant volume irregardless of where they are
in the water column. The mass of that air will be greater the deeper they
go, and therefore the scuba diver will consume more air the deeper they go.

The formula for how much air they consume is:

( volume consumed ) = ( surface RMV ) * ( atmospheres absolute ) * ( time )

For the US this looks like:

( cu ft ) = ( cu ft / min @ 1 ata ) * [ ( fsw ) / 33 + 1 ] * ( mins )

For the metric world this looks like:

( l ) = ( l / min @ 1 ata ) * [ ( msw ) / 10 + 1 ] * ( mins )


Standard Surface RMV Rates

The canonical standard surface RMV rate that we use in these examples is
0.75 cu ft / min for the 'average' diver and 2.00 cu ft / min for two
stressed divers sharing air. The actual values may differ greatly from
this. New divers may have surface RMVs of slightly over 1.00 cu ft / min
while experienced divers usually are closer to 0.60 cu ft / min with some
achieving surface RMVs of nearly 0.30 cu ft / min.


Rock Bottom Rules

The rules for Rock Bottom are that you should immediately begin ascending
when you hit the point where if your buddy had an OOA that you could get
both of you back to the surface while doing all your stops. Once you have
gone beyond the Rock Bottom limit if a failure occurs you could not handle
it, and you run an increased risk of DCS or death. When a diver hits their
rock bottom pressure they should immediately begin ascending to a shallower
depth. If you hit rock bottom and thumb or turn the dive to a DM and
they continue diving you should take your buddy and begin your ascent. If
you hit rock bottom and thumb and your buddy doesn't respond you should
read them the riot act when you get out, and re-consider diving with them.
The thumb sign isn't a question, its a statement. To prevent miscommunication
underwater these rules should be gone over prior to descending.


Halves and Turn Pressures

If you are doing a dive where you descend, swim out, swim back and ascend
(e.g. dive along a wall) then you are going to want to know your turn
pressure. If you would like to return to your starting point, but could
make an ascent at any time, your turn pressure is going to be half of the
gas you have available after reserving your rock bottom.


Multi-phase Diving and Turn Pressures

For a dive where the plan is to descend, swim out X minutes, swim around
and object (wreck, etc), turn, swim back and ascend the "rule of halves"
can be generalized into the principle that you always want to have enough
gas to swim back to the upline/shore without violating your rock bottom
pressures. If you will never be more than six minutes from your upline
then compute your gas consumption at depth for six minutes, add to your
rock bottom time and that becomes your 'turn pressure'. If you might
experience current, changing conditions or other difficulties you may
want to pad this number appropriately.


Thirds and Turn Pressures

If you are doing a dive where you have an physical or virtual overhead
and cannot ascend immediately, you are going to want to dive thirds or
sixths. You will also need doubles and other redundant equipment and
significantly more training.


Rock Bottom vs. 500 psi

The rule that you need to be "back on the boat with 500 psi" doesn't
help you know when to turn your dive. It also doesn't take into account
equipment failures that might cause your buddy to lose all their gas
at the worst possible moment. Rock Bottom times give you the information
that you need to make a decision about when to turn your dive. Rock
Bottom pressures will probably require turning a dive at a surprisingly
high pressure.


Rock Bottom - Ascents

To compute Rock Bottom, we add up the amount of gas we need to:

- take a minute at depth to solve the problem (start sharing gas,
communicate the plan to turn, collect wits, etc)
- ascend to the first stop
- do our stops
- ascend to surface

Individual divers should adjust their rock bottom calcs for how they
do their stops. I will be doing my examples assuming the ascent plan
is a pause at 80% ata or 50% max depth and stops for 1 min @ 30 fsw,
1 min @ 20 fsw, 1 min @ 10 fsw. The max ascent rate that should be
used is 30 fpm. For the purposes of the Rock Bottom calculations I'll be
ignoring the pause as not signficant.


Rock Bottom - Mathematical Simplification

All of the ascent phases can be combined together into a single computation
of the air necessary to ascent from depth to the surface. It doesn't matter
if the ascent phases have stops in between them, it can be treated seperately
as a direct ascent to the surface and the gas consumption at the stops can
be computed directly. For the depth of the ascent to plug into the formula
you can take the average depth of the ascent which is going to be the max
depth / 2.

For the stops, I compute them as a single stop at the time-weighted average
depth for the total time of the stops. For example:

( 1 min * 10 fsw + 1 min * 20 fsw + 1 min * 30 fsw ) / (3 mins) = 20 fsw

So I'll be doing a 3 min stop at 20 fsw. I've plugged thorugh the math and
shown that algebraically this is an identical computation to doing three
different computations for the three different stops. If your eyes
glassed over at the phrase "time-weighted average depth" have no fear and
either just use 3 min @ 20 fsw or the canonical 3 min @ 15 fsw that the
industry recommends.


Rock Bottom - Mathematically Rigorous Example

To figure out what the rock bottom volume is for a dive to 60 feet we have
three different computations to do and sum up. We need the value for the
'problem time' at the bottom, the ascent phase, and the stops. Those
computations are:

problem gas = ( 2.00 cu ft / min ) * [ ( 60 fsw ) / 33 + 1 ] * 1 min
= 5.63636 cu ft

time to ascend = 60 fsw / 30 fpm = 2 mins

ascent gas = ( 2.00 cu ft / min ) * [ ( 60 fsw / 2 ) / 33 + 1 ] * 2 mins
= 7.63636 cu ft

[ note that the depth used is the average depth of the ascent - 60/2 = 30 ]

stop gas = ( 2.00 cu ft / min ) * [ ( 20 fsw ) / 33 + 1 ] * 3 mins
= 9.63636 cu ft

Rock Bottom Volume = 22.9 cu ft



Rock Bottom - Mental Example

Another way of computing rock bottoms is simply to total up the entire amount
of time that you're spending in the water, take the average depth and
compute the gas consumption. This is very easy and not precise, but the whole
model of rock bottom times is not going to precisely model an actual emergency
anyway.

For the example above, you are spending 1 minute at depth, 2 mins going up
in the water column and 3 mins at your stops for a total of 6 mins. Your
average depth (just take max depth / 2 ) is going to be 30 feet or about
2 atmospheres. This gives:

2 cu ft / min * 2 ata * 6 mins = 24 cu ft

For a dive to 100 fsw you're going to spend 3 mins ascending for a total of
7 mins at 2.5 ata:

2 cu ft / min * 2.5 ata * 7 mins = 35 cu ft

For a dive to 130 fsw you're going to spend 4 mins ascending for a total of
8 mins at 3 ata:

2 cu ft / min * 3 ata * 8 mins = 48 cu ft


Rock Bottom - Volume to Pressure Conversion

To be mathematically exact we can take our rock bottom pressures in cu ft
and convert them to psi using as exact of values as we have for tank
capacities. For the standard AL80 those values are 77.4 cu ft @ 3000 psi.
Therefore the computation is:

24 cu ft * (3000 psi / 77.4 cu ft) = 930 psi
35 cu ft * (3000 psi / 77.4 cu ft) = 1356 psi
48 cu ft * (3000 psi / 77.4 cu ft) = 1860 psi


Rock Bottom - Tank Factors

We can introduce a concept known as a "tank factor" which is the number of
cu ft in the tank per 100 psi. In other words, every time your SPG drops
by 100 psi this is the amount of cu ft that you consume. For an AL80 this
works out to:

( 77.4 cu ft / 3000 psi ) * 100 = 2.5 cu ft

For a PST E8-130 tank this works out to:

( 130 cu ft / 3500 psi ) * 100 = 3.7 cu ft

We can use these values mentally to convert from volume to psi. For example
to convert from 24 cu ft to psi in an AL80:

24 cu ft / 2.5 is appx 10 => 1000 psi.

For doubles, it should hopefully be obvious that the Tank Factors are
multiplied by two (double E8-130s would be 7.4).


Rock Bottom - Lowest Pressure Rule

No rock bottom pressure should be lower than 500 psi to take into account
the possibility that an SPG doesn't read zero accurately. Even for a 30 fsw
dive on dual LP-120s the rock bottom pressure should be 500 psi.

 

[lamont]

Rock Bottom - Table Values

I would suggest using tables like the following for an AL80:

30 fsw - 700 psi
60 fsw - 1000 psi
100 fsw - 1300 psi
130 fsw - 1800 psi

This works just like dive tables in that if you are at 75 fsw you'd use
the 100 fsw value. The important point here is that the table is very
easy to memorize and use on a working basis. It doesn't require calculations
and doesn't require extensive wet-notes. You can easily shift where you
are in the table on-the-fly to adapt to changes in your dive plan.


Rock Bottom - Planning Note

It is common to hear people state "rock bottom for this dive is 1300 psi"
which is not an entirely rigorous statement. If you've turned the dive and
are back at 30 fsw your rock bottom is now 700 psi (assuming an AL80) and
if you are above that value you can swim around or do skills for awhile.
You don't have a rock bottom for a dive, you have a rock bottom for a
depth, and you have a planned max depth for a dive, and a rock bottom at
that depth.


Rock Bottom - Turn Pressures Example

If we're doing a wall dive at 100 fsw on a single AL80 our rock bottom will
be 1300 psi. If the plan is to descend, travel the wall, return on the
same path and ascend, then the turn pressure will be:

usable gas = 3000 psi - 1300 psi = 1700 psi
gas used on swim out = 1700 psi / 2 = 800 psi
turn pressure = 3000 psi - 800 psi = 2200 psi


In other words, we expect to use 1700 psi on this dive by the time we get
back to the up-line in order to still have our rock bottom pressure at
the up-line. We will therefore turn the dive after we have used half of
that.

If we had been doing a drift dive, we could have continued at 100 fsw
until we hit our 1300 psi rock bottom and then ascended. In either case
the actual total bottom time of the dive will be the same -- but in the
case of the wall dive it will be composed of two phases going out and back.


SAC rates - E-Bay your Air Integrated computer.

In addition to calculating your Rock Bottom times you can also plan and
calculate your SAC rates on-the-fly. Using tank factors we can convert
the 0.75 cu ft / min value into a psi / min value. For the example of
an AL80 with a tank factor of 2.5:

( 0.75 cu ft / min ) / 2.5 = .30 --> 30 psi / min

For the example of an E8-130 with a tank factor of 3.5:

( 0.75 cu ft / min ) / 3.7 = .21 --> 20 psi / min

This is the amount of air that you expect to consume on the surface. At
depth you just multiply this value by the atmospheres that you are at,
e.g. for an AL80:

30 psi / min * 2 ata = 60 psi / min @ 33 fsw
30 psi / min * 3 ata = 90 psi / min @ 66 fsw
30 psi / min * 4 ata = 120 psi / min @ 100 fsw
30 psi / min * 5 ata = 150 psi / min @ 133 fsw

You can then multiply this number by 5 or 10 to give you the amount of
gas that you expect to be using per a manageable time interval:

33 fsw = 600 psi / 10 mins
66 fsw = 900 psi / 10 mins
100 fsw = 1200 psi / 10 mins
130 fsw = 1500 psi / 10 mins

This can be very useful since if you're at 66 fsw with 1900 psi you know
that you've got another 10 mins left before hitting rock bottom. You now
don't need to be checking your SPG, but only need to check your BT/computer
for your dive time. You can also monitor your SAC rate underwater by
taking SPG readings at 5 or 10 minute intervals.

SAC rate calculations can be useful for planning a dive since they can tell
us how long we expect to be able to dive before hitting rock bottom. They
can also be useful while actually executing a dive since we can adjust our
expectations for dive time based on how rapidly we are actually using our
air.

Canonical RMV vs. actual RMV rates

Obviously, if you're a person that normally has an RMV rate lower than
0.75 cu ft / min you should adjust your SAC calculations accordingly.

You should not adjust the Rock Bottom RMV of 2.0 cu ft / min even if you
tend to use less gas. The assumption is that you may be buddied up with a
hoover like me on any particular dive who can easily exceed 1.0 cu ft / min
when I'm excited.


DIR / GUE

This is a compendium of my own thoughts that I've learned from various
sources. I'm not claiming or trying to offer exactly what GUE teaches.
Although I do owe GUE, fifthd and the DIR guys on scubaboard a debt of
graditude for most of this information.


Disclaimer

No Warantee expressed or implied, scuba diving is inherantly dangerous,
don't sue me if you get hurt. Don't blindly trust anything you read on
the Internet.

 

[lamont]

(i just wrote all that up today -- corrections are welcome -- post or PM)

 

[rainbowangel]

rainbowangel, here's the link to the post on rock bottom:

http://www.scubaboard.com/showthread.php?t=37506&page=7&pp=10

Thanks!!

 

[michaelaz]

Wow great post !!!!!!!!!!

Lamont you are a wealth of knowledge , thanks alot for the info.

 

[scubajane]

I faced a similar situation several years ago during a drift dive in a central american location. Confounding issues were weather( 7-10 ft swells diving with a tropical depression off shore) and the dive master was my buddy. At 150 psi after an 80 ft dive I had no choice but to go up for a safety stop. ...prior, he kept signaling me to stay down because the boat was no where to be found....if I came up alone in the waves, I knew I would never be found.....
It took me several years to feel comfortable on drift dives following a dive master.....particularly when the weather is not optimal.....I still have an issue diving deeper dives....someone says 80 ft and I will call the dive.......still working on this!

 

[pennypue]

lamont, you can dive with me. I've got enough air left for both of us! I've never gotten to the boat with less than 1000psi. Now granted I haven't done too many dives (12) but I think I can spare a little air.

Seriously, I love the info on rock bottom. I've read enough on these boards to know that I'm going to have to watch out for some bad judgement. I don't want to have three people try to share MY tank on an ascent!!!! (And no, I'm not a tiny petite girl that barely breathes. It's just that I barely breathe.) I can't wait to try checking on my consumption rates. I want to see "how low can I go?"

So much to learn and so little time to dive.......
I figure with a life expectancy of 92 years and a few days off for flying, I only have enough time for 37000 dives if I do two per day. Time's a wasting!

 

[Rick Inman]

(i just wrote all that up today -- corrections are welcome -- post or PM)

Don't ya love it? In their user profiles, some people have three lines listing every course, class or card they have. Lamont's just says, PADI OW.
Great post, Lamont, thanks!