Maximizing depth AND time under

[selytch]

I have a "problem" - relatively low SAC rate of about 13 lpm (0.4 cf/m) on an average dive (air).

I need help planning dive to maximum depth and longest duration based on my SAC rate, while preferably staying within NDL.

I usually dive Molokini - a crater on Maui. It has relatively steep descend from 40feet to 160+.

At the end of the dive I usually have ~1000psi after ~40 minute dive to max of 120 feet or so.
I have to ascend early either because of NDL, buddy running out of gas or being bored on the ascent forced to explore shallower depth (due to NDL again).

My typical profile is like this:
|...........................|
|........................__/
\................______/
.\....... ______/
..\....../
...\____/

When planning a dive I have to follow certain rules:
1. Stay within NDL
2. Stay within max depth (well, maybe a little extra)
3. Do deepest portion of the dive first
4. Come up with some air left (though for our type of diving I would rather come up with 50psi)

So again, how do I plan dive to maximum depth and longest duration based on my SAC rate, while preferably staying within NDL.

Software/tricks/suggestions are very appreciated!

 

[Codiak]

Trick? It's just math, no tricks

A dive computer will allow you to adapt your profile and be a bit more flexible then tables.

If the goal is to extend NDL, use Nitrox.
If the goal is to surface with an empty tank, use a smaller one. Or do some air sharing and let you buddy stay longer

 

[selytch]

Thanks for the reply!

Dive computer calculates time on the fly, but this does not really help with planning in advance.
Nitrox does not allow diving as deep as I usually do.

I've been diving Molokini more than 20 times and still cannot not figure out how to go deep and use all the air at the same time.

 

[DwayneJ]

Unusual question -

At depth (on the same mix using the same computer/conservatism) we are all going to hit NDL limits at the same time. You will have more gas in your tank.

1. Dive computers don't take your sac rate into consideration so having a low SAC rate only really helps at shallower depths (less than ~60) when you are not hitting NDL. For me I find around 60'-65'/45 minutes is a depth where gas and NDL are about the same so that I have 750-1000lbs when I surface (HP120). For you, that magic depth would be shallower but longer dive time.

2. Knowing an optimal depth with a single step or multi step profile may be great for you but never will match your buddies endurance.

3. Depending on fitness, water temp, vis, recent dive experience, sac rate can be quite different. I find I breathe like a pig switching back to Open Circuit after diving Closed Circuit.

4. There are plenty of software choices like V-Planner and such to calculate a multistep dive but ideally the team would be diving the same dive profile with the same levels of conservatism. Desktop computer and fixed tables do not allow for flexibility in your dive plan.... Dive The Plan! Ideally you wrist computer matches the algorithm on your Desktop Computer for backup.

I would recommend sitting down with a deco instructor. Every dive to the limit is an opportunity to get bent and if you are going to dive as such, you really need to understand the algorithm/level of conservatism you are using.

Dwayne

 

[Codiak]

So your example of max depth was 120... 32% Nitrox would do the trick.

The problem you have is the NDL is a fixed limit regardless of how much air is in your tank.
Exceeding NDL and going into deco is always an option just not recommended without the proper training. BTW part of that training is the gas planning your looking for.


So are you try to match available air to create the longest dive possible? If that's the case you can dive shallower extending the NDL.
Or are you trying to go deeper and want to calculate how deep you could go on that same tank?

 

[selytch]

Molokini has pretty much constant temperature year-round, some variation with currents but the underwater landscape is pretty well known - there are several directions but all of them have similar profile with shelf @ 40ft and rapid drop-off to 160ft.

Interesting things (for me) are at about 120-140 ft.

Options I was exploring:
1. vertical descent with gradual ascent (hit NDL fast at the bottom and it chases all the way up).
2. slower descent with some time at 80-90 ft (can't even get to the depth due to NDL).
3. brief stay at 140 feet with slow ascent - NDL catches up on the ascent.
4. brief stay at 140, quick ascent to 40-50 feet - boring end of the dive and left with plenty of air.

Is there a way to explore/experiment with these profiles (and plan new ones) before the dive? My computer software (pc logbook with archimedes II) allows some simulation but I cannot find optimal profile.

Or am I just wasting time and should either stick with Nitrox for shallower depth or start taking some tech classes for trimix/deco dives?

 

[Codiak]

Unusual question -
2. Dive computers don't take your sac rate into consideration

Air Integrated Computers such as the Galileo monitor remaining bottom time based on NDL and actual gas usage for the current dive.
So while it's true they don't use SAC, they go one better.
From the manual
"The RBT (remaining bottom time) is the time you can spend at the current depth and still have enough gas supply to make a safe ascent and reach the surface with the tank reserve. The RBT calculation is based on your current breathing rate, accounts for any existing and upcoming decompression obligation and for any temperature gradient in the water. It assumes an ascent at the ideal ascent rate (defi ned in section 3.2.1). A higher value of the tank reserve is more
conservative, but limits your diving time. A lower value gives you more diving time but the risk of running out of gas supply "


As the OP notes it's less helpful with planning the dive ;-)

 

[selytch]

So your example of max depth was 120... 32% Nitrox would do the trick.

The problem you have is the NDL is a fixed limit regardless of how much air is in your tank.
Exceeding NDL and going into deco is always an option just not recommended without the proper training. BTW part of that training is the gas planning your looking for.


So are you try to match available air to create the longest dive possible? If that's the case you can dive shallower extending the NDL.
Or are you trying to go deeper and want to calculate how deep you could go on that same tank?

I try both
I do go deeper than 120 (140 often, down to 170 rarely) so 32% can be too much O2. Though Nitrox is definitely an option @ 120ft.

@Codiak, I did consider blending class/deco diving class but could not find where to start. I know of IANTD, however on Maui, in absence of competition between courses, prices are quite high here. Not sure, if it is worth the investment...

 

[Codiak]

Molokini has pretty much constant temperature year-round, some variation with currents but the underwater landscape is pretty well known - there are several directions but all of them have similar profile with shelf @ 40ft and rapid drop-off to 160ft.

Interesting things (for me) are at about 120-140 ft.

Options I was exploring:
1. vertical descent with gradual ascent (hit NDL fast at the bottom and it chases all the way up).
2. slower descent with some time at 80-90 ft (can't even get to the depth due to NDL).
3. brief stay at 140 feet with slow ascent - NDL catches up on the ascent.
4. brief stay at 140, quick ascent to 40-50 feet - boring end of the dive and left with plenty of air.

Is there a way to explore/experiment with these profiles (and plan new ones) before the dive? My computer software (pc logbook with archimedes II) allows some simulation but I cannot find optimal profile.

Or am I just wasting time and should either stick with Nitrox for shallower depth or start taking some tech classes for trimix/deco dives?

Hey, your discussing diving so it's not a waste... the issue is understanding the goal to help find the solution.

When I dive my semi-closed rebreather, like you I'm always on the surface with gas to spare thanks to NDL.... there's no way to beat it without going into deco .

#3 is closest to how I dive it... pick the max bottom, stay just under NDL and ride the NDL-wave to the surface some time later. In essence your trading NDL time upwards but shallower.
A dive computer makes it very easy to stay within the limits.

Remember that NDL is calculated based on a DIRECT ascent... so once you hit the limit it's up to your safety-stop.


From what you describe I'd suggest the TDI Deco procedures and Advanced Nitrox classes.... and find a buddy with the same training ;-)

 

[selytch]

No TDI deco instructors on Maui... Any preference of TDI over IANTD?