"no fly" time

[Bismark]

If you read the large print in the owners manual you will find some pretty "cover your ass" statements by the manufacturer which basically states that you follow the computer at your own risk. I believe any computer manufacturer will state the same thing. Remember that the computer is creating working within a mathematical model and just using your input to adjust the model as you dive. (See the various posts above) No computer model is 100% safe for the simple reason that their are simply too many dynamic variables coupled with the fact that decompression science is not completely understood. Like Walter said, the only way to guarantee not to get bent is don't dive or don't come up. However, that being said, what most people do not understand is that DCS is not a simple "black and white" illness. It is a sliding scale of gray from sub-clinical symptoms all the way to the big one. And soooo.. much is dependent on individual physiology. We simply don't know everything about it. Computer software is intended to offer (in most cases) a degree of conservatism that is acceptable for the "average" diver. However, the best thing you can do as a diver is spend some time inserting your brain into the feedback loop and understanding what is happening and why to the best of your ability. This will help you understand what your computer is telling you and let you adjust your own diving habits even within the framework of what your computer is telling you. At the very least, learn what Deep stops are and incorporate them into your diving. Also, take great care with your ascent profile, including the last stages of your ascent, where many people simply feel the dive is over. Ask yourself what the relative change in ATA's is in the last 15 ft of your dive and understand why this last bit is important.

Above all else, if you are going to dive, you owe it to yourself to understand what is happening. Simply trusting a piece of software put into a mechanical instrument assembled by the lowest bidder with your life might not be the wisest choice. Computers are tools and should not be used as a substitute for your brain.

Best,

 

[Bill_PNW_Diver]

Greetings.

I'll be in the Hawaiian archipelago for 2 weeks in April. The trip will start on Oahu, them jump to Molokai, and then the Big Island. My question concerns the "no fly" period of 24 hours after a multi-day dive. What altitude is considered a "flight"?

The island hopper planes in Hawaii only travel between 60-120 miles. So it would seem to this casual observer that they don't obtain a very high flight altitude. My hopes, therefore, would be that I would not need to place much concern with these flights that are 18 hours after my last multi-day dive.

Is there an official altitude and/or duration that define a "flight"? Should I be concerned about these flights after my multi-day dive plan

So here's a surefire way to travel between the islands and dive when you want with no risk decompressions sickness....

Hawaii Superferry A Whole New Way To Get Around The Islands

- Nice boat ride
- $39 fares

 

[Damselfish]

So here's a surefire way to travel between the islands and dive when you want with no risk decompressions sickness....

Hawaii Superferry A Whole New Way To Get Around The Islands

- Nice boat ride
- $39 fares

well, except that it only goes to 1 of the OPs 3 islands and hasn't been able to run reliably since it started.
http://www.scubaboard.com/forums/hawai-i-o-hana/224327-super-ferry.html

 

[Thalassamania]

So here's a surefire way to travel between the islands and dive when you want with no risk decompressions sickness....

Hawaii Superferry A Whole New Way To Get Around The Islands

- Nice boat ride
- $39 fares

Not running at the moment.

Updated: March 8, 1:00 PM
We are extending the amount of time the Alakai will be in drydock because work to repair damage to the ship that occurred during the drydocking process is going to take longer than was previously projected.

Based on the latest repair schedule, reservations for sailings after April 22 are now being accepted. Updates on the sailing schedule will be issued as they become available.

Passengers holding reservations for affected voyages are being notified, re-accommodated on a future voyage or refunded.
We appreciate your patience and understanding while our vessel is out of service. We are very sorry for the inconvenience this cancellation has caused.

 

[Charlie99]

When I use my Computer as the yardstick for my bottom times, it gives me credit for an ascending profile, this technically violates my NDL Table limits. The reason that this is OK, is because my Comp is using a safe algorithm .... yes?

By the same logic, is it acceptable to follow the "No Fly" recommendation of my comp after single or multiple dives, even if they are lower than the DAN and/or PADI recommendations?

As others have noted, there is no "safe" algorithm, table limits, or no fly calculations. It is a sliding scale of relative risk.

Your computer uses the same model and to calculate the the no fly time as it does to figure out mandatory decompression stop times and NDL times. You will have roughly the same risk flying right at the minimum no fly time as the risk would have by surfacing right at NDL or right at minimum deco time.

=================================

Some background on models and how no-fly time is calculated to explain the basis behind my statement above ......

A dissolved gas decompression model (aka Haldanian, neo-Haldanian) such as the popular Buhlmann ZHL-16 model has limits for each compartment as to the maximum allowable dissolved gas tension one is allowed to have upon ascending to sea level after a dive. Most models (PADI/DSAT being one notable exception) also have "slope" information, which give the allowable increase in the tensions in each compartment as you go deeper. These sea level limits are used to calculate NDLs. The sea level limits, combined with the slope (or change of limits with depth) are used by the computer to calculated deco stop times in decompression diving.

After a deep dive, your computer may initially say that the shallowest allowable depth (ceiling depth) is 20'. Then after you have spent enough time relatively shallow, but deeper than 20', your computer will clear you to go up to 10'; and then later clear you move up to the surface.

The no-fly time is calculated exactly the same way as the length of a 10' or 20' decompression stops. The pressure decrease at 8,000' altitude (about 7fsw less than sea level) is multiplied by the slope factor and subtracted from the sea level limits to get the 8,000' altitude limits for dissolved gas tensions in each compartment. The computer than calculates how long it will take to offgas to those limits.

When you see how the no fly time calculation is related to the deco stop time calculations, it should be evident why I say that flying at min no-fly time poses approximately the same risk as surfacing at 0 NDL time, or doing a decompression dive with the minimum allowable deco stop time.

These same type of calculations are the basis of the NOAA Ascent to Altitude Tables.http://www.ndc.noaa.gov/pdfs/AscentToAltitudeTable.pdf. The repetitive letter groups are per the NOAA Air Tables. http://www.ndc.noaa.gov/pdfs/NoDecoAirTable.pdf

Charlie Allen

 

[rjack321]

nevermind

 

[Thalassamania]

The Navy tables are crap. If I did 60ft/min Navy ascents on air I'd be bent like a pretzel after just about any dive - no flying required. If you treat your recreational dives like deco dives you can fly just about as fast as you can get through TSA. I am a slow offgasser due to various scar tissues, no I don't know what stupid letter group those tissues are, but I do know proper decompression from all dives is the key to not bending yourself on airplanes reagrdless of how long you chose to wait.

While it is possible that you are out one extreme or another (and I'd be curious to know what data you have to indicate that), describing the U.S. Navy tables as "crap" and 60 ft/min as a problem does great violence to eternal truth that millions of divers, in and out of the service used them with great safety and continue to use them with great safety. That's an insult to those who developed the U.S. Navy tables.

 

[limeyx]

Ditto. Although you may not want me in any studies I know I'm a crappy off-gasser!

Oh, I dunno -- you seem to "offgas" on the internet pretty well (yes yes I know, pot, kettle, dark color blah blah blah)

 

[DocVikingo]

By normally you mean 8,000 feet?

Best as I know, that's the normal commercial aircraft cabin pressurization world-wide.

Regards,

DocVikingo

 

[Riger]

Wow !!

Lots of really good feedback and food for thought.

Thank you all for your commentary on my questions.

Best Regards
Richard (Riger)