Experienced gas swichers

[MikeFerrara]

Do you have the text book from your nitrox text book? Go back and look at the examples.

If you're using the DSAT tables, the directions are on the table. Assumiong that you are using copatable tables...like all DSAT as an example, you are right in that the pressure group tranfers from one table to the other. Procedurally it's going to be exactly the same as repetative dives on the air table so you might want to make sure that you remember how to do that before comfusing things with different gasses. Remember to take SI time and RNT into account and don't forget to check O2 exposure.

 

[fire_diver]

I am having a total brain lock-up!

I have been using computers toooooo long now i am going back to the tables and just realized that I have either forgotten how to do repetitive dives or the answer to this question is wrong in the book.

Here goes:


A diver completes a 70 foot dive for 30 minutes, using air. One hour later she plans a 60 foot dive using Nitrox-II. What is her maximum actual bottom time to avoid stage decompression?

I don't know what models your training agency is using. Here is what I get from the PADI tables I have:

a dive to 70ft for 30 rounds up to 31 minutes. This gives a pressure grouping of "O". After a 1 hour SI, the pressure group is now "E". Moving now to the back of the EAN 36 card, starting at the second dive pressure group of "E", so a 60ft dive has an NDL of 89 minutes.

On the other hand, if I look at my EAD table, I see that EAN36 at 60ft has an EAD of 50ft. Staying now with the air tables, a beginning pressure group of "E" yields an NDL of 59 minutes for the dive to (theoretically) 50 feet.

If I wasn't lazy, I'd work out the calcs by hand and give you a third, and different answer. This is why I dive with a computer.

FD

 

[Bigcape]

There is no correct answer because the table starts off with 170mins as the NDL in the first place. No matter how you do the math you cant get a number small enough to be ANY of those answers. However "none of the above" is not a choice!

 

[Jason Ooi]

You second dive use Nitrox 36, depth desire and have a EAD table conversion to normal air table
IANTD have a EAD/MOD table

 

[TomP]

My view... using Navy air table for dive 1 and NOAA Nitrox II table for dive 2.

Using the air table, you are an F diver after 30 minutes at 70 feet
After a one hour surface interval you are an E diver

Switching to the NOAA II table to plan dive 2, an E diver at 60 feet has a 38 minutes of residual nitrogen

Subtracting the 38 minutes of RNT from the single-dive NDL for 60 feet of 100 minutes, you get an NDL for dive 2 of 62 minutes

 

[fire_diver]

These responses have really opened my eyes. I was raised that a mathmatical problem has a set answer. I expected to see some varience, but the differences are staggering. Kind of makes me feel that it's all one big guessing game under the waves.

But I guess I shouldn't expect too much. The first few decades, divers were making dives, bottom times, and ascents that (according to modern thinking) should have bent the vast majority.

FD

 

[TSandM]

GUE: 60 fsw on Nitrox is approximately 50 feet on air; 50 feet has a MDL limit of 60 minutes. If surface interval less than 90 minutes, do 2 min at 30, 20 and 10; if greater than 90 minutes, do 1 minute at 30 , 20, and 10. This is what Decoplanner would generate (or close to it), and this is why I really like this system. No tables, and I did it all in my head.

 

[Bigcape]

My view... using Navy air table for dive 1 and NOAA Nitrox II table for dive 2.

Using the air table, you are an F diver after 30 minutes at 70 feet
After a one hour surface interval you are an E diver

Switching to the NOAA II table to plan dive 2, an E diver at 60 feet has a 38 minutes of residual nitrogen

Subtracting the 38 minutes of RNT from the single-dive NDL for 60 feet of 100 minutes, you get an NDL for dive 2 of 62 minutes

YOU ARE CORRECT! HOWEVER!!!!!!!>>>>>>


This question was asked in a test and a book that does not use the NOAA table with a 100 min NDL. It uses a 170min NDL limit to subtract your RNT from.

Therefore the correct answer cannot be chosen using the materials provided!


This made me think that there was a typo in the table and the book I was given and a serious NDL mistake made (think about seeing a 70min difference!!). Then it was brought to my attention that if you figure a the EAD of 60fsw with 36% mix you come up with EAD=42fsw @ 170mins!!!!


So after spending soooooo much time I now conclude that the the test question was written with correct answer taken from using a NOAA table NOT the one provided and printed with the book.

The finding: A 100 min NDL limit is actually a VERY conservitive number when in reality 170mins is correct too.

My Question now is why did the NOAA people find it nessecary to drop it to 100 mins on their table?

 

[Slipstrm]

A diver completes a 70 foot dive for 30 minutes, using air. One hour later she plans a 60 foot dive using Nitrox-II. What is her maximum actual bottom time to avoid stage decompression?

here are my choices:
a. 62 mins
b. 30 mins
c. 60 mins
d. 20 mins

PDIC Nitrox, One of my favorite flavors , I love mixed gas, Hmmm...Who is your Instructor?
I think you may have gotten these answers from another problem....Although I cant see this combination of answers on any of the other questions??......anyway that would go a long ways to explaining some of the responses you got.....and your confusion
If this is question # 22 in Nitrox Diving by Fred Calhoun, The standard PDIC Nitrox Text, then the possible answers "should" be
a. 124 mins
b. 60 mins
c. 120 mins
d. 40 mins

Here is how it works...using the PDIC supplied book with Navy Air Tables.
1st dive to 70' for 30 min.
This makes you a F pressure Group Diver (Navy Air Tables)
Out of the water for 60 minutes makes you an E pressure group diver. (still on air tables)

Now you want to plan a 2nd dive on 36%,
Take your E diver status to the residual/repetitive matrix on the 36% tables.
Plan as an E pressure group diver to 60' and notice you have 46 minutes of residual according to the tables.

Now look at the NDL for 60' on the 36% table, now....if you were a clean diver without nitrogen......WHICH YOU ARE NOT AT THIS POINT.....you would have an NDL of 170 minutes at 60 feet.
Take the NDL of 170 minutes and subtract the 46 minutes of residual and theres your answer.

There is a mistake in the test on another question, that always lets me know who really understands what is being taught, can you find it?

 

[TomP]

....So after spending soooooo much time I now conclude that the the test question was written with correct answer taken from using a NOAA table NOT the one provided and printed with the book.

The finding: A 100 min NDL limit is actually a VERY conservitive number when in reality 170mins is correct too.

My Question now is why did the NOAA people find it nessecary to drop it to 100 mins on their table?

The gap between 100 and 170 mins is pretty big. My Vytec gives me 94 minutes at 60 feet using 36%.

Getting back to the original source of your frustration - to be completely facetious, yet correct, you could argue that the answer cannot be determined. The question as posed

One hour later she plans a 60 foot dive using Nitrox-II. What is her maximum actual bottom time to avoid stage decompression?

is intriniscally flawed in as much as we are only told when she began the planning process and not what SI she used in the plan.