NDL's

[HeatCker]

hopefully i can put this together to make sense
i'm looking at my dive tables and depth starts at 35'
on my table (padi DSAT) the NDL for 35' is 205 minutes
why do the dive tables start at 35' ?
what would the NDL be at say 15 or 20' ??
i assume your off gassing at about the same rate as your on gassing but wouldn't there be a known limit ??

here's the reason for the question
when the NDL is greater than 99 minutes my suunto shows two bars " -- ", that's normal for my vyper
the oceanic geo continues to show actual numbers
the other day i borrowed my buddies oceanic geo dsat computer out of interest more than anything
i did 2 dives to 75' on a wreck following my suunto computer, i used his geo for comparison
heading up the rope at the end of the second dive i was in the 15-20' range
my suunto went into safety stop countdown mode before the geo
at that point the oceanic indicated i had somewhere in the area of 550 to 649 minutes left
do the NDL's increase that much in the shallow ranges ??
if so the curve plotted using depth, time and NDL's sure goes off on a tangent

 

[tbone1004]

you will see drastic differences between a Suunto and an Oceanic because the algorithms are completely different, so don't expect them to behave the same.

In the shallow area, you have functionally unlimited no decompression time. Once you get cleared to surface, which is when the safety stop countdown ends, or if you come back up and haven't hit NDL, then you are able to surface whenever you want. From shallow depths, you can stay there for essentially an unlimited amount of time and surface whenever you want which is why the tables don't usually start until 10m or 40ft. If you look at those numbers, you have to be there for a LONG time, and few people are really going to stay at 35 minutes for over 3 hours to hit the PADI table NDL.

The curve between depth, time, and NDL are not linear, they are very much exponential because pressure does not increase linearly. It's quite complicated and best to read up on a myriad of papers that are on the Rubicon database if you are that curious.

Computers will also show numbers quite a bit different in the shallow range than tables will because the tables have to be brought to some reasonable amount of time so you will never see 600 minutes on a dive table, or just "unlimited".

we could talk for hours about how and why, but hopefully this answers part of your question. If I didn't clarify something properly, feel free to ask

 

[Steve_C]

hopefully i can put this together to make sense
i'm looking at my dive tables and depth starts at 35'
on my table (padi DSAT) the NDL for 35' is 205 minutes
why do the dive tables start at 35' ?
what would the NDL be at say 15 or 20' ??
i'm assume your off gassing at about the same rate as your on gassing but wouldn't there be a know limit ??

Who cares. It is so long that you are not going to stay down that long.

when the NDL is greater than 99 minutes my suunto shows two bars " -- ", that's normal for my vyper
the oceanic geo continues to show actual numbers
the other day i borrowed my buddies oceanic geo dsat computer out of interest more than anything
i did 2 dives to 75' on a wreck following my suunto computer, i used his geo for comparison
heading up the rope at the end of the second dive i was in the 15-20' range
my suunto went into safety stop countdown mode before the geo
at that point the oceanic indicated i had somewhere in the area of 550 to 649 minutes left
do the NDL's increase that much in the shallow ranges ??

Yes it does.

Different computers go into countdown at different points. My Suunto does it at 20. My scubapro does it at 15 but then keeps the countdown as long as I am above 20.

if so the curve plotted using depth, time and NDL's sure goes off on a tangent

You are thinking of it backwards. What is usually important is to note how rapidly the NDL shrinks with depth.

Also note that different computers increase the NDL differently as you go up. My scubapro starts increasing it as soon as I start up. The suunto tends to make me go up some and then starts adding NDL sometimes in big chunks.

 

[Clark Fletcher]

Well, I'm newly back into diving but, first certified back in 1979. Even back then the PADI dive tables started at 35'. Our instructor back then told us that the reason they started at 35' is that there is no limit at depths shallower than 35' (I think he told us it's actually shallower than 33'). Basically, as long as you had an air supply, you could stay down as long as you wanted. I never bothered researching it to verify that what he told us was correct. I do know that if you run the PADI eRDP for anything 35' or less it will always say 205 minutes.

 

[tbone1004]

@Clark Fletcher the dive tables haven't really changed since they were first introduced as it is a very expensive process to change and with the advent of reliable dive computers, PADI is actually teaching OW courses where the tables aren't discussed, it is all computer based.

We teach through NAUI and teach that any dive shallower than 40ft *where our tables start as they were taken from the US Navy tables*, is treated as a 40ft dive and is subject to a 130minute run time for NDL diving.
That number is feasible if you have say double AL80's, or a steel 149 and stay at 20ft which gives a SAC of around 0.7cfm, but very few people are going to do that.

 

[BoltSnap]

why do the dive tables start at 35' ?
what would the NDL be at say 15 or 20' ??

Anything less than 40 feet will be considered 40feet and takes the NDL for 40 feet using dive tables.

when the NDL is greater than 99 minutes my suunto shows two bars " -- ", that's normal for my vyper

The "--" in the Suunto computer line means the NDL is much larger than what the display would display.

my suunto went into safety stop countdown mode before the geo

After you go deeper than 30 feet, you will need to make a safety stop between 19 - 10 feet and hence the Suunto computer will start the safety stop clock when you are shallower than 20 feet and had gone deeper than 30 feet prior.

 

[BoltSnap]

We teach through NAUI and teach that any dive shallower than 40ft *where our tables start as they were taken from the US Navy tables*, is treated as a 40ft dive and is subject to a 130minute run time for NDL diving.
That number is feasible if you have say double AL80's, or a steel 149 and stay at 20ft which gives a SAC of around 0.7cfm, but very few people are going to do that.

If making repetitive dives in one diving day, it is possible to exceed the NDL for dives 40ft and shallower :)

 

[tbone1004]

If making repetitive dives in one diving day, it is possible to exceed the NDL for dives 40ft and shallower :)

on naui tables of course. One of the things we talk about in our course is comparing dive tables from different agencies and the how and why they look so different and when you may want to use someone elses tables.
Doing two dives a day for one day off the coast of NC where it's relatively deep? Probably want to use NAUI tables.
Doing a liveaboard? PADI tables win.
They then usually decide that that is way too much work and end up getting a computer when they ask us what we use and tell them we all use computers

 

[Clark Fletcher]

@Clark Fletcher the dive tables haven't really changed since they were first introduced as it is a very expensive process to change and with the advent of reliable dive computers, PADI is actually teaching OW courses where the tables aren't discussed, it is all computer based.

For the most part, absolutely correct. But, they have had minor changes. I ran across my 1978 copyright PADI dive tables right after I started my new certification and compared them to the current PADI dive tables (just out of curiosity). Like you said, for the most part they're identical. However, the current dive tables are slightly more conservative at some of the deeper depths. I'm guessing those minor changes were in large part prompted by empirical data gathered over the years.

 

[tbone1004]

For the most part, absolutely correct. But, they have had minor changes. I ran across my 1978 copyright PADI dive tables right after I started my new certification and compared them to the current PADI dive tables (just out of curiosity). Like you said, for the most part they're identical. However, the current dive tables are slightly more conservative at some of the deeper depths. I'm guessing those minor changes were in large part prompted by empirical data gathered over the years.

more likely CYOA from the lawyers, but yeah, the tables themselves are the same, they just moved the NDL over a bit. NAUI did the same to the USN tables