New diver advice--what are the dangers?

[dishonored]

Those are your absolute no-deco limits for that depth.

Edited to add that one should always be aware, when using any tables, what the underlying assumptions of the table are. In the RDP, it is that bottom time is counted from the initiation of descent to the time of beginning a direct ascent to the surface, and that said ascent will be conducted no faster than 60 fpm (and generally closer to 30). Ascent time is important, because you are decompression during the ascent, and if you shorten the time, you do not do as much decompressing as the table model assumes you will do.

ok so I probably haven't gotten to that part of the training then yet, what is an absolute no de-co for that depth mean? sorry guys, just want to make sure I have this right. I don't know who my instructor is yet, just have a shop number and can call them, but this thread has helped me a ton so far!!

 

[TSandM]

You will discover in your further reading, that the time one can spend underwater is limited not only by how much breathing gas one can bring down, but also by the absorption of nitrogen into the body. Nitrogen is an inert gas, which means it isn't used up (unlike oxygen). So when you come back to the surface, the nitrogen you have absorbed at depth has to come out of your bloodstream. If it does it too fast, it causes an illness called "the bends".

Recreational diving, which is what you are learning to do, is diving where you can always abort a dive and proceed directly, at a controlled rate, to the surface. This is important, because if you ever enter a situation where you can't, you HAVE to have enough gas to deal with it, and you HAVE to have the skills to solve the problems underwater. That's advanced diving. For the general recreational diver, it's a good idea to have the surface as an option. If you stay down too long at any depth below about 30 feet, you will eventually hit a time when you have absorbed too much nitrogen to go directly to the surface without a high probability of developing symptoms. The amount of time you can spend at any given depth before incurring an undue amount of risk is called your "no-deco" time. It's a bit of a misnomer, as I think I said earlier, because the model assumes an ascent rate, so it factors in some decompression time as you go up. So you are actually doing a little deco on every dive, but you aren't having to do an mandatory stops where you have to wait at a certain depth for a prescribed period of time.

 

[dishonored]

You will discover in your further reading, that the time one can spend underwater is limited not only by how much breathing gas one can bring down, but also by the absorption of nitrogen into the body. Nitrogen is an inert gas, which means it isn't used up (unlike oxygen). So when you come back to the surface, the nitrogen you have absorbed at depth has to come out of your bloodstream. If it does it too fast, it causes an illness called "the bends".

Recreational diving, which is what you are learning to do, is diving where you can always abort a dive and proceed directly, at a controlled rate, to the surface. This is important, because if you ever enter a situation where you can't, you HAVE to have enough gas to deal with it, and you HAVE to have the skills to solve the problems underwater. That's advanced diving. For the general recreational diver, it's a good idea to have the surface as an option. If you stay down too long at any depth below about 30 feet, you will eventually hit a time when you have absorbed too much nitrogen to go directly to the surface without a high probability of developing symptoms. The amount of time you can spend at any given depth before incurring an undue amount of risk is called your "no-deco" time. It's a bit of a misnomer, as I think I said earlier, because the model assumes an ascent rate, so it factors in some decompression time as you go up. So you are actually doing a little deco on every dive, but you aren't having to do an mandatory stops where you have to wait at a certain depth for a prescribed period of time.

ok thanks man. I think I have a better understanding of it now. I have been reading most of the day, a little slow at work today. I have worked about 100 of those random series of question on the link posted above. the problems are not hard, its fairly easy. I was having trouble understanding how the chart flows and all works together. Using a chart isn't hard, understanding how to use and how it works is what's challenging sometimes.

 

[MrChen]

Other than sharks what are some other good things to watch out for.
Pretty much any advice that a new diver would need to know. Especially about sharks.

I don't watch out for sharks, I look for sharks! What do you need to know? They swim away really fast

Thinking about sharks is scary, but seeing them underwater is surreal. They don't seem to care that you are there. Just don't mess with them and you'll be fine.

 

[dishonored]

Just to make sure I am doing this right... (based on padi chart)
I came up with my own scenario...
Say I dive at 60' for 30mins (puts me in L group), I come up for 27 minutes (now in pressure group H), my next dive is 47' and I want to know how long I can stay down without crossing that no-deco limit. I find the H on table 3 and follow it down to 50' (rounding up) then look at the bottom number in blue (not to be exceeded), the . so my maximum time at that depth after my first dive and SI would be 24minutes?? table 3, find column "H" and track down to the 50' row to find a RNT number of 28minutes but a do not exceed limit of 52min (NDL). I would subtract 28 from 52 for a total bottom time of 24 minutes.
based off of RNT+ABT= TBT and that number should not exceed the number in blue on table 3 (NDL).
this would also change my pressure group from "H" to "F".

for a third dive say I wait at the surface for 2 hours. MY new group would be "A".
the last dive of the day is set to be at 70'.
that would mean my new RNT= 5 and the TBT= 35. 35-5= 30 minutes of actual bottom time available to me at that depth.
After that dive I would be in Pressure group "O"..... I hope I have this right lol

---------- Post added June 13th, 2013 at 07:50 PM ----------

after a couple edits adding to it and double checking my work I think I have it..

 

[Frosty]

Those are your absolute no-deco limits for that depth.

Edited to add that one should always be aware, when using any tables, what the underlying assumptions of the table are. In the RDP, it is that bottom time is counted from the initiation of descent to the time of beginning a direct ascent to the surface, and that said ascent will be conducted no faster than 60 fpm (and generally closer to 30). Ascent time is important, because you are decompression during the ascent, and if you shorten the time, you do not do as much decompressing as the table model assumes you will do.

Yea but it also assumes a square dive profile.
Sorry had to throw that in

 

[Dr. Lecter]

I think it's safe to say the main dangers are death and paralysis, though even a 'mild' Type I DCS hit is unpleasant enough to give you a whole new perspective on just how excruciating a certain hyperbaric chamber scene from Tom Clancy's Without Remorse would have been.

How you might blunder into those dangers is a very long list . . . though sharks are probably somewhere near the very, very bottom of it.

 

[dishonored]

Thanks.
I am becoming familiar with DCS.
Can anyone check my hypothetical dive plan??

 

[TSandM]

No, you have made a number of errors.

First off, if you are L at the end of the first dive (and you are) and you do 27 minutes of SI, you are an E, not an H.

But assuming you are an H, for the purposes of your further calculation, your no deco time at 50 feet is 52 minutes. The 28 minute number is the residual nitrogen time. If you'll notice, you started the day with an 80 minute no-deco time at 50 feet -- your first dive has taken 28 minutes away from that, so your no-deco time is 52 minutes now.

Where the RNT+ABT comes in is in figuring out your pressure group AFTER the second dive. You add the actual bottom time you did (which must be less than 52 minutes) to the 28 minutes -- the number you come up with will be less than 80, and you look at the first table and find that number under the 50 foot column. That will tell you the new pressure group.

 

[dishonored]

No, you have made a number of errors.

First off, if you are L at the end of the first dive (and you are) and you do 27 minutes of SI, you are an E, not an H.

But assuming you are an H, for the purposes of your further calculation, your no deco time at 50 feet is 52 minutes. The 28 minute number is the residual nitrogen time. If you'll notice, you started the day with an 80 minute no-deco time at 50 feet -- your first dive has taken 28 minutes away from that, so your no-deco time is 52 minutes now.

Where the RNT+ABT comes in is in figuring out your pressure group AFTER the second dive. You add the actual bottom time you did (which must be less than 52 minutes) to the 28 minutes -- the number you come up with will be less than 80, and you look at the first table and find that number under the 50 foot column. That will tell you the new pressure group.

Ok I see. I thought I subtracted 28 from the 52 to get my maximum time. Thanks.

I do have a question about the being an "E" instead of H.
If at the end of my first dive I am an L.. And I do :27 of SI. When I track left to find where :27 would be me it falls in line with the H column.
On my chart I got ^above anyway.