Varying NDL ascent strategies

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We were told that the tables were based on 10m/min until the first stop (perhaps it was 9m or 12m, I don't quite remember since I wont be doing deco dives and subscribe to Sherlock Holmes' theory about remembering useless stuff), otherwise we ran the risk of on-gassing too much for the table to be completely valid.

Given that nearly everyone is using a computer these days (at least for the kind of light backgas deco included in the 3* syllabus), and the computer tracks any ongassing during a "too slow" ascent, I assume that it was an exercise in following a predetermined ascent strategy rather than a real deco exercise.

The 88 tables solve the problem of variation in ascent rate by defining a dive as the time from leaving the surface to reaching the ascent check depth (6m on a no stop dive). So if you ascend really slowly, then your 'dive time' is longer, potentially moving you to a new surfacing code (or potentially compulsory decompression).

Now when I was teaching ERD, and Bhulmann tables (which are used by BSAC for Trimix courses, and ERD before it was dropped). The ascent speed is critical, if diving to table, rather than computer.
 
Someone mentioned riding the computer up. Makes me ponder at what rates various computers use as well.


Side note:
I thoroughly enjoy your articles John. Thanks for still being a contributing member. There are many here that appreciate your thoughts and opinions. Very concise and well put together. Thank you!
 
Someone mentioned riding the computer up. Makes me ponder at what rates various computers use as well.

Riding the computer is normally defined as staying just inside the 'no stop' time. i.e. having the computer at 1 minute of no-stop time all the way to the surface.
To my mind, that is aggressive diving.

I used to know an instructor who demonstrated it by standing right on the edge of a table.
 
Riding the computer is normally defined as staying just inside the 'no stop' time. i.e. having the computer at 1 minute of no-stop time all the way to the surface.
To my mind, that is aggressive diving.
I've heard that the use of computers and an increased tendency of riding the NDL may be responsible for a claimed increase in DCI in slow tissues, i.e. more spinal bends compared to skin and joint bends. While I haven't seen data supporting that claim, it seems plausible.
 
I understand the riding NDL. I am however referencing riding it as in ascent rate. I believe Sdi teaches not to exceed your computer ascent rate. (don't quote me on that)

Edit: they call it, computerize your ascent. Referencing never exceeding the computers recommend ascent rate.
 
I believe Sdi teaches not to exceed your computer ascent rate.
What's interesting is that when we did the 10 m/min ascent rate exercise, more or less everyone's computer was rather pissed off. And told us so in no uncertain terms. My Suunto sulked for several hours afterwards, so good thing that was the last dive of the day.
 
I understand the riding NDL. I am however referencing riding it as in ascent rate. I believe Sdi teaches not to exceed your computer ascent rate. (don't quote me on that)

Edit: they call it, computerize your ascent. Referencing never exceeding the computers recommend ascent rate.


I don't want to go off on a tangent from boulderjohns orignial request. BUT

Swimlikethefish, you need to understand how the tables are working.
A traditional Buhlmann table defines dive time as leaving the surface to leaving the bottom. It then calculates your decompression requirement based on maximum depth, dive time AND an ascent rate of 10m/min, either to the surface (No-Stop), or to the first stop depth (with the following stops).

IF you slow the ascent rate from (extreme example) 10m/min to 1 m/min. For a no-stop dive from 30m to the surface, the table dive assumed it will take you 3minutes to the surface, you have just taken 30 minutes! Even a less extreme example 5m/min means you add an additional 3 minutes to the total submerged time (dive time + ascent time)!

This is why if using a hard table (Buhlmann), divers ascended at a rate of 10m/min, to stay within the calculations of the hard table. On most dive computers, the maximum ascent rate was normally 10m/min. So we used to ascend right on the edge of tripping the fast ascent warning (Suunto 3 bars = 10m/min). If not, you where outside the table. (The other point - when doing Trimix or ERD dives, your first hard (not Pyle) stop was, say, 15m, the pressure drop from 50m - 15m is quite small compared with the pressure drop form 6 to the surface!)

It is about staying within the RULES of the TABLE you are using. To a greater degree this is now academic, because most diving is done via a dive computer. The dive computer RECALCULATES the required decompression if you slow the ascent rate. Which is OK, if you still have the gas to do the dive, but could invalidate your gas calculations if you add 15 minutes to the ascent time!

Sorry boulderjohn for the thread deviation
 
If you have not yet read the excellent "Deco for Divers" from Marc Powell, it explains the second part of your question ("why do they advocate it") very well, i think. It elaborates pretty extensively the history of development of decompression models till present time, which - again - explains the reasons behind why each agency advocate their strategies.
 
just wondering how the article's coming along @boulderjohn
 
What's interesting is that when we did the 10 m/min ascent rate exercise, more or less everyone's computer was rather pissed off. And told us so in no uncertain terms. My Suunto sulked for several hours afterwards, so good thing that was the last dive of the day.
I had a Suunto d4i and it would randomly add 1m deco stops if I did an ascent of 9m/min (if I remember correctly)
 

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