Deco with too less air, options from the book

[Gilldiver]

John, thanks for the clarification. I had thought that they entered and exited from the breach.

As for the Navy Tables, they work, but you have to follow them exactly. If you hit 120 feet for 30 seconds and then do the rest of the dive at 100, you have to do the bottom time as if you did it all at 120. You cannot cut corners with them.

A dive at 121 feet must be calculated as being at 130, etc. It also helps to pad them and round up to the next time.

But a hit rate of .06 to .1% is in the right range if you run the table exactly. If you pad them, it will be less.

The other thing to remember about them is that in the 60-90 foot range the deco penalty for going over time is greater then at any other depths.

 

[Tortuga68]

The other thing to remember about them is that in the 60-90 foot range the deco penalty for going over time is greater then at any other depths.

Could you elaborate? Thanks

 

[Caver95]

Could you elaborate? Thanks

I too would like to know.

 

[Gilldiver]

If you over stay the NDL limit on the Navy tables, you have to round up to the next factor of five or 10 depending on your depth. For 60 feet, the NDL is 60 minutes. So if you stay for 61 minutes you have to use the 70 minute table. The same thing for depth, if you go to 61 feet, you have to use the 70 foot table.

If you go down the table from 40 feet to 130 feet and go over that 1 minute (see the attachment) and then list the required deco, you will find that the deco obligation is the most for the 60 to 90 foot range. You end up having more deco owed for the 1 minute over in this range then you will have for over staying 1 minute at deeper depths. Now add in residual nitrogen time from a previous dive, it gets more complicated and we all know how hard tables seem to be for a lot of divers.

As 60 to 90 feet tend to be the most common rec depths, and rec divers tend to be a little loose in monitoring time, depth, and staying on the tables, you get a higher hit rate in this depth range. It is not the fault of the tables, the tables are just not forgiving if not followed correctly.

Pete Johnson

 

[scjoe]

I don’t know if the Navy tables are any more or less forgiving that any other set of tables. If you dive any set of tables or any computer algorithm to its limit, you are more likely to get a hit than if you give yourself a margin for error. Computers are easier to use provided you remember what all the little symbols mean.

It is possible there are more hits on dives in the 60-90 foot range because more recreational dives are conducted at that range, but I haven’t seen any studies examining the hit rates of various tables or algorithms for that dive range. Most rec divers don't use tables anymore so there may not much to compare in the way of tables.

 

[boulderjohn]

As 60 to 90 feet tend to be the most common rec depths, and rec divers tend to be a little loose in monitoring time, depth, and staying on the tables, you get a higher hit rate in this depth range. It is not the fault of the tables, the tables are just not forgiving if not followed correctly.

Since the overwhelming majority of dives occur in the 60-90 foot range, it is reasonable to find the majority of hits in that range, but it may not have as much to do with being loose with monitoring time, depth, and staying on tables as you might think. Whether you are using tables or a computer to guide your dive, a significant percentage of DCS hits (I have heard 50%) happen to divers who are within all limits.

There is more going on, frankly, than we understand. DAN has recently shown that that a huge percentage of hits happen on the first day of a trip, and the vast majority of those happen on the first dive of that first day. Why? No one seems to know.

 

[Tortuga68]

Well, from a strictly statistical viewpoint, that seems more likely, if you're predisposed to getting bent for some reason

 

[Gilldiver]

The thing is that computers and newer tables is that they tend to have more padding built into them. The old Navy tables not so much.

The Navy tables are very reliabal if you follow them exactly and are the most proven one ever used. In the 10 to 60 foot range no other tables have anywhere near the number of dives on them - why? - because that is the range for hull maintinance.

When you think about the number of dives that are done on ships hulls for inspcestion, cleaning, repair, and maintanince, no other table has the track record.

But these tables do not:

1 - Give you credit for multi-level diving
2- Will keep you in the water longer then some others because of #1
3 - Work on what some call a Bend Me/Fix Me process. You do the deco shallow where most modern theory has you doing it deeper to prevent micro bubbles

I don't dive Navy tables as a standard practice, but I have a copy on me for all deco dives. If things go real south - lost computer, lost buddy, lost anchor line, I still have a timer and depth gage and a set of tables.

Even for some mix dives they will work. As long as the O2 is 21%, (He% + N2%=79% with He less then 35%) and you have one deco gas, air tables will get you out of Hell.

 

[Tortuga68]

WAy off topic I know, but...

You don't use run time for deco dives?

 

[Chatterton]

And the point is that we have to manage more than just the gas, and dive profile.


Cheers

JC

Wow, I read that book a while ago and did not get that from the reading it. They both had air and options and couldn't use them.....