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

Inspired by a conversation here http://www.scubaboard.com/forums/ba...9-overheard-interesting-conversation-lds.html

Many times we read a post where a member says something to the effect of, "I have a computer but I also use the RDP tables as backup."

Computers have replaced the RDP (tables) and are not a back up to the computer, but the computer can be a backup to the RDPs.

In a post here I show two different dives.

Both dives exceed the RDP's no-decompression limits and would indicate that diving is now done for the day. However, if my computer failed during either dive, my diving would also be done for the day, RDP or not.

The point being that after-the-fact dive planing is not dive planing, it is simply reacting to the situation.

If you plan your dive using the RDP as your primary source for limiting depth and duration then your computer is the backup should you exceed the limits set forth in your pre-dive plan.

I was kind of thinking along those lines myself. It would seem to be more conservative to plan with the tables and dive watch and have the computer along as well--especially in case you are ascending too fast. The computer "rounds off" everything, unlike the RDP, so one might figure that if you go by your computer for the first dive and it bums out, your use of the tables for the second dive is based on the rounded off computer figures, rather than the RDP. A possible exception may be serious multi level dives. If the computer fails after the first it probably is best to cancel the second.

Teamcasa:

In a post here I show two different dives.

Both dives exceed the RDP's no-decompression limits and would indicate that diving is now done for the day.

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I'm not sure about that.

Bear with me since it's been a while since I've done anything with this table, but doesn't the RDP define "Actual Bottom Time" as from when you leave the surface to when you begin your ascent?

To me, the first profile looks like you began your ascent at about 6 minutes (from then on, you went up pretty much at a linear rate + the stop at 20 feet.

100 for 6 is well within NDL.

The second profile looks like you began your ascent at about 25 minutes.

(not sure if these dives were consecutive or not, but assuming at least 48 minutes of SIT)

80 for 25 is within NDL.

Blackwood:

I'm not sure about that.

Bear with me since it's been a while since I've done anything with this table, but doesn't the RDP define "Actual Bottom Time" as from when you leave the surface to when you begin your ascent?

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

Moral of the story-run 2 computers if possible?

Teamcasa:

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

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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.

jmorriss:

Moral of the story-run 2 computers if possible?

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Other possible morals of the story:

• develop an understanding of decompression theory adequate to allow you to use tables and computers interchangeably (may require a paradigm shift regarding table methodology)
• plan to sit out for 24 hours (or whatever your table of choice says) if your computer dies

Blackwood:

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.

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The PADI tables end the dive when the diver begins a DIRECT ascent to the surface. Goofing around at 60' on the way up from a 100' dive is NOT a direct ascent, it's a multi-level dive.

Richard

I agree with you in theory - the computer always makes a better back up to a table than vice versa - and encourages much better gas planning.

On the other hand, back in the day, shortly after the PADI Wheel was produced, some sharp peopel figured out how to use the RDP in the wame manner to plan multilevel dives using the same shorter NDL's and rules regarding minimum depth changes as the Wheel - and that still work with the RDP.

So in many cases with a bit of pre-planning of your profile to be multi-level table compliant, you could figure multi-level dive profiles and not be out of business.

You could also use a set of much less conservative navy tables and provided you made adequate safety stops you may find in most cases that you met the deco stops required even with the assumed square bottom time profile. Knowing for example prior to the dive that 30 minutes at 100' requires a 5 minute safety stop (I am pulling this out of my A$$ I am not sure off hand what stop is required as I don't have a table handy), you just ensure you make a 5 minute "safety" stop. Thenif you default to tables at some point, you have it covered.

You can also buy a cheap back up computer off e-bay and just keep it in a pocket. As long as it is more liberal than your primary, you can ignore it until you need it.

But personally, I like to do the dive plan first and get to know the expected deco schedule and gas requirements, then dive the computer as essentailly a backup.

rstofer:

The PADI tables end the dive when the diver begins a DIRECT ascent to the surface. Goofing around at 60' on the way up from a 100' dive is NOT a direct ascent, it's a multi-level dive.

Richard

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Personally, I wouldn't consider that little flattening of the curve at 70 feet to be a 'level'. The general trend of the right hand side of his profile from maximum depth on up is linear.

But hey, that time at 70' is about 13% of the total ascent time, so say the ascent started at 15 minutes. 100 for 15 doesn't exceed table limits either (based on my aforementioned understanding of PADI/DSAT bottom time).