Computer VS. Tables

[boulderjohn]

What about this, guys ("Dive Computer" vs Tables vs "Smartphone or Desktop Computer")?

1. Throw away the whole idea of NDL (consider all dives deco).
2. Plan max depth and max time (assume square profile).
3. Use software to calculate how much deco time you will need.
4. On ascend do the 1-minute mini-deco stops and deco stop at 15-20 ft according to the plan.
5. On second dive - repeat, but double up the mini-deco and deco stop at 15-20ft.

All this "NDL" model is purely theoretical anyway and it will differ vastly from person to person. Why not just treat every dive deco and stay at 15-20 ft long enough?

Any criticism?

1. Item 3 assumes all decompression dives will end with a deco stop of varying lengths at 15-20 feet. That is not true.
2. Items 4 and 5 contradict item 3, unless you write a new software program that tells you to do items 4 and 5.
3. True deco stops demand greater training than divers have in NDL diving. Because a safety stop in NDL diving can usually be safely skipped, a diver does not need to worry about staying at depth in an emergency situation. If things go bad, they can always go to the surface. Divers who do planned decompression bring equipment and training that allows them to deal with emergencies at depth without the need to surface immediately.
4. Not all dives look like the one you describe. There is a significant advantage to using multi-level principals on multi-level dives. Why throw that advantage away?

 

[mrdre]

1. Item 3 assumes all decompression dives will end with a deco stop of varying lengths at 15-20 feet. That is not true.
2. Items 4 and 5 contradict item 3, unless you write a new software program that tells you to do items 4 and 5.
3. True deco stops demand greater training than divers have in NDL diving. Because a safety stop in NDL diving can usually be safely skipped, a diver does not need to worry about staying at depth in an emergency situation. If things go bad, they can always go to the surface. Divers who do planned decompression bring equipment and training that allows them to deal with emergencies at depth without the need to surface immediately.
4. Not all dives look like the one you describe. There is a significant advantage to using multi-level principals on multi-level dives. Why throw that advantage away?

1. If we are talking about dives within recreational limits - they are (plus stop at 50% of the depth). I never said "ALL decompression dives".
2. No they don't. The software was written long ago. For iphone "Baltic Lite" for example (it's free, i'm not promoting the app - there are tons of these). Based on this model: Bühlmann decompression algorithm - Wikipedia, the free encyclopedia Pick ZHL16-B
Using this strategy: Ascending From A Dive (ascent rates, deep safety/deco stops & time limits) (for example, just first thing came up on google, there are a lot of articles like that)
The mini-deco stops are basically making you to ascend at 30 feet per minute. I don't see any contradiction.
3. True, but we are not talking about "true" deco stops. We are talking about recreational dives, where people more likely run out of air before reaching NDL and even if they do they don't cross it significantly to require "true" deco stops. Not staying in depth in emergency is same for tech divers too. If there is a choice between ascending with DCS risk and drowning then obvious choice would be ascending. Any diver with 100 dives should be able to perform slow ascent and mandatory stop at 20 - they suppose to learn that in OW class.
4. The main disadvantage with multi-level principals is that you can't really plan your dive (unless you know your profile ahead, but if you do, then there is no difference from using desktop software). Your computer calculates theoretical levels of nitrogen on the fly based on your depth/dive time. You just dive watching your computer until it tells you "it's enough, end the dive". Of course it's a safe strategy and it works for most of the divers. But if you treat every dive as deco and assume conservative square profile, then you will be required to do mini-deco stops (think of it as very slow ascend) and mandatory 15-20 ft stop. This way you can plan your dive 100% ahead. That's the advantage I see.

 

[Storker]

The main disadvantage with multi-level principals is that you can't really plan your dive (unless you know your profile ahead, but if you do, then there is no difference from using desktop software).

I plan multilevel dives all the time. Shore dives on steep or sloping bottoms and wall dives are extremely simple to plan as multilevel dives. More often than not, a multilevel profile is the best plan for these dives. Deep section on the first (outgoing) leg, shallow section on the second (return) leg is a great way to ensure you don't swim over the same terrain. And, as a bonus, you get more bottom time than if you're diving a square profile.

I might be a bit weird, but I usually don't bring my desktop computer on a diving outing. I have no problem planning those dives on my PDC and/or my RDP.

 

[boulderjohn]

1. If we are talking about dives within recreational limits - they are (plus stop at 50% of the depth). I never said "ALL decompression dives".
...
3. True, but we are not talking about "true" deco stops. We are talking about recreational dives, where people more likely run out of air before reaching NDL and even if they do they don't cross it significantly to require "true" deco stops.

I see. So when you said

1. Throw away the whole idea of NDL (consider all dives deco).

what you really meant was "throw away the whole idea of NDL except in the case of NDL dives. Keep the idea there. Consider all dives deco, except for deco dives--we're not talking about them." Glad you straightened that out. (OK, I have to admit that I am now thoroughly confused.)

4. The main disadvantage with multi-level principals is that you can't really plan your dive (unless you know your profile ahead, but if you do, then there is no difference from using desktop software).

Sure you can. There are lots of ways to plan a dive. You just can't do it the way you know how to do it. You don't say where you are from. If you are anywhere near Colorado, I teach a distinctive specialty course on dive planning, and I will be happy to show you more options.

 

[mrdre]

Storker and boulderjohn, how often you actually PLAN a multilevel dive? Meaning how often you actually plan like this "I will descend to that depth and stay there that long, then I will ascend to that depth and stay there that long". And how often you actually FOLLOW such plans? Also, how often you think the recreational divers plan such way and actually dive their plans? How often you use any kind of software to enter all these different depths to calculate your NDL? If you don't, how do you know when are you gonna hit your NDL during the dive? If you do plan that way, how exactly you do it?

Basically my question is: how do you know ahead of the multilevel dive when are you gonna hit NDL? What sorts of calculations you do?

You don't carry a desktop computer, but a lot of people carry smartphones, it's not the point.

---------- Post added August 25th, 2014 at 12:13 PM ----------

boulderjohn, I updated my profile if it makes any difference. I'm in LA.

 

[diver 85]

OP........IF I did this, it'd just show me how much BT I missed out on over the 20+ years prior to me using a computer-------& I'd get very mad....

 

[boulderjohn]

Storker and boulderjohn, how often you actually PLAN a multilevel dive? Meaning how often you actually plan like this "I will descend to that depth and stay there that long, then I will ascend to that depth and stay there that long". And how often you actually FOLLOW such plans?

As I said, there are many ways to plan dives, and I am sorry that appears to be the only one you know. I am also sorry that you seem to consider all plans that don't match yours to be not planning.

Also, how often you think the recreational divers plan such way and actually dive their plans?

On a multi-level dive? I don't think many do that at all, and I do not think they have to.

How often you use any kind of software to enter all these different depths to calculate your NDL?

On a planned decompression dive? Every time. On a planned NDL dive, never.

If you don't, how do you know when are you gonna hit your NDL during the dive? If you do plan that way, how exactly you do it?

Basically my question is: how do you know ahead of the multilevel dive when are you gonna hit NDL? What sorts of calculations you do?

You don't carry a desktop computer, but a lot of people carry smartphones, it's not the point.

As I said repeatedly, there are lots of ways to plan a dive, and you do the way that is most appropriate to the situation.

Here is an example of a dive plan for a site like Bonaire or Curacao, which have a slightly sloping walls with a light current: As we start our descent, note a distinguishing characteristic at a specific depth with safety stop range. Descend to a maximum depth of 85 feet and start heading into the current. Ascend a little at a time, based upon what we are seeing, planning to be at about 50-60 feet when one of us hits half of our PSI. We will turn and ascend a little and return with the current, ascending a little at a time until we are sure we are at the depth of our specific landmark before we hit it. When we hit it, check our gas situation. If we still have 1,000 PSI or so left, poke around the area and then head back to shore when one of us hits 750 PSI. As always, give the deco signal and ascend a little if you get within 5 minutes of NDL.

Here is an example of a dive plan for a shore dive to a known wreck or something else of interest at a known depth: Head down the slope following the known landmarks until we reach the wreck. Start circling the wreck near the sand line and then check out the main deck. Go to the wheel hose and then start our return when one of us hits [XXXX] PSI. Follow the landmarks back to shore. As always, give the deco signal and ascend a little if you get within 5 minutes of NDL.

Here is an example of a wreck dive from a boat in current with a descent/ascent line: Descend down the line and check out the current situation at the wreck. If it is heading toward the descent line, start exploring into the current and then turn the dive at 1/3 of our PSI, not counting 1,000 for ascent. Return with the current. If we get near the ascent line with lots of gas, explore that area near the ascent line more thoroughly. Begin our ascent on the line when one of us hits 1,000 PSI. When we hit it, turn and head to shore. As always, give the deco signal and ascend a little if you get within 5 minutes of NDL.

The course I teach has an entire chapter on different ways to plan dives. If you want to play the "well, what about this situation" game, I can cover whatever you want to throw at me. the key is to be a thinking diver and make a plan that works for your situation rather than be bound by some one size fits all rule that may make no sense for the dive you are planning to do.

 

[mrdre]

...

Here is an example of a dive plan for a site like Bonaire or Curacao, which have a slightly sloping walls with a light current: As we start our descent, note a distinguishing characteristic at a specific depth with safety stop range. Descend to a maximum depth of 85 feet and start heading into the current. Ascend a little at a time, based upon what we are seeing, planning to be at about 50-60 feet when one of us hits half of our PSI. We will turn and ascend a little and return with the current, ascending a little at a time until we are sure we are at the depth of our specific landmark before we hit it. When we hit it, check our gas situation. If we still have 1,000 PSI or so left, poke around the area and then head back to shore when one of us hits 750 PSI. As always, give the deco signal and ascend a little if you get within 5 minutes of NDL.

Here is an example of a dive plan for a shore dive to a known wreck or something else of interest at a known depth: Head down the slope following the known landmarks until we reach the wreck. Start circling the wreck near the sand line and then check out the main deck. Go to the wheel hose and then start our return when one of us hits [XXXX] PSI. Follow the landmarks back to shore. As always, give the deco signal and ascend a little if you get within 5 minutes of NDL.

Here is an example of a wreck dive from a boat in current with a descent/ascent line: Descend down the line and check out the current situation at the wreck. If it is heading toward the descent line, start exploring into the current and then turn the dive at 1/3 of our PSI, not counting 1,000 for ascent. Return with the current. If we get near the ascent line with lots of gas, explore that area near the ascent line more thoroughly. Begin our ascent on the line when one of us hits 1,000 PSI. When we hit it, turn and head to shore. As always, give the deco signal and ascend a little if you get within 5 minutes of NDL.

The course I teach has an entire chapter on different ways to plan dives. If you want to play the "well, what about this situation" game, I can cover whatever you want to throw at me. the key is to be a thinking diver and make a plan that works for your situation rather than be bound by some one size fits all rule that may make no sense for the dive you are planning to do.

OK. One more time. How do you know with "a little more time" "lots of gas" "still have 1000 PSI" sort of "planning" when are you gonna hit your NDL? let's say on the third dive with 1 hour SI between the other two? How do you know when you are going to hit NDL AHEAD of the dive? You are gonna watch your dive computer to see if you hit NDL, right? You are not gonna KNOW when you are gonna hit it right? So you aren't gonna KNOW ahead of the dive when you will HAVE to ascend, right?

 

[boulderjohn]

OK. One more time. How do you know with "a little more time" "lots of gas" "still have 1000 PSI" sort of "planning" when are you gonna hit your NDL? let's say on the third dive with 1 hour SI between the other two? How do you know when you are going to hit NDL AHEAD of the dive? You are gonna watch your dive computer to see if you hit NDL, right? You are not gonna KNOW when you are gonna hit it right? So you aren't gonna KNOW ahead of the dive when you will HAVE to ascend, right?

No. I was in a hurry and left some information out. The chapter is a long one, and I can't write it all down in a post. I didn't mention knowing your SAC rate so that you are sure you have enough gas, either. You are focused on NDLs, so that is where I focused.

Your computer has a planning function. You go to that. You put in your planned maximum depth. It tells you how much time you have before you hit NDLs if you entered the water at that time. That tells you how long you have if you stayed at that maximum depth the whole time. If You know that you are going to be above that maximum depth for quite some time, so you aren't going to stay down that deep that long, but if it is cutting too close, you don't do it. You wait until you see you have enough time.

Think some of this through for a moment. In one of the examples I gave above, we are talking about being at safety stop depth at around 1,000 PSI and poking around looking at stuff until 750 PSI. That much planning is not precise enough for you, by golly. You want to know precisely when I will hit NDLs. Well, at 15 feet, that will probably be some time the next day. I don't think it is necessary to know that. In the other case you ridicule, I mentioned hanging out near the ascent line until we hit our planned ascent pressure. You indicate that isn't enough planning for you. I think that if I have planned to start the ascent at 1,000 PSI and I am 25-30 feet from the ascent line looking around with the current behind me, I think I have the ability to calculate on the fly when I need to start swimming those 25 feet in order to get there in time. You evidently don't have that ability yet, but if you keep working on it, you, too, will be able to do it with confidence.

I am a cave diver. We do very precise planning in cave diving. We match tank volumes if we are using dissimilar tanks so we can be most accurate. We cannot fail to get back to the mouth of the cave at the end of the dive. When we do that, we usually use the rule of thirds. We use 1/3 of our gas going in. If we use the same amount coming back (and we should usually use less because of flow), we have 1/3 left for emergencies. When we are planning the dive, after we match tanks we determine what the turn pressure is for each diver, and we turn the dive whenever one of the divers hits that pressure. That's really all there is to it. We do not do what you seem to want us to do--predict how many minutes into the dive that will happen, and how many feet we will have progressed. Yes, we have a rough idea of that, and we use that rough idea if we are trying to hit a certain place in the cave, but we can't predict it all that perfectly. The current may be higher than expected. We may just not be feeling it that day. If we have a goal in mind and don't reach it, we don't reach it. We turn on pressure--not time--not distance--not a predetermined goal.

What about NDLs? In some caves, like ini Mexico, there is not much chance of that if we are diving nitrox. In other cases, there is a chance or even a certainty, and we are prepared for that via our training in decompression diving.

If you look at what I just wrote, you will see that even in the most precisely planned dives, there is a certain amount of adjustment based on the situations that arise. You have to be able to do that. I once did a dive in Hawai'i. We had a specific plan for the dive that went out the window the minute we looked down and saw a manta ray in a cleaning station below us. We went down to it and watched as the cleaner wrasse worked it over. The ray left, and we were about to do the same when an eagle ray took its place. We watched that for a while, signaling back and forth to each other about our air supply and NDLs. We signaled to go up, and we stopped at about 50 feet where we saw something else of interest--a helmet conch successfully stalking a decorator urchin. Again we communicated our situation and ascended to the top of the reef, where we checked things out until our gas supplies ran low and then ascended. At all times we had enough gas to do what we were doing and ascend with a buddy in an emergency. At no time were we in danger. I assume you would have stuck with your original plan and ignored all of that. In contrast, I think what we did was perfectly safe, and I would happily do it again. It was one of the best recreational dives I have ever had.

 

[Storker]

Storker and boulderjohn, how often you actually PLAN a multilevel dive? Meaning how often you actually plan like this "I will descend to that depth and stay there that long, then I will ascend to that depth and stay there that long".

Very often. I prefer walls, steep or at least sloping bottoms to flat bottom sites. Multilevel dives are perfect for such sites.

And how often you actually FOLLOW such plans?

Ever heard the quote 'plan your dive, dive your plan'?

Also, how often you think the recreational divers plan such way and actually dive their plans?

I don't know about "the recreational divers", but I and some of the guys I dive with do. I might have a skewed impression of diver competence, though. You see, up here, divers generally are competent to plan and execute a dive without relying on a DM or guide to babysit them.

How often you use any kind of software to enter all these different depths to calculate your NDL?

It ain't rocket science. You can plan a simple two-or three level dive easily with your PDC, PADI's eRDPml or even the standard PADI table. Like the "25m 25min, then 15m 25min" example in one of my previous posts. And those depths are max, just as with a square profile dive. I might descend to 25m rather quickly and then just go gradually shallower. I just make sure I'm at or above 15m after 25min, and I'm within my plan. I still get a lot more bottom time than if I were to treat that dive as a square profile, without having to spend a lot of time doing unnecessary deco

If you don't, how do you know when are you gonna hit your NDL during the dive? If you do plan that way, how exactly you do it?

See above. Anyway, I usually bring a neat little device with me underwater. It's called a personal dive computer. It does a good job of tracking my bottom time and calculating my nitogen loading, and so far it hasn't given me any nasty surprises.

Basically my question is: how do you know ahead of the multilevel dive when are you gonna hit NDL? What sorts of calculations you do?

You know, even PADI gives a specialty on multilevel diving. You can also have multilevel as one of the five adventure dives in your AOW.

I'm sorry that the subset of the diving population you know isn't able to plan and execute a dive without risk of getting bent, because up here it's a requisite to get certified. It seems as if the diving education in your corner of the world is sorely lacking.


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