Decompression

[SLOdiver]

Hy guys

i'm new to diving and i have a quetion.
I got the us army charts for how long you can stay at certan depths.
but the time is really low(42 meters-5minutes)
so if you navigate based on this charts and you are at 42m for 5 minutes you don't have to do any decompression stop except the safety obviously.
but if you are under longer i don't know for like 1 hour at wich depths you have to start making decompression stops and how long. is there any rule like every 5 meters, 9 meters?
thanx

 

[Rupert Vidion]

All diving is "decompression" diving to one degree or another by dint of the fact that one exposes their body to an increased pressure of inert gas (most commonly nitrogen). When ascending from a "No Decompression Dive" one decompresses by ascending at a safe rate and making a "Safety Stop". Just because your computer or tables say that you are within "No Decompression Limits" doesn't mean that you have not absorbed extra inert gas. What you appear to be talking about is "Formal Decompression Diving" which is a different kettle of fish.

There are lots of rules and procedures for formal decompression diving. None of which can suitably or safely be learnt over the internet.

If you are considering moving into decompression diving you should find a technical instructor who can take you through the process of decompression certification.

To answer your question about times and depths more directly - it all depends on what gasses you breath, how deep you went and for how long and which decompression algorithm you follow.

But consider that if you breath air at 40m you will suffer an element of narcosis and you will empty a 80cuFt tank in less than 20 minutes, the logistics of carrying enough of the correct gas become complex.

If you really want to know more on a purely academic level can I recommend "Deeper into Diving". It is a very good book with lots of information about a variety of diving topics including a run down of the major decompression theories and their application.

I am sure someone else will chime in with a comment about the suitability of military dive tables for recreational use.

 

[Chugwhump]

 

[DevonDiver]

The tables are based on an algorythm that predicts the absorbtion of nitrogen into various tissues of the body. Different tissues absorb and release nitrogen at different rates.

the depth you reach on a dive, coupled with the time you spend there, will determine how much nitrogen is absorbed into the various tissues. After a certain time (dictated by the tissue absorbtion speed), the tissue will become full of nitrogen at that pressure (depth). This is called Saturation.

When you begin to ascend from the dive, your tissues become 'super-saturated'. This means they contain more nitrogen than the external pressure. Our bodies can tolerate a small level of super-saturation...and this allows the nitrogen to slowly leave our bodies (again, this is determined by the tissue speeds). However, if we ascend too quickly, then the super-saturation becomes to great and the nitrogen in our tissues will expand and form bubbles. This is known as Decompression Illness...and can be very damaging or fatal.

The tables used for recreational diving keep the level of nitrogen saturation within our tissues to a minimal level...meaning that a slow ascent to the surface is enough to off-gas the excess nitrogen.

However, if you stay at depth beyond the limits of the tables, then your body will contain more saturated nitrogen than can be safely released on a slow, direct ascent to the surface. In order to allow the release of this nitrogen, the decompression diver must make numerous 'stops' or pauses on his way to the surface.

The deeper that you go and the longer you stay, the more nitrogen will be absorbed into your very slow tissues. This means that the process of letting the nitrogen out will also be very slow. This, in turn, means very long stops on your ascent.

The algorythms used to predict the behaviour of nitrogen absorbtion and release on your body will provide the times and depths for the stops that you require.

decompression diving requires considerable extra equipment, skill and training. This is for 2 main reasons.

1. The dive profile must be planned and run precisely.
2. It is very dangerous to ignore/miss the required stops on your ascent...so if you have a problem on the dive, you must resolve it there...as ascent to the surface is not an option.

 

[SLOdiver]

Yea i know all that i just don't know at what depths and for how much time you must stop (so the body looses the gas).
so if i am diving at 30 meters the maximum dive time for my chats is 10 minutes.
if i stay down for 20 minutes what should i do?
on my charts it says that if i exceed the max dive time i must stop at 5 meters for at least 15 minutes if i exceeded the max time for more than 5 minutes.
but is that really all or do i have to stop i don't know at 30(1 minute), 25(3 mins), 20(4 mins), 15(5 mins), 10(6 mins), 5(15 mins) meters?

 

[vjanelle]

The 42M limit is for contingency purposes only... I don't have those charts in front of me but they probably also say something about those depths being something you'll want to avoid... For example you went to 130' in cold water(sorry, I dive imperial), and the charts would recommend I calculate my pressure groups for 140'.

At 42 msw you're going to get a very limited run time on your gas also(depending on your sac rates).

Take it slow, there's (usually) no reason to be diving at that depth, especially on plain air.

To answer your question more specifically though, what you're looking into is something akin to multi-level diving, which usually requires training(and planning!) if you're going to be doing it with charts, or a dive computer. Yes, you'd just ascend to 5 meters, and hover for 15 minutes(air permitting). Environmental conditions and ascent rates will of course determine what a bubble model would say, but the charts always hover around the outsides of these.

 

[Kern]

Yea i know all that i just don't know at what depths and for how much time you must stop (so the body looses the gas).
so if i am diving at 30 meters the maximum dive time for my chats is 10 minutes.
if i stay down for 20 minutes what should i do?
on my charts it says that if i exceed the max dive time i must stop at 5 meters for at least 15 minutes if i exceeded the max time for more than 5 minutes.
but is that really all or do i have to stop i don't know at 30(1 minute), 25(3 mins), 20(4 mins), 15(5 mins), 10(6 mins), 5(15 mins) meters?

You're using NDL tables to do NDL dives. You should not be using NDL tables, limited experience & no knowledge to do staged deco dives.

 

[CardShark]

Another excellent book to read about this is Deco for Divers, by Mark Powell. ( You'll be surprised at what you didn't know - I was ! ) If you read that book, you'll understand that you really do need formal training, not just a quick fix type of answer.

 

[gcbryan]

Hy guys

i'm new to diving and i have a quetion.
I got the us army charts for how long you can stay at certan depths.
but the time is really low(42 meters-5minutes)
so if you navigate based on this charts and you are at 42m for 5 minutes you don't have to do any decompression stop except the safety obviously.
but if you are under longer i don't know for like 1 hour at wich depths you have to start making decompression stops and how long. is there any rule like every 5 meters, 9 meters?
thanx

It would help if you filled in your profile and told us a little more about yourself.

You are new to diving. Why would you think or be thinking about going to 42 meters for an hour?

What table were you trained on? I'm not aware of any Army charts.

There is no rule such as you suggest. Any such rules listed on a table are just in case you exceed the table by a few minutes...by carelessness and not by an hour.

If you are going to do dives of that depth/length then you need a lot more training and equipment.

There is no other answer to your question but your question raises many others.

Welcome to the board by the way.

No question is a bad one I suppose but don't expect responses to stick strictly to your question because as I said it raises many other questions.

 

[Rupert Vidion]

To answer your second post:
There is no "one-schedule-decompresses-all". Each decompression schedule needs to be calculated on the basis of the dive's profile. This is one of the (many) skills learnt on a decompression course. It would be dangerous and irrisponsible for anyone to just hand out decompression schedules and I strongly doubt anyone on here would ever do so.

If you find yourself beyond your NDLs by accident, make a slow controlled ascent to the depth advised by your training agency/tables and make a stop for the prescribed time or as gas supply permits. Finish the dive by making a slow controlled ascent to the surface and abstain from diving for 24hrs.

Deliberately exceeding your NDLs is a dangerous game, not least because pushing your NDLs will more than likely involve pushing your gas supply as well.

I think I speak for everyone SLOdiver in saying that the internet is not the place to learn formal staged decompression. If you want to learn decompression procedures there are many agencies and instructors you can look to.