91 replies to this topic

[pir8]

In the case of the Cochran you used it was set fairly conservatively. That feature ranges from 0 to 50 and yours was set on 25. (I set them all on 25 assuming we are mostly ahem...middle aged and that is the middle number.) I have discovered that if I set my Cochran on 25, and dive 21% mix it will track perfectly with the Adladin Pro set to 30% EAN.

And yes the Cochran adapts the algroythm for warm water, salt vs fresh water, flying before diving etc. It also takes a snapshot every second to update info rather than every 8 seconds as many computers do.

Some divers do not mind that their computer assumes worst case scenario ie. assumes cold vs warm water or fresh vs salt, or recalibrates every 8 seconds instead of every second since it limits bottom time and that means it is therefore 'inherently' more conservative.

As long as you know, then its a matter of your own choice.

I expected that the Cochran would be more liberal for a couple reasons, nuber one, I did a dive before I got the Cochran, but with my own computer, and number two, the warm water credit, the flying credit, etc.

I was surprised that this was not always the most liberal, but that on some dives it wanted me to surface as much as 3 minutes before my other computer.

If the Cochran was allowing you to surface 3 minutes before your other computer then it was taking into account other factors such as water temp, real time deco processing etc. As Kim mentioned it either processed info more accurately therefore not requiring the 3 extra minutes your computer did or it accounted for deep stops you made. This can be been seen as 'being more liberal' meaning you have more bottom time or it means you did not have a deco obligation that other computers might.

As for your computer making an extra dive...all the Cochrans made the same number of dives. It just dove in my pocket when you were not using it on that first dive.

Several of made an afternoon dive on our arrival day before you got there.

Edited by pir8, 15 January 2008 - 04:46 PM.

[WreckWench]

Several of made an afternoon dive on our arrival day before you got there.

Ah yes...I forgot about!

[peterbj7]

I haven't read all this thread - no time - but the first page or two is what I expected when I saw who had started the thread! There is indeed nothing sacrosanct about NDL, nor is its value fixed in stone. It is derived from empirical data, and roughly applies to the average person with a healthy dollop of conservatism added in. Sure you can breach it, and the more intelligently you do so the more you're likely to get away with. And of course you may be a person who is far more tolerant of nitrogen than the "average" person envisaged.

But equally you may be more susceptible than the average, in which case you may need to take greater note of published NDLs. The NDL is almost the only readily available indication of safe diving practice that is derived from actual observation. You may choose to break it, but in my opinion only a fool would simply ignore it.

How you recognise it is of course up to you. You may use a computer, which of course measures depth (pressure) and time very precisely and applies its own in-built algorithm to that data, or you may choose the much coarser tool of tables derived from much the same raw data, with your own observations of depth and time. I've done both, and so long as the computer is doing what I want it to I prefer that. I'll still follow along with the tables just to be sure.

IMO, it's good that you question everything and don't follow the norm blindly. But simply denigrating what many people do and have done successfully for years, even if it is "tongue in cheek" as I suspect it often is, can nonetheless be tiresome and come over as arrogant. There many things that I (for example) often do by short cuts - eg. taking photographs with the camera set on "automatic" - when I know perfectly well how to delve into the detail. I still do delve into detail as much as is necessary, but when a shortcut produces the desired result I have no problem using it.

So I'd say that an NDL is most certainly not arbitrary, even though it may not be well defined. And although it may be bent it is very foolish to ignore it.

Edited by peterbj7, 16 January 2008 - 12:39 AM.

[PerroneFord]

I haven't read all this thread... but the first page or two is what I expected when I saw who had started the thread!

...simply denigrating what many people do... can nonetheless be tiresome and come over as arrogant.

Thank you for your thoughts. Over my time here, I've asked several members of the staff whether it was appropriate for me to initiate discussion about certain topics. Especially if they might be seen as controversial. Some of those topics have included things from computer choices, to decompression, to gas management practices, to buddy/team principles. In nearly all cases I was encouraged to talk about these things.

In each case the controversial topics drew heated discussion and of course some was directed at me. It's been some time since I initiated a thread talking about a controversial topic. But as you have observed, tolerance for it has not grown with absence.

I had no desire to denigrate the concept of NDL. I would suppose since you hadn't read all this thread or the context from which it was derived, split from another thread, that you would have missed how the conversation started. But nevertheless, it is clear that this venue is not the correct one to examine diving practices common to divers and perhaps get people to think about them.

So this will be my last foray into trying to stimulate discussion on matters related to diving. It's probably best left to discussion diving trips and matters related to single divers as the name would imply. I apologize if my topics come across as denigrating and arrogant. And I'll bid you adieu.

[shadragon]

The answer you're looking for is the 60 ft dive and that is most likely the case but is dependant upon the deco done on the 240 ft dive

If done correctly, both divers blood gasses should be more or less equal. Whatever benefits the special gas mixtures give you are offset by the severe depth so with appropriate deco there should be no real difference as both would be under the threshold for contracting DCS. That is subjective, of course.

[shadragon]

I was taught that the "surface interval" was the time spent on the surface between dives taken within a defined time, such as a day or 24 hour period.

A "Surface Interval" is the period of time starting at the end of a dive and ending at the start of the next dive. This can be minutes, hours or days... Each PADI log book has an SI block to indicate the time between dives. This value is used to determine your adjusted bottom time of any subsequent dives using the back of your PADI tables. Not sure what the other agencies use, but I suspect it is compatible.

So if you did three dives in a day such as:

Dive 1:
Time In : 1115 (11:15 AM)
Time Out: 1200 (Noon)

Dive 2:
Time In : 1400 (2 PM)
Time Out: 1500 (3 PM)

Dive 3:
Time In : 1600 (4 PM)
Time Out: 1630 (4:30 PM)

Then the SI between dive 1 and 2 would be 2 hours and the SI between dive 2 and 3 would be 1 hour. If you did a dive #4 the next day at 0830 (8:30 AM) in the morning then your SI would be 16 hours between dive 3 and 4.

The longer the SI, the more off gassing you do.

[secretsea18]

I was taught that the "surface interval" was the time spent on the surface between dives taken within a defined time, such as a day or 24 hour period.

A "Surface Interval" is the period of time starting at the end of a dive and ending at the start of the next dive. This can be minutes, hours or days... Each PADI log book has an SI block to indicate the time between dives. This value is used to determine your adjusted bottom time of any subsequent dives using the back of your PADI tables. Not sure what the other agencies use, but I suspect it is compatible.

So if you did three dives in a day such as:

Dive 1:
Time In : 1115 (11:15 AM)
Time Out: 1200 (Noon)

Dive 2:
Time In : 1400 (2 PM)
Time Out: 1500 (3 PM)

Dive 3:
Time In : 1600 (4 PM)
Time Out: 1630 (4:30 PM)

Then the SI between dive 1 and 2 would be 2 hours and the SI between dive 2 and 3 would be 1 hour. If you did a dive #4 the next day at 0830 (8:30 AM) in the morning then your SI would be 16 hours between dive 3 and 4.

The longer the SI, the more off gassing you do.

Yes Simon, I know that. However, while it is not unusual to consider 23 hourish surface intervals, it is highly unusual to consider them in terms of week long SIs as was noted in the original example from Perrone. That was the point of the question... there is no "surface interval" as used in the standard dive lingo, when the dives are really a single dive with a week between them....

Perrone, don't let anyone chase you off, your comments are welcome and DO stimulate thinking, even if there are people who disagree with you. You would disagree with my standard diving practices and routines, but that doesn't mean that we can't talk about those differences.

[ScubaDrew]

Perrone, don't let anyone chase you off, your comments are welcome and DO stimulate thinking, even if there are people who disagree with you. You would disagree with my standard diving practices and routines, but that doesn't mean that we can't talk about those differences.

Absolutely. Good conversation is hard to come by. These little back and forth discussions are great ways to expand our thinking and learn new things. I would hope you would stick it out...

[peterbj7]

On surface intervals, the PADI tables would have it that after 6 hours you're back in nitrogen equilibrium, while the rec computer I bought 5 years ago reckons that doesn't happen until 48 hours. Shows how thoughts have changed with the intervening years of research. Diving is a very new sport and it doesn't pay to have fixed ideas about anything.

Perrone, come back! I didn't mean it. These pages wouldn't be the same without your constant questioning. It's thought provoking, which I know is your intention.

Edited by peterbj7, 16 January 2008 - 08:24 AM.

[annasea]

Perrone, don't let anyone chase you off, your comments are welcome and DO stimulate thinking, even if there are people who disagree with you. You would disagree with my standard diving practices and routines, but that doesn't mean that we can't talk about those differences.

Well said, Robin!

[pir8]

Perrone, don't let anyone chase you off, your comments are welcome and DO stimulate thinking, even if there are people who disagree with you. You would disagree with my standard diving practices and routines, but that doesn't mean that we can't talk about those differences.

Well said, Robin!

I couldn't agree with this more.

[ScubaDrew]

I think that politics has shown us all that it is much easier and faster to discredit an idealogy by attacking the person behind it, rather than the ideas it is founded on. These tactics are so well known and played out in the media that it becomes almost an instictive reaction to insert an ad-hominem when you are making your case. We are all guilty of it from time to time.

So I apologize to anyone I may have attacked, in this thread or others before it. And if I haven't attacked you yet, put this in the bank for when I do...

Edited by ScubaDrew, 16 January 2008 - 09:43 AM.

[finGrabber]

I think anything that gets divers thinking about what they are doing to their bodies underwater and how their actions affect them, is good

and actions they can take to lessen the affect of nitrogen is good conversation too

[WreckWench]

Perrone, don't let anyone chase you off, your comments are welcome and DO stimulate thinking, even if there are people who disagree with you. You would disagree with my standard diving practices and routines, but that doesn't mean that we can't talk about those differences.

Well said, Robin!

I couldn't agree with this more.

Trust me...Perrone does not run easily!

[WreckWench]

On surface intervals, the PADI tables would have it that after 6 hours you're back in nitrogen equilibrium, while the rec computer I bought 5 years ago reckons that doesn't happen until 48 hours. Shows how thoughts have changed with the intervening years of research. Diving is a very new sport and it doesn't pay to have fixed ideas about anything.

Perrone, come back! I didn't mean it. These pages wouldn't be the same without your constant questioning. It's thought provoking, which I know is your intention.

I don't think you have run him off...he has succeeded in what he has hoped to acheive...very serious discussion on a very serious topic.