RDP's can not be the backup to a computer.

[Blackwood]

Nope, the tables are designed (for safety) to include total dive time and maximum depth for the planing.

FYI, I don't believe I own an RDP any longer (at least, I wouldn't know where to look for one), let alone the instruction manual, but this thread appears to confirm my recollection.

It is however beside the point since you can easily run a profile that will in fact break the square table limits.

 

[Steve50]

Weird. I know they work of max depth (i.e. "square profile"), but I would have sworn that bottom time ends when the ascent begins.

PADI tables = yes

 

[Steve50]

If I know my equivalent letter group from the RDP when I begin my computer dive, and the max depth on the dive that my computer failed on, and if using that max depth and the elasped time doesn't blow me out of the water tables-wise, I continue diving tables.

 

[Diver0001]

Weird. I know they work of max depth (i.e. "square profile"), but I would have sworn that bottom time ends when the ascent begins.

I think you're right.

I'm pretty sure that the BT is defined from the moment the descent begins to the moment the diver begins an uninterrupted ascent to the surface, not including time spent at the safety stop. That's the reason that the 5pts descnet includes setting your watch and the 5pts ascent includes the step of looking at the time.... it's because that's the end of your 'bottom time' assuming you intend to make an uninterrupted ascent.

It's practically never taught like this though and for good reason. Given how most people tend to dive (ie. what someone *thinks* was an unterrupted ascent often isn't) I suspect that a lot of instructors tell their students that their dive time should be the same as their total dive time not including the safety stop.

An easy rule of thumb for people to remember is that BT ends when SS starts, even if it adds a few extra minutes to your BT.

R..

 

[gcbryan]

I'm not sure of the details in the original scenario...two dives both exceeding the RDP. So they had a deco obligation...did they fulfill the obligation? If so, why is diving done for the day. If not then of course it's done.

Regarding a computer crapping out and diving being done for the day...only if that was also your only time and depth information.

I agree that the RDP isn't really a backup to a computer and that a computer could backup the RDP but to me both (tables and computer) should really just be a backup to what's in your head.

I agree that it's not dive planning if it's not done prior to the dive. I would think that regardless of RDP or computer usage that one would run over the likely dive scenarios in their head before the dive with certain dive perimeters in mind...max dept, approx time, approx profile.

If a computer fails you shouldn't now be wondering can I stay here for another 60 minutes or another 10 minutes. To me you would just be more conservative and not try to get every last minute out of your remaining dives.

Two hour surface interval, reduce max depth and times, and keep on going (if you have time and depth info). When in doubt just do 30 foot dives.

I don't get people talking about using tables as backup either but the real issue to me is someone feeling that they are being conservative lugging the tables around when in most cases they can't be used properly after using a computer and more importantly it sounds like they are being totally reliant on either tables or computers to give them any info...as in no idea what would be correct until it's read from a table or viewed on a computer screen.

If you know time and depth then a conservative educated guess should keep you in the water if you want to be there. If you don't think you can make an educated guess practice with your computer while it is working and see if your guesses would have worked. There are other ways to get this skill or info of course.

 

[Blackwood]

I'm not sure of the details in the original scenario...two dives both exceeding the RDP. So they had a deco obligation...did they fulfill the obligation? If so, why is diving done for the day.

RDP (I believe) refers to a specific table, the one published by PADI/DSAT.

On the back, it tells you how to handle dives past NDL.

If a no decompression limit is exceeded by no more than 5 minutes, an 8 minute decompression stop at 15ft is mandatory. Upon surfacing, the diver must remain out of the water for at least 6 hours prior to making another dive. If a no decompression limit is exceeded by more than 5 minutes, a 15ft decompression stop of no less than 15 minutes is urged (air supply permitting). Upon surfacing, the diver must remain out of the water for at least 24 hours prior to making another dive.

So I guess if you plan to use the RDP to handle a broken computer (implicitly you don't depth average), if you come up from a dive to say 100 for 26 minutes, the table instructions end your diving for the day (and then some).

Yes, deco isn't black and white and the arbitrary decompression stops are patently ridiculous, as is the increase from a 6 hour surface interval to a 24 hour surface interval because of one extra minute of bottom time on a 130 foot table.

Personally, I believe that tables and computers can be used interchangeably, but it takes a paradigm shift in how the tables are used, and it takes a level of awareness (regarding profile) that many divers simply don't exhibit. Using The RDP as it instructs to cover dives that exceed its limits is absurd at the very least.

 

[Mr Carcharodon]

Moral of the story-run 2 computers if possible?

A several other conclusions could be drawn:

It would be a better idea to have a plan on what you are going to do if you run over your NDL whether that value comes from a computer, or table or whatever (seems like a better option that ending your diving day to me).

One way to build conservatism is to do stops on the way to the surface (pretty clear from the two examples given). Of course if you can not reliably do stops or know how long or where to do them this might not be a good plan. Still the examples show safety stops are a good start.

And that using max depth to calculate NDLs for the two example profiles is wrong.