Question about the PADI Dive Table

[-hh]

Keep in mind the history of this... In the "good ole days" bottom time started at descent, and ended when your head hit the top... Then guys would look at their tables and watches and say "Gee... Were at 100 feet, and we have to be back at the surface in 1 minute... Let's hurry up!

The USN Tables from 20+ years ago stated BT as being from the surface to depth, and ended when you started your ascent back up...NOT the surfacing time.

They also included a miscellaneous provision that a slow ascent would have all additional ascent time spent deeper than 50fsw added to the bottom time. In those days, the perscribed ascent rate was 60ft/min.

(Edit: An example: dive to 110fsw for 10minutes BT, followed by a slow ascent at 30ft/minute from 110fsw to 15ft (safety stop). The ascent time from 110->50 was 2 minutes actual, but should have only been 1 minute, so BT is increased from 10 to 11 minutes. The additional slow ascent time from 50->15 is neither debted nor credited.)

Other tables have used the surface-to-surface definition, presumably in an attempt to add a little more conservatism to the same basic table model. But they can encourage "rushing up" as described here.

The rule here is that every individual Dive Table has to be specifically researched for what its particular definitions are, and its instructions for use, because they can -- and do -- differ.

More people get bent within table limits that outside table limits.

IMO, this is a self-fulfilling statistics because most people dive profiles that are within the Limits...same principle behind the usless statistic that most accidents occur within 5 miles of home.

-hh

 

[MikeFerrara]

And they assume safty on their side of the NDL and don't bother to learn or practice good ascent stratagies or monitor behavior before and after the dive. What does DAN say that the average ascent rate from the safety stop to the surface? I don't remember but from what I see it's pretty fast and that can do it.

Also many are on vacation so there may be issues with rest, hydration, diet and exertion.

 

[Dr Deco]

Hello Scuba Board Readers:

Tables, NDLs, and DCS

Readers will know that this is my favorite topic. Up until the NASA studies on DCS in null gravity, it was thought that there was just one “set” of no decompression limits and adding work to the end phase would reduce these NDLs. Today, some of us recognize that the NDLs are dependent on the post-dive circumstances. If the test subjects sit still, the bottom time will be longer (greater NDLs) than if the subjects are walking around. NDLs are reduced more if the subjects then perform some physical activity (such as lifting or climbing).

Testing

The interesting point about testing is that it essentially involves dive subjects who were sitting down following the exposure. Basically, that is the only thing that they have to do. No table is tested by requiring the subjects to carry heavy objects. It is known that this would lead to DCS and result in shorted NDLs. Should anyone wish to make a table this way, their table would be very conservative in comparison to others. a diver looking at such a table would think, “ Who wants to shorten the dive so that they can carry heavy tanks?” They think, “I simply won’t do that.”

Quite logical until you then consider that divers are lugging equipment during the surface interval and using a table where the subjects rested between dives. This is true also of the PADI/DSAT tables since the effect of walking (gravity) was not known at that time. The “smart money” simply does not use the surface interval to accomplish all kinds of physical activity. Sure, some things need to be done but try to avoid straining and keep it down to a minimum.

Otherwise we will continue to see divers with DCS who were still within the table limits. :sprite10:

Dr Deco :doctor:

 

[fins wake]

More people get bent within table limits that outside table limits. So don't hurry the ascent... you can make a nice slow ascent with multiple safety stops - and it is doing more help than harm.

I agree on the nice and slow ascents and multiple 'safety stops' (good multilevel profile, preferably with deeper stops not too deep for ordinary recreational divers for ongassing purposes). You might even be right about being "bent within table limits". The only thing to remember here is the amazing number of 'lost' deco or depth meters when victims arrive at the pot (fact!) and the amazing number of people who claim to have been within limits without offering any proof thereof ...

I know of cases where attendants have been able to sneak off the victim's computer and running the profile on desktop software. How many here think that the original statements from the victim - about being within limits - were truthful? :winky: :54:

 

[hockeyplay]

While It's stupid to lie to your hyperbaric doc about where you have been and for how long, with the amount of macho crap that floats (pun intended) around the diving community I can see people doing it to save face when they F'ed up.

I don't mind being tough about most things, but bubbles in my blood reeking havoc on my nervous and musculo-skeletal systems doesn't happen to be one of them.

 

[fins wake]

While It's stupid to lie to your hyperbaric doc about where you have been and for how long, with the amount of macho crap that floats (pun intended) around the diving community I can see people doing it to save face when they F'ed up.

Precisely! I share your sentiments about being upfront with your hyperbaric chamber staff. This is not a good time to lie.

 

[DepartureDiver]

I think it´s important for new divers that when someone talks about deep stops or veryslow ascent rates, it has to be carefully performed so you don´t start ONGASING WHILE ASCENDING

For table dives (computers take care of themselves), any on-gassing from slow ascents is so negligible, it won't really bring the compartment much closer to it's allowable tension, but the benefit from the slow ascent and the off-gassing of "faster compartments" is of incredible benefit.

 

[iainwilliams]

For table dives (computers take care of themselves), any on-gassing from slow ascents is so negligible, it won't really bring the compartment much closer to it's allowable tension, but the benefit from the slow ascent and the off-gassing of "faster compartments" is of incredible benefit.

I agree with departure diver (Brian). However, there has to be some rational thought process to the ascent. It has to be an ascent, not just a rise of so many feet before looking around some more, then another rise -this is mulitlivel diving - which is safter than square table diving.

With regard to activity during surface intervals. We NEVER climb onto the boat with tanks on. We always clip them to a shot line at the end of the boat for the surface girl to pull up. We ALWAYS drink 2 cups of water before decending and three cups when back on the boat. Divers are BANNED from lifting the anchor - this is the surface girl's job. These are just a few of the things we do to minimise DCS risk......Iain

 

[DepartureDiver]

With regard to activity during surface intervals. We NEVER climb onto the boat with tanks on. We always clip them to a shot line at the end of the boat for the surface girl to pull up. We ALWAYS drink 2 cups of water before decending and three cups when back on the boat. Divers are BANNED from lifting the anchor - this is the surface girl's job. These are just a few of the things we do to minimise DCS risk......Iain

Good for you. Hopefully others do the same .. or will start. We have been during altitude deco dives to 400+ feet here (okay ... not within the RDP's range) and we always let our support team do all post dive activity for us ... pull all tanks in, carry them to the car, etc. It's simply not worth the risk. It doesn't take much of a shearing force to make bubbling worse.

Brian

 

[iainwilliams]

Good for you. Hopefully others do the same .. or will start. We have been during altitude deco dives to 400+ feet here (okay ... not within the RDP's range) and we always let our support team do all post dive activity for us ... pull all tanks in, carry them to the car, etc. It's simply not worth the risk. It doesn't take much of a shearing force to make bubbling worse.

Brian

Yes Brian so true. I'm amazing at what some people do and actually get away with it. I beleive that all the DCS risk minimalisation standards should be taught on all courses. Unfortunately this is far from what actually happens - but I guess those keen enough will teach themselves!Enjoy..........Iain