Another Tables vs. Computers Thread

[TheRedHead]

This is the definitive method right here Folks, what all OW divers should be learning the rudiments of, right after AOW, during Basic Nitrox and continuing on with a full understanding in Adv. Nitrox/Deco & Trimix

I think OW divers should skip AOW and just go to Nitrox, Adv. Nitrox and Deco. You learn a lot more and you don't necessarily have to dive doubles.

 

[HowardE]

I think OW divers should skip AOW and just go to Nitrox, Adv. Nitrox and Deco. You learn a lot more and you don't necessarily have to dive doubles.

I believe that PADI has AOW as a pre-req for Rescue and/or DM... Don't they?

 

[TheRedHead]

I believe that PADI has AOW as a pre-req for Rescue and/or DM... Don't they?

Yes, but you can do SSI Rescue without AOW. DM is not the way I want to go anyway. I don't think AOW is a useful certification and I don't you do either.

 

[HowardE]

Yes, but you can do SSI Rescue without AOW. DM is not the way I want to go anyway. I don't think AOW is a useful certification and I don't you do either.

You know I don't think it's useful as it stands now. I only got mine because they ask for it in Key Largo.

 

[Charlie99]

Ratio deco helps you plan the ascent from a given dive, not necessarily the dive itself.

You plan the dive based on some kind of time at some kind of avg depth (within some fairly narrow window of depths, BT's) then you can use ratio deco to plan the ascent.
.........
Deco is based on the average depth, so if I was doing a dive with some time at 100 and some at 40, I would actually (since there is a big variance in the depths) do the deco for 100 feet. If I was diving between 80 and 100 for an average of 90, then do the 90 deco.

That's pretty much my understanding also, that ratio deco is useful for planning deco from a significant deco dive, but that it doesn't have much relevance to the NDL or minimum deco (or in a few cases minimal deco) dives that we have been discussing.

One question still on the table is how well depth averaging works on dive profiles with a wide spread in the bottom depths.

For the 100' + 40' dive, as I noted above, I just treat it as a single level dive at 100' with a long hang time afterwards. As you note, a 100' + 80' dive can be treated pretty much as a 90' dive.

What happens though in a true multilevel dive, such as when one pops down a wall, then spends the entire dive slowly ascend back up. Is depth averaging applicable to planning such a dive?

I don't have enough experience with depth averaging of that sort of dive, to definitively say one way or the other. If I had to guess, from the few cases I've looked at in detail, I would say that it does work, but becomes unecessarily conservative when the depths involved have a wide range.

Of course, failing by becoming too conservative is a good failure mode. OTOH, until I get a better handle on how depth averaging treats a variety of profiles, to use it would be much like doing a "trust me" dive.

 

[Kevrumbo]

Read pp. 5-6 on Depth Averaging.
FWIW, this particular computer/bottom timer has a convenient & useful depth averaging feature:
http://www.scubapro.com/products/comps/al_tec_wrist/al_tec_wrist.asp

 

[limeyx]

That's pretty much my understanding also, that ratio deco is useful for planning deco from a significant deco dive, but that it doesn't have much relevance to the NDL or minimum deco (or in a few cases minimal deco) dives that we have been discussing.

One question still on the table is how well depth averaging works on dive profiles with a wide spread in the bottom depths.

For the 100' + 40' dive, as I noted above, I just treat it as a single level dive at 100' with a long hang time afterwards. As you note, a 100' + 80' dive can be treated pretty much as a 90' dive.

What happens though in a true multilevel dive, such as when one pops down a wall, then spends the entire dive slowly ascend back up. Is depth averaging applicable to planning such a dive?

I don't have enough experience with depth averaging of that sort of dive, to definitively say one way or the other. If I had to guess, from the few cases I've looked at in detail, I would say that it does work, but becomes unecessarily
conservative when the depths involved have a wide range.

Of course, failing by becoming too conservative is a good failure mode. OTOH, until I get a better handle on how depth averaging treats a variety of profiles, to use it would be much like doing a "trust me" dive.

I think we mostly agree. I would just say minimum deco is a result of the ratio deco "collapsing" down to a more simple form (just as a tech1 dive uses a more collapsed version of it than a tech2 dive)

Depth averaging indeed generally will be more conservative for a lot of multi-level profiles, and the further spread out the depths, the harder it gets to maintain. But if you choose the right gases, then it gets a bit easier.

 

[limeyx]

This is very true, but many of my dives in Cozumel are of the type where we drift until until gas dictates the DM blows the bag and we make our way up. I'm looking at my Suunto dive manager and see I did a dive in April with BT of 1:15 max depth of 106 ft, avg. depth of 55.8 feet on EAN32. The deepest part of the dive occured at 31 minutes into the dive and we were 53 minutes into the dive before we ascended to 49 feet. Most of the dive was spent between 50-60 feet. We gradually ascended and at 69 minutes into the dive we were at 20 feet and spent 5 minutes at 20-15 feet and a minute at 10 and ascended. We did this dive on 100s and 120s and this was the second dive after a 2.5 hour SI. I incurred no deco on this dive with my conservative Suunto and I don't see any way it could have been planned this way or dived without a computer.

How did you plan your bailout gas etc? You had no deco owing at any point during the dive? Not even when you were at your deepest point and starting to ascend?

I would think you need to have a profile in mind before you get in the water to plan your dive. I dont think you can rely on your dive computer.

what if you have a problem at minute 31 (your buddy goes OOA) and you have to ascend, so dont have the luxury of that nice long slow ascent? How was that planned for? Did you really have enough gas for 2 divers to do proper 'deco' on ascent?

If you know your profile, I dont think a computer is needed in the water. If the profile is too variable to predict from the surface and you rely on a computer in the water, what happens if you ascent to 30 feet and it says you have 10 mins of deco left? and then at 20 you have 20 mins? how do you know in advance that you wont be in that situation?

I'd honestly like to hear more details on how you are doing it.

 

[TheRedHead]

How did you plan your bailout gas etc? You had no deco owing at any point during the dive? Not even when you were at your deepest point and starting to ascend?

No deco at any point. I'm looking at the profile right now. For bailout gas, we adjust depth according to gas supply. This was an odd profile and that is why I offered it as an example. We generally do the deepest part of the dive first. This may have been a case where we were diverted by the current into an unusal profile. I honestly don't remember.

I would think you need to have a profile in mind before you get in the water to plan your dive. I dont think you can rely on your dive computer.

We do have a profile in mind when we start the dive, but it doesn't always work out because of the currents. For example, the plan may be to dive Devil's Throat, but we can't make the entrance in time because the current is ripping, or someone in the group doesn't feel right about it, so we abort and go over Punta Sur instead of through the throat. That would change things somewhat.

what if you have a problem at minute 31 (your buddy goes OOA) and you have to ascend, so dont have the luxury of that nice long slow ascent? How was that planned for? Did you really have enough gas for 2 divers to do proper 'deco' on ascent?

I had over 2000 psi at that time from my high pressure 100 cu ft cylinder that was filled to 3538 at the start of the dive. If I didn't have the gas I wouldn't have done it. There are many times when we swim over the group. And it's usually because the conservative Suunto doesn't like the profile not because of gas. My SAC is .37 on most dives.

If you know your profile, I dont think a computer is needed in the water. If the profile is too variable to predict from the surface and you rely on a computer in the water, what happens if you ascent to 30 feet and it says you have 10 mins of deco left?

I don't ge myself into those situations - that's just carelessness, not paying attention to your gas. I don't go into deco on these types of dives unless it is a planned deco as with some of the deep dives. We plan those and do deep stops as well as the deco make our computers happy. However, there is a real concern about down currents on deep walls which can put you into unplanned deco and change the plan considerably. We don't dive Nitrox on walls for exactly that reason and we do the wall at the beginning of the dive and then move up to a shallower reef with a bottom.

This type of drift diving isn't for inexperienced divers. I've done over a hundred dives in Cozumel and feel comfortable with this. I certainly didn't as a new diver. I dived square profile tables for my first 30 dives. I didn't even own a computer.

 

[limeyx]

No deco at any point. I'm looking at the profile right now. For bailout gas, we adjust depth according to gas supply. This was an odd profile and that is why I offered it as an example. We generally do the deepest part of the dive first. This may have been a case where we were diverted by the current into an unusal profile. I honestly don't remember.



We do have a profile in mind when we start the dive, but it doesn't always work out because of the currents. For example, the plan may be to dive Devil's Throat, but we can't make the entrance in time because the current is ripping, or someone in the group doesn't feel right about it, so we abort and go over Punta Sur instead of through the throat. That would change things somewhat.


I had over 2000 psi at that time from my high pressure 100 cu ft cylinder that was filled to 3538 at the start of the dive. If I didn't have the gas I wouldn't have done it. There are many times when we swim over the group. And it's usually because the conservative Suunto doesn't like the profile not because of gas. My SAC is .37 on most dives.



I don't ge myself into those situations - that's just carelessness, not paying attention to your gas. I don't go into deco on these types of dives unless it is a planned deco as with some of the deep dives. We plan those and do deep stops as well as the deco make our computers happy. However, there is a real concern about down currents on deep walls which can put you into unplanned deco and change the plan considerably. We don't dive Nitrox on walls for exactly that reason and we do the wall at the beginning of the dive and then move up to a shallower reef with a bottom.

This type of drift diving isn't for inexperienced divers. I've done over a hundred dives in Cozumel and feel comfortable with this. I certainly didn't as a new diver. I dived square profile tables for my first 30 dives. I didn't even own a computer.

You still aren't providing any details of how you actually do the planning though.
Saying I had 3400 psi to start and 2000 when I left the bottom is kind of vague.

and how do you adjust depth according to gas supply? inquiring minds want to know!