Nitrox Help

[munkispank]

Hi there,
I am not sure where to post this, but here goes.

I go Padi Nitrox certified today, but I am left confused even after asking my Instructor the question.

Basically- the Nitrox manual had a typo stating that air dives to under 50 ft register as GREATER than 0.5 ata for oxygen exposure. That shouls read less than, but my question is, how do monitor oxygen exposure for air dives deeper than 50ft?- Instructor says there is no oxygen concern, but I wanted to check with everyone else.

ex: Dive 1 to 80ft on air, dive 2 to 70ft on air, dive 3 and 4 say 60-50ft on nitrox- how do I calc the times for oxygen exposure to see if I could do a 5th???
(I get the RNT etc but was confused about this)

I guess ultimately I am not sure Nitrox will be worth the effort for me. Any hints?

 

[TwoBitTxn]

The lowest O2 mix on my TDI PO2 table is 25%.

This matches what your instructor told you. The O2 time limit on air doesn't factor in.

TwoBit

 

[Don Burke]

ex: Dive 1 to 80ft on air, dive 2 to 70ft on air, dive 3 and 4 say 60-50ft on nitrox- how do I calc the times for oxygen exposure to see if I could do a 5th???
(I get the RNT etc but was confused about this)

According to the EAD/Oxygen Exposure Table from DSAT/PADI:

80 feet on air is .72 ppO2
70 feet on air is .66 ppO2
60 feet on EAN36 is 1.01 pp O2
50 feet on EAN36 is .91 pp O2

Air oxygen partial pressures are on the EAD table just to the right of EAN40. In my PADI Nitrox book (Version 1.1), this is noted on page 44 in the same paragraph with the typo you referred to.

If we can get some times for these dives, we can compute the percentage of exposure.

If we ignore the RNT, going to the no-stop limits on all of these dives will take you to 145% exposure.

In real-world terms, doing those first two dives on air is going to cost you a bunch in terms of RNT. It will also take a frightful amount of gas to get to the no-stop limits on those last two dives if no RNT is assumed.

If you use EAN36 on all four dives, this exercise will mean something.

 

[matt_unique]

Hi there,
I am not sure where to post this, but here goes.

I go Padi Nitrox certified today, but I am left confused even after asking my Instructor the question.

Basically- the Nitrox manual had a typo stating that air dives to under 50 ft register as GREATER than 0.5 ata for oxygen exposure. That shouls read less than, but my question is, how do monitor oxygen exposure for air dives deeper than 50ft?- Instructor says there is no oxygen concern, but I wanted to check with everyone else.

ex: Dive 1 to 80ft on air, dive 2 to 70ft on air, dive 3 and 4 say 60-50ft on nitrox- how do I calc the times for oxygen exposure to see if I could do a 5th???
(I get the RNT etc but was confused about this)

I guess ultimately I am not sure Nitrox will be worth the effort for me. Any hints?

For the depths you are referring to you would incur lots of decompression obligations due to nitrogen long before you would need to worry about 02 toxicity. You reach the contingency pp02 of 1.6 ata at 218' breathing air.

--Matt

 

[MikeFerrara]

I'm not 100% sure I'm clear on what the question is but...

When diving air within recreational limits you run into N2 limits before O2 limits so they don't bother to teach you to track O2 exposure.

As you know that isn't the situation with nitrox. If you're mixing air dives with nitrox dives you track O2 exposure for the air dives just as you would for the nitrox dives. You do it exactly the same way. CNS exposure is dependent on PPO2 and time.

 

[munkispank]

thanks for the replies guys,

"If you're mixing air dives with nitrox dives you track O2 exposure for the air dives just as you would for the nitrox dives. You do it exactly the same way. CNS exposure is dependent on PPO2 and time."

this is what I wanted confirmed. The book mentioned that below 50ft it is not neccesary to track oxygen exposure with air, but I wanted to clarify the peocedure for air below 50ft. The book suggested the above calcs, but as my Instructor said I didn't need to track at all, I was a little confused.

thanks for the clarification, I re-read the book a couple of times but wanted to make sure I had it right.

thanks
Jackie

 

[sharkmasterbc]

It is a good idea to track O2 exposures especially if you are doing multi day and or multi dives. If you have a computer, you could turn it onto the nitrox mode and then set it to 21 % when you are diving air, this will track o2 exposure for you. It would be better to figure it out in your head and do the long calculations (this should be in the manual). I usualy dive nitrox 32 for dives to 100 ft but use air tables and track o2 exposure manually. The o2 exposure on air is minimal unless you are doing long deep air dives which IMHO is not a safe diving practice. eanx 32 to 100ft, anything deeper requires further training and different breathing gasses.

 

[Don Burke]

The book suggested the above calcs, but as my Instructor said I didn't need to track at all, I was a little confused.

My guess is that your instructor thought you were asking about days when all of the dives were on air.