DivingDoc,
Disclaimer: This is intended for discussion purposes only and any examples of no decompression limits should only be used for this discussion. For planning dives or calculating pressure groups please refer to the material supplied by your training agency or by your computer manufacturer.
Glad you asked. Your question immediately made me notice the advantage of the bar graph. I shall explain. Where should I start? Let's start with tables. In fact, let's start by noting that the time you can stay under water safely depends on the amount of nitrogen that your body can absorb. There is no way to calculate that it has to be done by experiment and all that your computer or tables do is work out, based on the results of those experiments, if you have been down too long or if you can stay a bit longer. Computers do that using a model, which is simply a way of describing the results of the experiments as an equation or series of equations that allow the results to be calculated by a computer. Computers and tables are based on essentially the same experiments. So now we know the rules come from experiment and computers and tables are just to help us follow the rules.
So what affects how much nitrogen we absorb? Basically it is the pressure and the amount of nitrogen. The amount (or fraction) of nitrogen is always the same (*1), it is always 79% of the gas in your tank the rest of it is oxygen. (*2) So since the amount of nitrogen cannot change, the only thing that can is the pressure and we learnt about that in our open water course, 1atm pressure at the surface (from the air) plus one more for every 33 feet we go down. If we go deep the pressure is high and it "pushes" lots of nitrogen into our bodies and it pushes it in quickly. That is why if you look at your tables, you see that at 35 feet (2 atm) you can stay down for 205 minutes, at 70 feet (3 atm) for 30 minutes and at 130 feet (5 atm) for only 10 minutes. When you are shallow it doesn't push much nitrogen in and it only goes in slowly. When you go deep you get much more nitrogen much quicker. Now I hope it is obvious that in order to know how much nitrogen you have in your body, you have to know how deep you have been and for how long. Because normal people can't keep track of their depth every five seconds or so the easy thing to do is to assume that they were at the maximum depth for the whole dive. We assume the deepest because that gives the shortest no decompression time and is therefore the most conservative. The great thing about computers is that they measure your depth continuously throughout the dive so that you don't have to assume that you were at the maximum depth for the whole dive.
To recap:
1) time under water is limited by nitrogen absorbtion
2) the main factors affecting the rate of nitrogen absorbtion are depth and the amount of nitrogen in the gas you have in your tank
3) if you are deep you absorb nitrogen quickly
4) if you are shallow you absorb nitrogen more slowly
So there is a maximum amount of nitrogen that can be absorbed (*4) before we get the bends and that is what the NDL is. If you get to the NDL you have absorbed as much nitrogen as you can and if you stay longer you have a much higher risk of getting decompression sickness. The bar graph at the side of the Cobra/Viper is just a measure of how much of that NDL you have used. When it gets to the red you have used it all up.
Using a dive to 130 feet as an example, we have an NDL of 10 minutes (*3). If I stay for 2 minutes I have used 20% of the total time so the bar graph will show 20% of the segments lit up. After 5 minutes half of the total time, the NDL, is used and so the bar graph will be half way up to the red. At the same time the no decompression time will be counting down. After 2 minutes it shows 8, after 5 minutes it shows 5 minutes and so on. The bar graph shows the fraction of the total no decompression time that you have used. The good thing about the bar graph is it is easy to see what fraction of your total NDL you have used. That is much harder with the numbers. I still use the numbers though, for some reason I like them (part of it is that they are easier to see!)
Now, lets say that I do a dive to 130 feet for five minutes and then move up to 50 feet for the rest of the dive. In the first part of the dive I have used up half of my NDL, that means I have absorbed half of the nitrogen that I am allowed to absorb. Now I go up to 50 feet. At fifty feet the remaining half of my NDL will last me much longer than five minutes because I won't absorb nitrogen so fast. If I am using tables I can only stay five minutes though, because I have to assume I am at 130 feet. If I have a computer I can stay longer because it can measure that I have gone to a shallower depth. Thus the total time is longer. As you go up your remaining nitrogen limit lasts you longer so while 5 minutes at 130 feet would be half of your time, 5 minutes at 50 feet is nothing like half because you are now absorbing the nitrogen more slowly. That is why some of the markers disappear when you ascend.
So you asked how many minutes do I ascend at. Well, never 0. In most open water courses, they tell you not to dive to the limits of the tables. The same thing applies to computers. Don't dive to the limit. If I dive to 130 feet the no decompression limit (NDL) is 10 minutes according to the PADI tables. I might wait until my computer has 1 minute of no decompression time left. That gives me about 90% of my NDL. That 90% is my own decision and is arbitrary if I were cold, dehydrated or just wanted to be more conservative I might ascend with 2 or 3 minutes left. On a dive to 50 feet I can stay for 80 minutes according to the tables. I might ascend when my computer has 10 minutes or so of no decompression time left. This gives me a total again of about 90% of the NDL of dive time.
Interestingly, now that I have written all of this - I hope you are not bored - I have convinced myself that I should use the bar graph more. I hope that you understood what I was trying to say. It got a bit long. I will write some more about nitrox sometime. All of this is just for air. Perhaps someone else could take on the nitrox bit...??
Piscean.
(*1) - unless you are using nitrox but I will assume you are using air for all of this discussion.
(*2) - For those amongst us who have to be exact, there are some other things too, like carbon dioxide, argon, etc. but they are present in such tiny amounts I am going to ignore them.
(*3) - I have PADI tables so my numbers come from there.
(*4) - in fact this maximum depends on the depth so it is much more complicated than described here. Please do not use this description to calculate or plan dives - use the tables, a wheel or your computer.
