Rock bottom, 500 PSI, or something else?

[Lobzilla]

(depth x 10) + 400 psi and don't dive deeper than the volume of your tank. Easy peasy for us RECREATIONAL divers to figure on the fly. Luck has nothing to do with it. Do your math and tell me how reckless I'm being. And yes I am having fun.

Interesting, I have never heard that one. SSI where I got AOW cert from certainly did not mention it.

This is pretty reasonable for a recreational dive if you can pull off a controlled(!) air-sharing ascent.

I guess we have found one answer for the "or something else" option of this thread's question.

 

[PfcAJ]

(depth x 10) + 400 psi and don't dive deeper than the volume of your tank. Easy peasy for us RECREATIONAL divers to figure on the fly. Luck has nothing to do with it. Do your math and tell me how reckless I'm being. And yes I am having fun.

That’s fine, but results in kinda silly reserve volumes in big tanks and maybe small volumes in small tanks.

As always, rote learning and overly simplified rules don’t lead to actual understanding.

 

[Altamira]

That’s fine, but results in kinda silly reserve volumes in big tanks and maybe small volumes in small tanks.

As always, rote learning and overly simplified rules don’t lead to actual understanding.

Yes, but most rec divers will only see one tank size during their entire diving history--an AL80, so a couple of numbers for them to remember might be the best thing for new divers, and those that don't dive very often. This discussion easily demonstrates that there is no universally agreed upon way compute actual gas requirements, expected SAC rates, ascent rates, etc., because of diverse assumptions taught by competing agencies. These assumptions, like most plans, will likely go out the window during an actual emergency or OOA situation with your buddy or another panicked diver that pulls the reg out of your mouth during a 100' dive. What the typical rec diver on a NDL dive needs to know is how much gas in psi/bar on an AL80 he/she should have at 100'/60/30 if an safe ascent is going to be made at 30fpm with a buddy sharing gas, and to realize that the ascent rate can be increased to 60fpm without a significant DCS risk if the gas is lower at reference points, for example your OOA buddy is sucking gas like a 61 Corvette. After all, a 60fpm ascent rate with no SS had been the safe standard for decades, and will likely be good enough in an emergency.

 

[Kevrumbo]

Yes, but most rec divers will only see one tank size during their entire diving history--an AL80, so a couple of numbers for them to remember might be the best thing for new divers, and those that don't dive very often. This discussion easily demonstrates that there is no universally agreed upon way compute actual gas requirements, expected SAC rates, ascent rates, etc., because of diverse assumptions taught by competing agencies. These assumptions, like most plans, will likely go out the window during an actual emergency or OOA situation with your buddy or another panicked diver that pulls the reg out of your mouth during a 100' dive. What the typical rec diver on a NDL dive needs to know is how much gas in psi/bar on an AL80 he/she should have at 100'/60/30 if an safe ascent is going to be made at 30fpm with a buddy sharing gas, and to realize that the ascent rate can be increased to 60fpm without a significant DCS risk if the gas is lower at reference points, for example your OOA buddy is sucking gas like a 61 Corvette. After all, a 60fpm ascent rate with no SS had been the safe standard for decades, and will likely be good enough in an emergency.

I tried to find the stuff that Lamont posted years ago but can't link to his website. Anyone have a copy of his calculations/explanation?

Here (going way back almost 14 yrs):

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.

 

[Kevrumbo]

Continued:

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.

 

[Altamira]

@Kevrumbo. Those numbers for rock bottom time on an AL80 are pretty close to what boat sju said he used in post #81. Maybe what is old still seems to be pretty good, at least that is what I keep tell myself.

 

[Kevrumbo]

@Kevrumbo. Those numbers for rock bottom time on an AL80 are pretty close to what boat sju said he used in post #81. Maybe what is old still seems to be pretty good, at least that is what I keep tell myself.

(depth x 10) + 400 psi and don't dive deeper than the volume of your tank. Easy peasy for us RECREATIONAL divers to figure on the fly. Luck has nothing to do with it. Do your math and tell me how reckless I'm being. And yes I am having fun.

Here's Lamont's old school "Rule of Thumb Formula":

Depth (in feet) * 10 + 300 works for Al80s, HP100s,LP85s and similar sized tanks
Depth (in feet) * 10 works for 130s/119s/LP104s/etc

Also @Altamira , this is why you don't gas plan a minimal volume for an air share emergency fast ascent rate of 60 fpm -->especially from 100fsw/30msw:

Consider the case where you encounter something unexpected at 100 feet on a wreck dive and you may be significantly delayed at depth dealing with the problem, and then may need to be sharing gas during some or all of the exit.

500 psi will last for a long time when you're hanging at 10-20 feet. 500 psi will get sucked out of your tanks real fast at depth when you've got two excited divers breathing off of it. 2 cu ft / min for 2 divers at 100 feet on an Al80 is 310 psi per minute. So if you're at 500 psi and your buddy goes OOA on you, and things get exiting, you run a good chance of being OOA again in another few minutes. 1300 psi in an AL80 at that rate will last 4 minutes at depth. If you assume maybe a more reasonable 0.8 cu ft / min breathing rate (two fairly typical male divers kind of excited), then you get a little over 5 mins. If you are both in reasonable shape and can stay calm and breathe at a combined 1.0 cu ft / min then the Al80 wil last 8 minutes.

 

[Lorenzoid]

30second stop and 30second slide up to the next stop comes out to be 10fpm.

Isn't that what I said? "That's not 10fpm between successive stops, but it evens out to 10fpm over multiple 30-second stops."

 

[Lobzilla]

Someone may wonder why to choose stops and slides instead of a constant ascent rate if it averages out to the same.

To achieve a constant ascent rate we need to dump gas from our BC (and drysuit if applicable) continuously but at an increasing rate as we approach the surface.

To slide up 10 feet and stop all we need to do is inhale deeply to set us in motion, exhale to slow down and then dump a little gas at the stop. Then rinse and repeat.

With stops and slides there is less of a chance that the ascent rate runs away if we don't dump enough or stalls and reverses if we dump too much. This becomes more critical as we approach the surface.

In an emergency, stops and slides will require more mental fortitude to resist the urge to reach the surface but it will make it easier to ascend at a sane, survivable rate and stay together without having to grab each other.

Just practice both and see what works best for your team. The time to surface should be the same.

I found a continuous ascent to half or 1/3 of the depth followed by slides and stops to the surface to be the most robust approach.

 

[tbone1004]

A little before your time tbone NASDS 1973 before AAS and spg's we used j valves with a 300psi reserve. Depth ,Time ,Sac and be shallow before before your need for the 300 psi reserve. Safe diving.

I have some old books from J-valve time that teach it, the curious bit is why they stopped with the advent of the SPG.... We actually have our students start calculating sac rates in the pool and they have to have them for all of their training dives before we sign log books.