Just got my AOW and might plan a deep dive. Things to make sure I cover off?

[xdjio]

Duhh, of course. You can blame not enough coffee!

 

[DivemasterDennis]

A recreational dive to 90 or 100 feet does not have to be as complex as some posters want to make it. The rule of thirds is a good rule. The original question, what are the different or additional focuses on the dive as compared to a shallower dive, are these:

Check you air levels regularly- more often than at shallower depths
Start you ascent with enough air to make appropriate safety stop(s) 800 PSI would be a minimum
Pay attention to ascent rate- SLOW. This is true for all dives, but especially deep dives
Be attentive to any symptoms of narcosis in yourself or your buddy
Be sure to have a buddy
Dive with someone who has dove to these depths before and has experience
Use a computer- a good one

So dive, be safe, have fun, and log all dives and include gas consumption data in you log entry so you can benefit from the experiences in planning future dives.
DivemasterDennis

 

[Bubbletrubble]

Hey guys: I just calculated a recent dive that I did (from memory of my logs):

(3000 psi in - 500 psi out) / 27 minutes = 92.59 psi per minute
15.7 m / 10 + 1 = 2.57 ATA
92.59 / 2.57 = 36 psi per minute

These calculations are fine, assuming that the average depth was 15.7 m.

the RMV that we could use for this (a 100 ft3 tank): baseline 95 cuft / 3000 working psi (leave a 500psi buffer) = 0.031 cuft per psi

36 * 0.031 = 1.116 RMV

There are a couple problems with this:
1. I don't know of any 100 cuft tanks that have a service pressure of 3000 psi.
2. You shouldn't allow for a 500 psi buffer/reserve when calculating your RMV.

Re-working your numbers with a more realistic tank choice...

Assume AL80 with capacity of 77.4 cuft and a service pressure of 3000 psi.
77.4 cuft / 3000 psi = 0.0258 cuft/psi
36 psi/min * 0.0258 cuft/psi = 0.93 cuft/min

An RMV of 0.93 cuft/min is rather high for an experienced diver under low work-load conditions, but 0.93 cuft/min is very reasonable for a novice. You didn't mention the work-load for this particular dive, so that may have elevated the RMV somewhat.

You should calculate RMV for all of the dives in your divelog. This will give you a handle on the range of RMV under various dive conditions. Expect the RMV to be elevated for higher work-load dives.

Use these data to arrive at a guesstimated RMV for dive planning purposes. When working the numbers, you should allow for sufficient gas reserves for an estimated "stressed" RMV -- something that might occur in a true emergency. Also, consider that you are carrying your buddy's gas reserve. You should have enough gas to get you and your buddy
to the surface at any point during the dive.

Now I know that I breathe about 1.116 cubic feet per minute. I'm going to dive a 100 cubic foot tank (so call it 95 feet). I know that my SAC is going to be around 36 psi/min at the depth I plan to visit, so should I just say 95 cubic feet / 1.116 cubic feet per minute and solve for minutes? Because that says I have 85 minutes of air!

You need to correct your gas consumption for the ambient pressure at which it is breathed.

Don't forget that your gas isn't just for the bottom phase of the dive. You need gas to descend, problem-solve an issue at depth, ascend, conduct stops, and serve as an ample reserve for your buddy.

 

[xdjio]

I just like to be cautious, of course. I'm also the sort of guy that likes to have a thorough grasp on the mechanics of things. In this case, I'm really digging having numbers laid out.

Here's what I have learned. My proposed dive to 30m (we'll call it 30m average depth) based on 1 data point for SAC (not enough) suggests that my SAC of 1.046 and a 100cuft tank gives me enough air for 23 minutes. Let's be conservative though - I have to get down there, and then back up, right? And make a safety stop. I would think that staying at 30m for no more than 8 - 10 minutes allows a comfortable safety margin, just based on breathing volumes.

That said, I'd want to establish a sane turn pressure too.

I think the plan is - get some more skill and comfort down around 60-90 feet. Get more dives logged, and log the SAC for them too. Home in on a range of SACs for various depths and profiles so I have a good intuitive feel for SAC and RMV when considering future dives, turn pressures, rock bottom, etc.

One step at a time!

But those are all great points to bring it back to the original question. I think I absolutely want to build a better sense of gas planning into my brain though.

 

[xdjio]

These calculations are fine, assuming that the average depth was 15.7 m.

There are a couple problems with this:
1. I don't know of any 100 cuft tanks that have a service pressure of 3000 psi.
2. You shouldn't allow for a 500 psi buffer/reserve when calculating your RMV.

Re-working your numbers with a more realistic tank choice...

Assume AL80 with capacity of 77.4 cuft and a service pressure of 3000 psi.
77.4 cuft / 3000 psi = 0.0258 cuft/psi
36 psi/min * 0.0258 cuft/psi = 0.93 cuft/min

An RMV of 0.93 cuft/min is rather high for an experienced diver under low work-load conditions, but 0.93 cuft/min is very reasonable for a novice. You didn't mention the work-load for this particular dive, so that may have elevated the RMV somewhat.

You should calculate RMV for all of the dives in your divelog. This will give you a handle on the range of RMV under various dive conditions. Expect the RMV to be elevated for higher work-load dives.

Use these data to arrive at a guesstimated RMV for dive planning purposes. When working the numbers, you should allow for sufficient gas reserves for an estimated "stressed" RMV -- something that might occur in a true emergency. Also, consider that you are carrying your buddy's gas reserve. You should have enough gas to get you and your buddy
to the surface at any point during the dive.

You need to correct your gas consumption for the ambient pressure at which it is breathed.

Don't forget that your gas isn't just for the bottom phase of the dive. You need gas to descend, ascend, conduct stops, and serve as an ample reserve for your buddy.

Yeah, corrected that in my little excel spreadsheet I whipped up just now. Thanks for all your input on this - it is well and truly helpful (and interesting to boot).

 

[rhwestfall]

see if you can get a copy of DivePal. Great little simulator to play out some of what can/should happen.

 

[xdjio]

Oh man - they make divepal for iphone? I'm all over that!

 

[dumpsterDiver]

You've dove to 120 feet for advanced certification. You are a self described math nerd. You have no idea what your air consumption rate is or was or how to do calculations based on it. Seriously, did the idea cross your mind to ask the instructor something like...I trust you a lot, but" how do i know this dive is not going to kill me?"

Rules of thumb, rules of thirds blah blah... If it were me, i would be damn sure i know what my SAC rate is on an easy shallow dive before I consider going deep.

Also, and I don't want to sound overy critical, but since your Advanced training left you without the most basic knowedge of how to not run out of air... are you at all worried that your knowledge of other aspects of deep diving might be similarly deficient?

 

[Jim Lapenta]

Just caught that DD. If he went to 120 feet on an aow training dive the instructor seriously violated standards.

Sent from my DROID X2 using Tapatalk

 

[bpotkin]

On the same subject but slightly different, I'm an OW diver who uses the following rule:
Example- depth + 0 + 500. ( 60 ft dive = Start ascent with 600 + 500)
(50 ft dive = start ascent with 500 +500). In addition, I subtract the ascent gas
then use 1/2 out and 1/2 to get back to anchor line. So far I've returned to the
boat with well over 500 lbs. On a 100 ft. dive I would start the ascent with 1500.
Any oppinions?