Dir Divers!!!

[conallmeehan44]

How on earth do you calculate your NDL without a computer?

 

[lowwall]

How on earth do you calculate your NDL without a computer?

I'm not DIR. But if you don't have a computer, there's a thing called a dive plan in which you calculate your dive profile in advance. This is how everyone dived before the computer, even if the dive plan was simply taking a small enough tank that they would run out of air before running out of NDL time.

There was also the "120 rule" that you could use to figure out an NDL dive on the fly. This was loosely based on the Navy Tables and said that you would be OK as long as your max depth in feet + minutes was less than 120. So you could do 60' for 60 minutes or 100' for 20 minutes. To make it more conservative, you could turn it into a 110 or 100 rule. Note that the 120 rule was for single dives only, not multiple dives on one day.

I do know that some early DIR training groups (not all were full fledged agencies) taught shorthand methods for calculating deviations from your initial dive plan on the fly when desired or necessary. These methods also produced on the fly deco schedules. I don't know how prevalent that still is given their general acceptance of computers.

 

[SpeedyJ]

Rules of Thumb: The Mysteries of Ratio Deco Revealed

Is it a secret algorithm developed by the WKPP to get you out of the water faster sans DCI, or an unsubstantiated decompression speculation promoted by Kool-Aid swilling quacks and charlatans? British tech instructor/instructor evaluator Rich Walker divulges the arcane mysteries behind GUE’s...

indepthmag.com

This has most of the info you likely want in a digestible format.

 

[crofrog]

For nx32 130-depth = max bottom time. It’s fairly aggressive, I’d do a long (5min) 20ft stop and a 6 min ascent to the surface if diving to that limit.

For repetitive diving there are some additional rules.

 

[crofrog]

I do know that some early DIR training groups (not all were full fledged agencies) taught shorthand methods for calculating deviations from that plan on the fly when desired or necessary. I don't know how prevalent that still is given their general acceptance of computers.

GUE still teaches the ratios as a way of sanity checking dive computers and tables. If either one of those gives you a number significantly shorter than the ratio number there has probably been some input errors somewhere in the system

 

[DiveLikeAMuppet]

30 minutes at 30 meters with 32 % nitrox and then a specific ascent protocol is the GUE way for NDL fun diving.

40 minutes at 27 meters, 50 minutes at 24 meters and so on. There is a table if you care to write it down into wetnotes that gives you more time in the shallows.

You need a bottom timer as a minimum though.

I suppose if you were really really really hardcore and there was a bottom never deeper than MOD (or you dragged a predetermined length of string from a DSMB on the surface), you could track your time with just your turn pressure but I doubt that’s a clever idea.

 

[Lorenzoid]

How on earth do you calculate your NDL without a computer?

First, let me say that this question could be interpreted as implying that the DIR diver is not using a computer. It's been a long time since DIR divers truly shunned computers. But even today there are some shortcut rules of thumb that they might use in some circumstances instead of or as a complement to a computer. This is one of them:

There was also the "120 rule" that you could use to figure out an NDL dive on the fly. This was loosely based on the Navy Tables and said that you would be OK as long as your max depth in feet + minutes was less than 120. So you could do 60' for 60 minutes or 100' for 20 minutes. To make it more conservative, you could turn it into a 110 or 100 rule.

Yes (although 120 is for air, while 130 works for the EAN32 that DIR divers prefer).

@conallmeehan44 , this rule of thumb was developed when someone looked at the rows and columns of the (air) table right in the central area of the table where most recreational dives are done and noticed a pattern. In this limited region of the table, they noticed that the depths and times added up to approximately 120. This only works neatly for the first dive of the day; for repetitive dives there are adjustments based on surface interval that seem to work.

 

[conallmeehan44]

Thanks for the responses everyone.

 

[LFMarm]

How on earth do you calculate your NDL without a computer?

1. "DIR" is not equal to "no computer". I always dive with 2
2. Before every dive, you do a dive plan. At that time it's very easy to calculate NDL from tables, software, dive computers or rules of thumb

 

[Lorenzoid]

1. "DIR" is not equal to "no computer". I always dive with 2
2. Before every dive, you do a dive plan. At that time it's very easy to calculate NDL from tables, software, dive computers or rules of thumb

Your second point brought to mind the possibility that the OP may have been under the impression that divers might somehow mentally compute no-deco time remaining during the dive, i.e., on the fly, based on actual depths, like a computer does, not to estimating a no-deco limit (NDL) as part of pre-dive planning. If that was the impression, then no, that isn't done.*

* There is the ratio deco technique linked to in post #3, but that's for dives exceeding NDL, not for "no-deco" dives.