TheDivingPreacher:
What exactly should this mean to the average recreational diver?
I think I understand the intent of your statement but where does one get the skills to "shape their dive curve"? :huh:
Yeah. That's the crux, isn't it. It's easy to understand that the "right" curve, whatever that it, is better for you but when you ask the question "How" the answer is usually that it's either too subtle or complicated to explain on the internet. At least the technique that UP is talking about won't get explained to you.
Let me just try to give you a rough sketch of something that you could apply to most of your dives. It's not quite what UP is talking about but it's easy enough to understand and is probably a big improvement over what you were already taught. Keep in mind that this is the Rob Turner method and has nothing at all to do with what UP is talking about despite certain similarities.
The first and most important thing is that the graph of your dive profile should look a little like a big check-mark. The profile that Snowbear posted (the second one) is an example but the tail (shallow bit) could be longer. If you don't have the discipline to make your profile look like that then you have to work on this step first. Connected to this is the realisation that off-gassing is a process that will happen on every dive. and that firm control over your ascent is the key to getting the most out of this process. Some folks say taht "every dive is a deco dive", which isn't true but to get the most out of your ascent every dive should should be *approached* like a deco dive. That means lots of time shallow to compensate for the time you spent deep.
The second thing is to realise that ascents are all about ascent rates. Not stops. ascent rates need to differ depending on your depth. The deeper you are when you start your ascent, the faster you can ascend to a point. The closer you get to the surface, the slower you need to go.
So two questions remain. How much time shallow (and a related question, *what* is shallow)? and how to stage the ascent to make it look like a big check mark. There are various different ways to do that. Some of them depend on certain strict parameters like choice of gasses at different depths but to make things simple lets just assume you're making no-stop dives (not over the NDL) using air.
On recreational dives, lets define a couple of "bands" or bandwidths of depths. 40-18 metres, 18-9 metres and 9 meters to the surface. These bands correspond fairly well to the on/off gassing that happens in these areas. from 9 metres to the surface you can spend nearly all day without getting into a deco situation. From 6 metres to the surface you can literally spend all day (which we'll get back to in a moment). From 9 to 18 metres you start to ongass but not drastically and deeper than 18 metres ongassing accelerates dramatically.
The converse is also true. Ascending from 40-18 metres the amount of offgassing you do is limited. It really starts to pick up from 18-9 metres and from 9 metres to the surface you're offgassing fairly rapidly.
So how to make a check mark. The first order of business is to get yourself off the bottom and up to the 18 metre zone. You can't do this any faster than 10 metres per minute or you're going too fast. If you go slower than 10 metres per min, you're also going too slow and will continue to on-gass.
From 18 to 9 metres you should ascend slower. The difference in depth is 9 metres so 3 metres per min is a nice round number. You can go faster, up to 9 metres per min but there is no reason and it doesn't help you.
From 9 metres to the surface you need to slow your ascent even more. Let's just say for the sake of argument that it should take you as long to go from 9 metres to the surface as the total time you spend deeper than 18 metres. So on a dive for 20 minutes to 30 metres, you should spend 20 minutes in the shallow zone. Of course on a no-stop dive the model says you don't *need* to spend this time in the shallow zone but you will feel better after the dive if you do. Just picking that number gives you an easy handle for making the tail end of the profile look like a nice checkmark.
So..... now the mysterious part..... how do you guarantee these ascent rates?
Answer: you stop along the way.
Say you start from 40 metres and you're on your way to 18. Say you start your ascent at 8 minutes. When you pass 30 metres (10 metres shallower) your clock should flip to 9 minutes. If it doesn't, then you wait at 30 metres until it does. At 20 metres you wait until the clock flips to 10. If it were me I would probably just go straight from 20 to 18 and wait there but that's just me being lazy. If you're too slow and your clock says 12 when you get to 18 metres then you have to add this to your bottom time. That's why it's never good to leave the bottom when your NDL says zero. Leave when it reads 2 minutes and you'll have a little buffer.
Now from 18 to 9 metres you want 3 metres per minute. Do the same thing. Go from 18 to 15 to 12 to 9 and put 1 minute stops in at each of these depths. Guaranteed 3 metres per minute.
From 9 metres to the surface you want to spend 10 minutes because that's how long you were deeper than 18 metres. You can spread this over 2 or three depths depending on how you like to do it. You could spend 1/3 of the time at 9 metres (3 minutes) and 2/3 of the time at 6 metres (7 minutes) or split it in 3 stops of 3 or 4 minutes and spend 3 min at 9 metres, 3 meters at 6 meters and 4 at 4.5 metres, for example. The idea is taht you spend more time at about 5 metres than you do at 9. Remember, once you're shallower than 6 metres you can extend the stop for as long as you like without additional ongassing.
Another example.
Making a checkmark from a 17 minute dive to 30 metres.
30 metres - 17 min
20 metres - 18 min
18 metres - 19 min
15 m 20 min
12 m 21 min
9 m 22 min
6 m 28 min
4.5 m 34 min
ascend to surface at 42 min.
So this is a 17 minute dive to 30 metres. You ascended with 10 metres per min to 18 metres, then with 3 metres per min to 9 metres and then spend 19 minutes from 9 metres to the surface; 6 min at 9 and 6 metres and 7 min at 4.5 metres.
Of course it's nice if you're going to dive like this to have something to look at along the way. It's clearly easier to do ascents like this if you're diving along a shoreline than it is if you dive, for example, on a wreck.
But that's the *basic* idea of one possible way to make clean check-mark type ascent. As I said before there are other ways, including the way UP alludes to but it's more specific and more generalized that what I'm describing. The advantage of making your ascent like this is that you'll generally feel better at the end of the dive. You'll also note that such profiles are easily done on the tables and that you'll soon start to view your computer, if you dive much like this, as nothing more than an elaborate bottom timer. The last thing to be said about this is that if you commit to making your profile like this then it would be nice to know that you're not going to run out of air before you're done.... that's chapter 2.
R..
Before I read that first article I had no idea the differences between algorythms was so great.
, would be much more liberal. How can it not be?!?
