Nitrox and Suunto Vyper computer

[Piscean]

DivingDoc,

Please forgive me for being so blunt, but on a board like this it is the responsibility of each user to read it! You can't just keep asking the same questions even though they have already been answered in this thread.

For example, TexasMike told you that in Nitrox diving nitrogen still limits your time under water and yet in your last post you write

So in Ntrox diving -- you only need to look at the OLF bar graph -- Nitrogen loading isn't important??

Also I explained the rationale behind the two measures of oxygen toxicity, cumulative oxygen exposure in the OLF bar graph and max ppO2 exposure in terms of a maximum operating depth and how you know if you are approaching the limits. Yet you asked again about that.

Also -- I would like to see where I'm at in terms of moving up to Oxtox before I get an alarm that tells me that I've reached it!

In fact, the answer to this question was in the text you quoted immediately under it. I will attribute that to your not understanding "cumulative central nervous system oxygen exposure" because you have not yet done your Nitrox course (though I feel sure it must have been mentioned at Med School if nitrogen loading was). Finally, I told you that the O2 exposure when calculated from tables is done the same way as nitrogen loading, i.e. by assuming the max depth for the whole dive. That is not time independent, it is the O2 exposure per unit time at max depth times the length of time of the whole dive.

As for the computer thing, that is personal taste. I told you that the alternate display will display the nitrogen loading bar graph when you are diving nitrox if you press the time button, so on the vyper (which you told us in your first post that you have - so infact, you could read the manual and find it out yourself - there are only 3 pages on displays while nitrox diving) you also have all three indicators, central nervous system oxygen toxicity (=OLF), consumed bottom time indicator bar graph (=CBT) and the ascent rate. In my long post I explained the relationship between the CBT bar graph and the no-decompression time number (which is always displayed in both air and nitrox modes) so now it is up to you to decide what is important to you. You have all of the information in our answers, you just have to read them!

If I am way off base in my tone - someone please let me know so I can vent in more appropriate ways in the future.

Piscean.

P.S. If you really want to choose a computer why don't you post a question on the computers board. "I do diving type blah, blah, I want to do nitrox and air (not on the same dive). Which features are important on a computer for this type of diving?"

 

[The Iceni]

Piscean wrote
You do not really need to know the current ppO2 IMHO. . . . . If my actual depth is less than my MOD, then my ppO2 is less than my max ppO2 and that is all I worry about. . . . .

Dr. Paul, please read through my long post earlier and see if you don't agree, that the real advantage of the bar graphs is that they show you your current percentage of your total allowable exposure (either to O2 or N2) corrected for that particular depth. That corrected for depth thing is important because if I ascend with 2 minutes of no-stop time left from 130 feet that was quite conservative as I have only used 80% of the no stop time. . . .

and
In fact, the answer to this question was in the text you quoted immediately under it. I will attribute that to your not understanding "cumulative central nervous system oxygen exposure"

I completly agree with your first comment, Piscean. I have to admit that I do not really understand the mechanisms behind the concept of "cumulative CNS oxygen exposure", having done a considerable amount of reading on the subject. As long as the diver does not exceed a pp O2 of 1.4 (at the planned the target maximum depth) at any stage of the dive he is very unlikely indeed to approach, let alone exceed, the NOAA recommendation of 3 hours at a constant pp O2 of 1.3, particularly in the sort of dives Elaine is proposing.

On the other hand, if, like me, you intend to use increasing high Nitrox mixes for decompression the pp O2 MAY approach or even exceed 1.6 bar (for example 100% oxygen at 6 metres (20 feet)) when the oxygen clock then very rapidly ticks away. I suggest Elaine should take a very close look at the NOAA tables for oxygen exposure limits. - 45 minutes at a continuous 1.6 bar.

My recommendation for Elaine is to forget about cumulative CNS oxygen exposure (at least for now), since as her posts confirm, this only serves to confuse. If a diver has difficulty getting to grips with such concepts on the surface I would suggest that confusion will reign underwater.

ON THE OTHER HAND

Now this is not the same as planning for nitrogen loading and diving conservatively in order to increase the safety margins for the risks of DCI which is another matter entirely and far more important. It is vital fo all divers to stay well within these limits. - This is what decompression therory is all about! - Each diver will decide on the level of conservatism he wishes to use and the best way to achieve it. A common example is to use a Nitrox and dive as if you are using air.

At all stages of the dive the actual nitrogen loading will be less than the computer believes (When using EAN 32 you have 68% Nitrogen (with a pp N2 of 0.68 bar on the surface) but the computer thinks the surface pp N2 is 0.79 bar - a safety margin of 16% at all stages of the dive.

This brings me back to one of my original comments. If she plans her dives properly and is not going to be doing any form of accelerated decompression will Elaine really need all this additional information, particularly about oxtox, DURING the actual dive?

If she does, in my humble opinion of course, I submit this represents excessive task loading at Elaine's stage of training.

Elaine, how about this for a way to enjoy your ascending profile dives, rather than looking at your computer every five seconds? Before you dive calculate your gas requirements for a square profile dive to your target 130 feet (40 metres) and see how long the gas will last at that depth and plan to surface before or at that time if you remain within the no stop limits as subsequently shown by your computer? If not ascent earlier as indicated by the time to surface value. Diving style 1 in my first post.

I am not familiar with the Vyper but surmise that the bar is simply one way of looking at the time to the no-stop limit. Unfortunatley, as Piscean demontrates, it is proportional, not absolute and to my mind of little practical use. Two mins of bottom time is 20% of the no stop limit at 130 feet. At 60 feet 2% of bottom time will be much, much longer.

What the diver needs to know is the actual time left in minutes before he needs to surface - TTS. Thats what I would keep my eye on!

I am not a diving instructor and make to claims as such. Your instructor clearly has a duty to ensure you understand the benefits and risks of using Nitrox. If you remain confused during your course ASK HIM!

 

[Zept]

Originally posted by Dr Paul Thomas
I am not familiar with the Vyper but surmise that the bar is simply one way of looking at the time to the no-stop limit. Unfortunatley, as Piscean demontrates, it is proportional, not absolute and to my mind of little practical use.

According to my Vyper manual, the bar graph is not proportional. It displays the amount of bottom time remaining at the current depth, but in reverse (ie more segments means less time). The manual says:

"Segments start to appear when the available no-decompression time decreases below 100, 80, 60, 50, 40, 30 and 20 minutes.

"As the bars reach the yellow zone, your no-decompression time is less than 10 or 5 minutes and you are getting very close to no-decompression limits. At this point, you should start your ascent towards the surface.

"As all the bars appear (red zone), your no-decompression limit has become zero and your dive has become a decompression dive."

If I am interpreting this correctly, the graph is absolute. If you start the dive by dropping to, say, 18m, you should immediately see several bars, because your no-decompression time is less than 60 minutes. When the first block appears in the yellow zone, you have 10 minutes left at your current depth. When the second block appears, you have five minutes. When a block appears in the red zone, you've blown it (assuming you're a rec diver).

All the same, I agree with your comment about the graph being "of little practical use". I find it much, much easier to look at the numeric display of no-decompression time, which tells me exactly how many minutes of bottom time I have left.

Zept

 

[DivingDoc]

Originally posted by Zept


All the same, I agree with your comment about the graph being "of little practical use". I find it much, much easier to look at the numeric display of no-decompression time, which tells me exactly how many minutes of bottom time I have left.

Zept

Thankyou, Zept, for your comments. They were very lucid. But this highlights the problem I have with the Vyper (and why I returned it -- along with the manual, so I can't refer to it anymore).

The problem is this: the manual suggests that if you are going to dive both air and nitrox that you keep the computer set to nitrox mode. In Nitrox mode, the computer's only visible bar graph is the OLF (oxygen limit fraction). I am assuming here, as Picean pointed out, that the OLF is the fraction of your oxygen exposure limit that you have used up. The numeric value continues to display your remaining NDT. So if you are using the computer for air, you have the OLF bar graph (which is useless for air diving) and the numeric NDT display. From what you have said above, it seems like the bar graph is useless in an air diving situation also when you have the computer set to air.

I am a very visual person, so right now the Pelagic computers (Aeris and Oceanic) are appealing to me more because they display both nitrogen loading and oxygen exposure in nice big colorful bar graphs that can both be seen at the same time.

I only wish they had as much memory and as good a software package as the Suuntos.

ET

 

[The Iceni]

DivingDoc said
OK -- here's what I mean. We are going to be doing nice warm tropical diving in Cozumel. In multilevel diving, you start deep -- up to 130 fsw, but only stay there briefly -- you then gradually move up, decreasing your depth in a stepped fashion, going over gradually rising terrain, all the time keeping within no-deco limits. You need a computer for this because if you just use dive tables, you would have to count your deepest depth as the depth for the whole dive. The computer can give you extra no-deco time based on your gradually decreasing depth.

Dive computers do cost money and Elaine does need carfully to consider how to spend her money.

The first observation I would make from the above is that anyone purchasing a dive computer should, as far as possible, ensure it is future proof. What I mean by this is will it be suitable for the style of diving expected in three years time.? To that end I would suggest that all computers should be set for Nitrox use.

Secondly, the ascending profile described above is effectlively a long, slow conservative ascent from depth, the equivalent of deep Pyle stops most technical divers add into their deco profiles to reduce the formation of micronuclei. At all stages of the "ascent" the faster and medium tissues - which are closer to saturation - will be offgassing while the slower tissues, which contain very little nitrogen at all - will continue to dissolve more nitrogen but never enough to pose a problem.

It is therefore, perhaps, the most conservative method of diving. It is certainly safer than a square profile.

I must say I remain puzzled as to why computer manufactuers are adding anologue displays (bar charts) when digital information is much more readily assimilated and the space could be used to make the digits larger. I am all for redundancy but, as I have said before, to my mind this is duplication of information (on a samll screen) which risks causing confusion. On the other hand many of my friends use Vypers so they are very popular.

However, Elaine, now that you have done your homework and consulted the NOAA tables could you explain to me why you think you NEED a graphical representation of oxygen exposure (and TTS) for your proposed multilevel dives?

 

[DivingDoc]

Originally posted by Dr Paul Thomas


I must say I remain puzzled as to why computer manufactuers are adding anologue displays (bar charts) when digital information is much more readily assimilated and the space could be used to make the digits larger. I am all for redundancy but, as I have said before, to my mind this is duplication of information (on a samll screen) which risks causing confusion. On the other hand many of my friends use Vypers so they are very popular.

However, Elaine, now that you have done your homework and consulted the NOAA tables could you explain to me why you think you NEED a graphical representation of oxygen exposure (and TTS) for your proposed multilevel dives?

Simply put, I am a visual person. I assimilate information much faster when I can look at a graphical representation rather than a number or a bunch of numbers.

On Pelagic computers, the numbers are big and bright and bold, and there are bar graphs for O2 exposure, Nitrogen loading, and ascent rate. There are big numbers in the middle showing the remaining NDT, and your elapsed time. Check out the demo for the Versa Pro at:

http://204.247.176.228/PDF_Docs/Versa_Pro_Interactive.html

Part of my problem is that I AM trying to plan out 3 yrs ahead on my computing needs. I am a relative newbie diver. Although I received my open water cert in 1993, I had only dove about once every 3 years or so, and then for only a 2 tank dive off of Florida or something. Then last January, we stopped at Cozumel for 2 days prior to a meeting I had in Cancun and did 5 more dives. Even with those, it only brings my total dives up to 12 (not counting the ones I did in the Bahamas with my Dad as a 12 year old, when there wasn't such a thing as certification.) We loved it and plan to dive more often in the future -- more like twice per year.

I am just starting now to read the material for my Nitrox certification class and am having an "aha" experience. Since us doctors normally deal with pO2 in arterial blood gases, I have thought all the time that the pO2 that was talked about with respect to Nitrox diving was also a blood value, though estimated from mathematical models and in different units from what I'm used to.

Now I see that the pO2 they are talking about is simply the partial pressure of O2 in the air you are breathing -- not the estimated blood value. Stay tuned for more eureka events as I read the Nitrox course material.

 

[Piscean]

Zept,

Your post made me check my Cobra manual and then try it out in planning mode and you are completely correct. The bar graph is absolute and thus as far as I am concerned utterly useless and I will continue to look at the numbers just as I have always done. I have to agree with Dr. Paul, it is duplication and the space could be better used.

Piscean.

 

[O-ring]

Weren't you going to bungee a PADI Wheel on your wrist and calculate your multi-level profile on the fly in the water? You could use wetnotes for the calculations in case your calculator isn't watertight or your primary light isn't strong enough to activate the solar cells on your solar calculator