Deep Dive Strategy

[mccabejc]

Wow. Thanks for all the info.

-hh, I think your discussion can be summarized in the bar graphs of the changing compartment saturations, no? That's why I personally think the simulation software is so much better to learn from than just the tables. You can get a better sense of some very complex interactions. But again I'm just guessing.

Anyway, I'm still not clear on the practicality of managing those dives where NDL, not SAC rate is a controlling factor. When you drop down to 90ft. for 10 or 15 minutes, and your NDL is down to 5-10 minutes, and you're trying to figure out how to maintain a no decompression dive, you need to have some sense of how to gain NDL time while at the same time not ascending too quickly, and not ultimately running out of air, etc.

 

[opiniongirl]

I haven't gone through all of these posts, so forgive me if someone's said this.

A little bit of knowledge can be a dangerous thing? Outstanding if you are using such information to expand your knowledge, but this board is no subsitute for training if you want to start pursuing more advanced techniques..which will ensure you have all the information required to adequately plan such dives...

Again, compliments on your search and thirst for knowledge...just a caution.

 

[lostinspace]

Anyway, I'm still not clear on the practicality of managing those dives where NDL, not SAC rate is a controlling factor. When you drop down to 90ft. for 10 or 15 minutes, and your NDL is down to 5-10 minutes, and you're trying to figure out how to maintain a no decompression dive, you need to have some sense of how to gain NDL time while at the same time not ascending too quickly, and not ultimately running out of air, etc.

You can't gain NDL time. And you should certainly never plan on having a sawtooth dive profile. If your NDL is at 5 minutes, and you have been diving for 10-15minutes, then it is time (if you are on 10L air) to start ascending slowly up to 60ft/20m. Check your air pressure - still more than 30% left? If yes, check your NDL time and swim around for a while. Ascend - slowly - when you reach 30% of air left, and if your buddy has more air, more power to them but you still ascend together. Assuming you are still on NDL time then do your 3 minute safety stop.
If you are on holiday and your dive group is still down deep, don't be afraid that you & your buddy swim shallower than the group, just follow their bubbles. Don't feel the peer pressure of remaining deep if you are not comfortable with it.
One thing to be aware of it that air consumption increases disproportionately from 20-30m (60-90ft). For your deeper dives, be obsessive with your buddy about checking each other's air consumption.

 

[mccabejc]

You can't gain NDL time.

I think you can. If you ascend, in many cases your NDL time will increase as you ascend. At least my simulator shows that. That's what decompression is, no?

 

[Xman]

You're right, you can "gain" NDL time as shown on your computer / simlutaion. I think the point he was quite rightly making is that you shouldn't. If you have been at depth (30m/90ft), and are getting close to requiring deco stops (running out of NDL in your terms I guess), then to keep within safe limits, you should not just swim shallower until you have more time. The point is, if you approach the NDL it is time to start up SLOWLY to a depth where you will actually be decompressing in a meaningful way (as opposed to just putting off you NDL).
It really is worthwhile getting some deep dive training before you start making statements about deep dive planning that others might actually listen to.
Having said all of that, it is fantastic that you are wanting to gain further dive knowledge, and increase your understanding of the theories involved. Maybe buy a copy of the PADI Encyclopedia of Recreational diving and give it a read. It will help give you a basic understanding of the relevant principles.

 

[Dive-aholic]

I think you can. If you ascend, in many cases your NDL time will increase as you ascend. At least my simulator shows that. That's what decompression is, no?

Buy the dive wheel and read the manual that comes with it. It's a little pricey, but it will explain the concept of multilevel diving, which is what your simulator is doing. You cannot gain NDL time at the same depth unless you are breathing a higher concentration of oxygen (ie. nitrox--this is another class well worth taking) However, you can increase your NDL time for the entire dive by ascending to a shallower depth. There are limitations on this based on what's been tested. For instance, spending 5 minutes at 100 feet doesn't mean you can ascend to 90 feet and gain time. You need to ascend to 60 feet (I believe-I don't have my wheel with me so can't say for sure) before you can truly call it a multilevel dive and give yourself more bottom time at the shallower depth than if you just stayed at the deeper depth. It's true that OW classes can be very basic; they're designed that way. The deep class offered at your lds might not provide much more either. But the Deep diver book, though, and read through it. It does provide some good information. It even has a SAC rate table. Again, I also recommend the wheel. It sounds like you'd have more fun playing with it than your simulator.

 

[scottfiji]

I think you can. If you ascend, in many cases your NDL time will increase as you ascend. At least my simulator shows that. That's what decompression is, no?

Of course you can gain NDL time. Jim, you are correct, when you ascend to a certain depth, the NDL time on your computer time goes up. When I dive to 90 feet, I try not to get close to my NDL time, and I SLOWLY swim up to 40 feet, as X-Man said.

I think LostInSpace meant to say that you can't (or shouldn't) increase your NDL at a specific depth, meaning you can't have 5 minutes left at 90 feet, swim up to 40, and then swim back down to 90 and have more than 5 minutes of NDL. (Which I know is not what you wanted to do).

 

[lamont]

I was futzing around with my Cobra simulator on the PC, and simulating dives with max depth of around 90 ft. I've never gone deeper than 60ft., so I'm trying to learn how to plan for a deep dive.

Anyway, it seems to me that if you're going down that deep, your plan at the outset has to be something like "we'll hang around at 90 ft. no longer than 5 minutes or so and then start back up" if you don't want to make it a decompression dive.

Also, when your max depth is 90 ft., it looks like you need to ascend to about 40ft. before you start adding/recovering NDL time.

In contrast, when your max. depth is only 60ft., it looks like you begin adding/recovering NDL time as soon as you start ascending above 60ft.

This simulator gizmo is cool....

Here are some things that I plan out for a 100 ft dive:

- I know my rock bottom at 100 ft on the tank (1000 psi)
- I know how much gas I expect to use in the first 10 mins (500 psi)
- I know how much gas I'll use at depth every 10 mins (800 psi)
- I know that my SAC is 200 psi / ATA every 10 mins and can adjust
my gas consumption expectations based on my depth.
- I check at 10 mins to make sure that I'm not consuming gas at an unusually high rate
- I know what my NDL is at 100 ft (20 mins)
- I know at what depth I'll be limited by NDL and at what depth I'll be limited by air.

 

[Charlie99]

I think LostInSpace meant to say that you can't (or shouldn't) increase your NDL at a specific depth, meaning you can't have 5 minutes left at 90 feet, swim up to 40, and then swim back down to 90 and have more than 5 minutes of NDL. (Which I know is not what you wanted to do).

Thinking and talking about NDL instead of compartment loadings can lead you to making a lot of wrong assumptions, such as the one above.

Ignoring the sawtooth profile for a moment, what you describe above is not that much different than going to depth for an NDL period, surfacing for an SI, then returning to depth. OF COURSE you will have longer NDL after your SI.

Similarly, if you go down to 90' until your computer reads 5 minutes NDL, ascend to 40' for several minutes and then for some reason (to help someone, to retrieve lost gear, etc) you return to 90', you will indeed have greater than 5 minutes NDL.

Hopefully, you will also have enough gas to do a proper slow ascent with deep stops.

-------------------

The ability to think in terms of and mentally track compartment loadings may change your willingness to dive various profiles. For example, exceeding NDL at 100' by a couple minutes is of less concern to me than being at 60' with a couple minutes of NDL remaining.

Getting a good margin of safety before surfacing is lot easier coming from slightly over NDL at 100' than from near, but still within NDL, at 60'. This difference is that for an immediate descent to a flat 100' dive, the limiting compartments are the fast ones. OTOH, if I'm at 60' and my computer is saying 5 minutes NDL left, then the problem is that the medium speed (30 and 40 minute HT) compartments are near the limits. Obviously, those 30 and 40 minute compartments take a lot longer to offgas than the 10 minute compartment that controls the 100' dive.

Once you get a feel for what is going on in the various speed compartments, you will also see the problem with "riding the NDL" back up. What happens as you slowly ascend while keeping the computer a few minutes inside NDL is that the controlling compartment shifts to the slower ones and you end up at 40' or 50' feet with a large nitrogen load that will be slow to offgas.


Dive safe, dive often,

Charlie

 

[DepartureDiver]

Does anyone understand the concepts and equations well enough to give me a short synopsis?

The others are right. You do gain NDL time. Just like a safety stop does your body good, and doesn't penalize you, ascending to a shallower depth starts giving you time back. At the deeper depths, your "fast compartments" limit the dive. When you ascend, they start offgassing and losing nitrogen, i.e. they gained nitrogen fast and may be in equilibrium at the deep depth. When you ascend, they start losing nitrogen and start going to equilibrium at the shallower depth. The slow compartments are always ongassing until you surface and will limit your shallower depths. In reality it is not quite this way and is more complex, but it will give you an idea. If you play with your tables, you will notice a trend and you can even make mental calculations underwater. You will notice that if you cut your depth in half, the NDL time is quadruple (actually a little more) For example, if you have an NDL of 12 min. at 120', the NDL can be 48 min. at 60'. You can take this and put it into practice. You can take your actual time at 120', double it, and then subtract if from the 60' NDL to calculate how much time you will have when you ascend to this depth. So, if you dive to the full NDL of 12 minutes at 120', then you half one-half of the 60' NDL time left, i.e. 24 minutes (48 minutes minus [12 minutes times two]). If you dive to 120' for 6 minutes, then you would have 36 minutes left at 60'.