Diver Training: How much is enough?

[Diver0001]

More recent research says that 30 FPM is a safer ascent rate, but if you ascend at that rate, you should leave the bottom sooner than indicated on the RDP because of your slower ascent. For PADI to change to 30 FPM, they would have to redo the study. And they don't have to. Their study showed that diving within the parameters of the RDP, ascending at 60 FPM, is extremely safe. The studies that showed that 30 FPM is better also showed that 60 FPM is very safe, just not as safe as 30 FPM. Those studies also showed that doing a safety stop was more valuable than either ascent rate.

I would question whether or not it's really necessary to leave the bottom earlier when using 10m/min ascent rates. I did a little study based on running 100 or so profiles through a planning tool that visually displayed compartment loading and reached the conclusion for myself that using the same NDL's as the PADI tables, that 10m/min in no case gets the diver into a decompression situation during the ascent.

That said, what I tell my students is this:

1) the official definition of the bottom time means that you can leave the bottom at the NDL provided you start an uninterrupted ascent to your safety stop. HOWEVER, since ascents don't work in the real world they way they did them while testing the PADI tables (the tests were all done in a pot) that my advice would be to leave the bottom 1 min earlier than the NDL for every 10 metres of depth and ensure that you reach the *safety stop* when you reach your NDL. This is with using tables, not with using a computer.

2) If the diver is using metric meters then to precisely control ascent rates using 10m/min is dead-easy. you look at your clock when you start to ascend. You go up 10m. If your clock has already ticked on to the next minute, you keep going. If not, you stop until it ticks and then move on (essentially waiting until your clock catches up with your depth). This is a lot easier than working with 18m/min because (a) for some people subtracting 10 from your depth is easier on the fly than subtracting 18 and (b) because of the finer resolution the diver has finer control over the ascent speed/profile. (and -c- because it prepares their manner of thinking about ascents for things they'll probably encounter later in their diving career, but that's a lower priority than the other items).

3) If the diver reaches the safety stop LATER than the clock said they should have then the additional ascent time is added to the bottom time. If this means that the officially go over the NDL (even though I'm working with getting to the saftey stop at the NDL) then the safety stop is extended according to the rules listed on the table. The recommendation to leave the bottom a bit early gives them some slack to accommodate this scenario so you don't end up extending 1/2 of your safety stops.

4) once at the safety stop, bottom time, as far as tables are concerned, stops accumulating. The SS can be as long as you want. Although 3min/5metres is a *minimum* safety stop, there is no maximum. Many instructors don't seem to even realise that if you extend the safety stop past 3 min that there is no additional risk of DCS provided you are careful not to descend deeper than 5m again.

So in main lines, I move the moment of reaching the NDL from the bottom to the safety stop and then tell them HOW to make sure they get there on time (and what to do if they don't). For me that makes a LOT more sense than waiting on the bottom until you hit deco and then having zero flexibility left for the ascent.

Getting back to a previous point I made, these kinds of "tips and tricks" are what I meant that the instructor might lack if they don't have experience diving on tables. If I hadn't done so many dives on tables I might not have developed this "simplified" way of dealing with the ascent that takes the whole time-pressure element out of the last phase of the dive.

R..

 

[DCBC]

Suppose a diver on vacation has been diving for 3 straight days doing 4 dives a day and then suddenly the computer craps out. What would be your strategy for "falling back" to tables? ...The one with the computer, or the one with bottom timer?

If you log the dives, you have all the requisite information to project the next dive with the tables. I'm afraid that this "best practice" isn't always followed when using a computer, as it often has a multiple dive memory... Neither a computer or BT is required; a dive watch is a third option.

 

[Martigan]

I know they know how to use tables. They also know that there is a difference between teaching deco theory and using the tables. Some people seem to think that you can't teach deco theory without teaching the tables. They showed that you can. Deco theory would be deco theory if no one had ever invented tables and had instead found a different way to deal with it.

Well, this post is little by little evolving into a classical tables vs computer thread...

I understand your point Boulderjohn, and I agree that tables and deco theory are two different things, and tables are not necessarely required to learn deco theory.
But I found (even if for my dives I am using a computer) the educationnal strenght of tables quite useful.
When you see the tables and see that, at depth, 1 more minute of bottom time extends by several minutes your deco stops, it makes (or should make) you think about the danger of deep diving, the dynamics of gas loads and offloads and the absolute necessity to plan your dive and to dive you plan when you are not in "simple tropical shallow recreational diving" mode.
It is a clear and graphical representation of the deco theory.
Your computer, in simulation mode, can show you this, but you need to play with it, and the comparison between two dive profiles is not as straightforward as it is comparing two line of a table.

So yes, you can learn deco without tables but I, personnaly, on a practical point of view, would recommend it.

 

[DCBC]

Random thoughts on some of the most recent twists in this thread....

I always dive with a dive watch in pool instruction. I have several relatively expensive dive watches. On two different occasions, at the end of the pool session I saw that the watch had somehow lost time--10 minutes one time and 20 minutes the other. The actual time of day was later than my watch indicated, and my watch had been correct when I started the sessions...

Buy a Rolex.

...You can do it with tables, or you can do it with computers.

All computers are based on tables (an applied algorithm). You are always free to build into the profile a degree of conservancy, so any user who knows how could safely project a safe decompression profile (even if you were saturated).

---------- Post added January 14th, 2013 at 06:58 AM ----------

False, the maximum ascent rates have been decreased from 60 t0 30FPM. As for yo yo diving messing you up, just read BoulderJohn's post. I don't promote yo-yo or bounce diving. You are the only instructor I know who does and without any support for the practice.

So when your asked for proof, you refer to someone else's post???? Well to me that just demonstrates that you have none and are just blowing hot air....

 

[Diver0001]

If you log the dives, you have all the requisite information to project the next dive with the tables. I'm afraid that this "best practice" isn't always followed when using a computer, as it often has a multiple dive memory... Neither a computer or BT is required; a dive watch is a third option.

Wayne,

If you log the dives and all of those dives are multi-level dives (because that's just how a computer works) then within a few dives you'll be off the tables in terms of bottom times vs. depths (ie, you'll be beyond "Z" in terms of what the table can handle in terms of input). Logging simply isn't enough.

It's kind of like having two maps of the same area on different scales. You might be able to line them up on one point, but then they won't line up on other points.

So once again, falling back to tables from the computer isn't something you just do without putting in a long surface interval to get you "back on the map".

As for dive watches, those are just simple bottom timers. In fact, speaking of flooded bottom timers, I mentioned the one purpose built bottom timer I had but I've also managed to flood two watches over the years. The point I keep trying to make about this is that just because you're using tables to plan your dive does NOT mean that your instruments are any more reliable. People keep talking about "what if the computer dies" but bottom timers flood too and the fact that you use tables to do your planning doesn't make the bottom timer any more reliable in terms of failure than computers.

Really, the only thing that using tables does for you is give you shorter bottom times because of the lack of adaptation for multi-level profiles.

Your point about planning ahead is well taken and it is a weak spot in most computers but there are still ways to do that. One obvious way is to use an app that you can download on your smartphone or computer/laptop etc. In lieu of that you can always use the planning or simulator features of the computer itself but I'll admit that I don't find them particularly user friendly on the whole, especially when you're looking for the minimum surface interval.

R..

---------- Post added January 14th, 2013 at 12:10 PM ----------

All computers are based on tables (an applied algorithm). You are always free to build into the profile a degree of conservancy, so any user who knows how could safely project a safe decompression profile (even if you were saturated)

Computers are not based on tables. Both computers and tables are based on the same/similar algorithmic foundations, whereby one is analogous to making a "still photograph" of the dive where the other is like making a "movie". I think that's probably what you meant.

R..

 

[DCBC]

...I don't know of any research saying it will harm me to conduct those exercises during OW instruction in shallow water, and I am quite sure students can tell the difference between what I do while supervising a CESA and what they should do during a dive.

My comments to NetDoc were based upon him pointing a finger to Instructors who may at times do multiple ascents during the course of CESA training. I am not promoting multiple ascents, but don't believe it beneficial for anyone to say that an Instructor is "showing a bad example" by doing so. There is NO evidence that this is poor diving practice. Moreover, if we teach the tables, they tell us quite clearly how to deal with any multiple ascent in a safe manner (to consider it as part of the same dive). We can't tell a Student that the tables are right and disregard the rules in-which they are intended to be applied. Again this is in the context of the OW diver and in consideration of the recommended limitations of the dive profile.

---------- Post added January 14th, 2013 at 07:34 AM ----------

If you log the dives and all of those dives are multi-level dives (because that's just how a computer works) then within a few dives you'll be off the tables in terms of bottom times vs. depths (ie, you'll be beyond "Z" in terms of what the table can handle in terms of input). Logging simply isn't enough.

I understand your perspective; but to clarify mine, my original question was focused on the training requirements of an OW diver (withstanding the recommended maximum depth limit of 60 or 70 FSW). We were talking about a computer failing and the Diver having to sit out for 24 hours. My point is that I believe that this would seldom be required if the diver had logged his dives and used the appropriate tables to safely plan the next dive. I've done saturation dives in over 1000 FSW without the use of a computer. Logs and tables can be enough; although you may have to wait some period of time (not necessarily 24 hours).

 

[Diver0001]

I understand your perspective; but to clarify mine, my original question was focused on the training requirements of an OW diver (withstanding the recommended maximum depth limit of 60 or 70 FSW). We were talking about a computer failing and the Diver having to sit out for 24 hours. My point is that I believe that this would seldom be required if the diver had logged his dives and used the appropriate tables to safely plan the next dive. I've done saturation dives in over 1000 FSW without the use of a computer. Logs and tables can be enough; although you may have to wait some period of time (not necessarily 24 hours).

I think you're right. It would seldom be "required". However, the problem isn't if it's required or not, the problem (continuing with my previous analogy) is how to get yourself back to a known point on the map.

The best I could think of, just speculating about this (this not a recommendation to try in practice) would be to assume you are in group Z, since it represents the highest level of tissue loading you can achieve using the PADI tables on a non-decompression dive, and then do a 3 hour surface interval as required by the tables and then carry on from there.

But it's only speculation and there will still be very real differences in the *actual* tissue loading you achieve using the computer as compared to using the tables....

The *safe* bet if we're looking for a one-size-fits-all recommendation would still be to sit out for 24 hours (as they recommend for incursions into deco). At that point you can be sure that you have gotten yourself back on the map and can start with a clean slate, as it were.

R..

 

[DCBC]

Computers are not based on tables. Both computers and tables are based on the same/similar algorithmic foundations, whereby one is analogous to making a "still photograph" of the dive where the other is like making a "movie". I think that's probably what you meant.

Yes, an algorithm is used to create a table and the same algorithm may be applied by a computer to calculate a result.

---------- Post added January 14th, 2013 at 08:11 AM ----------

I think you're right. It would seldom be "required". However, the problem isn't if it's required or not, the problem (continuing with my previous analogy) is how to get yourself back to a known point on the map.

We are speaking hypothetically; but in reality it would depend. Most OW Divers after being out of the water for 12 hours or so, don't have much residual nitrogen to deal with. If you follow the log and create the diver's profile with the tables, you are indicated to possess more RN than you would actually accumulate if you used a computer. Even so, you could identify a repetitive group and dive accordingly.

 

[Diver0001]

We are speaking hypothetically; but in reality it would depend. Most OW Divers after being out of the water for 12 hours or so, don't have much residual nitrogen to deal with. If you follow the log and create the diver's profile with the tables, you are indicated to possess more RN than you would actually accumulate if you used a computer. Even so, you could identify a repetitive group and dive accordingly.

I see what you're getting at. It's well beyond the scope of what most people are trained for but it could be done.

R..

 

[DaleC]

I know they know how to use tables. They also know that there is a difference between teaching deco theory and using the tables. Some people seem to think that you can't teach deco theory without teaching the tables. They showed that you can. Deco theory would be deco theory if no one had ever invented tables and had instead found a different way to deal with it.

I think we are now arguing small potatoes but I think, at some point, if discussing deco theory, one would probably refer to some sort of visual, tabular representation; be it a table of stops for various depths and times or a curve. How else would one describe the variances these two factors (depth/time) make.

The tables are just a tabular expression of the decompression theory used for pre-mandatory dives.

______________________________________

I see where part of the argument comes from in the comp vs tab debate. I use a computer when diving but still see the benefit of learning tables. It doesn't mean I need to dive the tables. As someone said above, they (the tables) put all the data in one place wherein different factors can be evaluated as to their effect on planned dives. They also show the interrelationship between dives and surface intervals - air, nitrox mixes and BT's.

To me they are a planning/perception tool. My computer is a diving tool.