It's been years since I have been here! And have a day of scuba coming up.

I booked some diving in Key West. But this is booked as 2 separate
tours, 2 dives in morning (Vandenburg! so 30m max) and a reef dives in afternoon (10-15m I was told.)

They'll rent me a computer for the morning, but said I won't need on for
the afternoon.

By the book, the loss of computer is akin to forfeiting remaining diving
for 24 hours, unless you can reconstruct your PADI dive profiles and use
the good old RDP tables. (and I have seen a divemaster do that once).

As the computer will (hopefully) allow me more than 20 minutes at the Vandenburg, converting that dive profile to the PADI tables would
result in exceeding no decompression limits and I get put in penalty box
for 6 to 24 hours.


HOWEVER:
I am not in a case where the computer failed.

Is there a way to use the computer's "desaturation time" to lookup the
residual nitrogen time on the PADI tables?

For instance, if computer tells me I have "34 minutes", and I plan to
dive to 16m, I can go through table 3 and find 34 minutes, pressure
group K with 38 minutes permitted as bottom time at 16m.

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JF Mezei <jfmezei.spamnot@vaxination.ca> wrote:

It's been years since I have been here! And have a day of scuba coming up.

I booked some diving in Key West. But this is booked as 2 separate
tours, 2 dives in morning (Vandenburg! so 30m max) and a reef dives in >afternoon (10-15m I was told.)

They'll rent me a computer for the morning, but said I won't need on for
the afternoon.

By the book, the loss of computer is akin to forfeiting remaining diving
for 24 hours, unless you can reconstruct your PADI dive profiles and use
the good old RDP tables. (and I have seen a divemaster do that once). --snip--
Is there a way to use the computer's "desaturation time" to lookup the >residual nitrogen time on the PADI tables?

There's a more direct way. Almost all dive computers have a dive
planning mode. As you're pulling back to the dock, check the max
available bottom time for your scheduled dive. Then, dive it.
If you don't exceed the planned depth or time, you should be fne.
In fact, since your surface interval will be longer than at the
time you consulted the computer, you've added some extra
margin into the plan.

--
Ron

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On 2017-11-29 21:47, Ron wrote:

There's a more direct way. Almost all dive computers have a dive
planning mode.

Thanks. That is what my refresher course instructor suggested as well.
Since the dive computer can predict my max bottom time for my 3rd dive
at depth X, I can look for the Actual Bottom Time on the dive card
(back) for that depth and get the residual nitrogen and then continued
as per the good old fashioned dive tables.


Question:

Say I did a computer dive to max depth of 30m (100 feet I guess) for 30
minutes (since I wasn't at 30m the whole time) and I still have spare
minutes by the time I end dive. (aka: did not exceed no decompression
limits).

Could I correctly assume that at worse case, I did a 20 minute dive at
30m (max time for that depth) ? That puts me in P ad can continue with card.

(All of the "documented" scenarios involve loss of computer during a
dive, which means there is no assurance THAT dive was within
no-decompression limits). Am thinking more of scenarios where computer
is lost after succesful dive.

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JF Mezei <jfmezei.spamnot@vaxination.ca> wrote:

Thanks. That is what my refresher course instructor suggested as well.
Since the dive computer can predict my max bottom time for my 3rd dive
at depth X, I can look for the Actual Bottom Time on the dive card
(back) for that depth and get the residual nitrogen and then continued
as per the good old fashioned dive tables.

Not safely. You don't get a safe 4th dive. See next part.

Say I did a computer dive to max depth of 30m (100 feet I guess) for 30 >minutes (since I wasn't at 30m the whole time) and I still have spare
minutes by the time I end dive. (aka: did not exceed no decompression >limits).

Could I correctly assume that at worse case, I did a 20 minute dive at
30m (max time for that depth) ? That puts me in P ad can continue with card.

No. The computer assumes you have slow tissue and fast tissue
accumulation of inert gas. It monitors your exact depth and adds
gas accumulation to the various speed-buckets appropriately. The
tables use only your max depth to do this.

Your own tables will tell you this plan won't work. I'll illustrate
using DCIEM tables, but you can follow the same approach
using your own.

At 100 feet (30m), max no-decompression time is 15 minutes and
places you in group D. Suppose, on the same dive, you spent time at
40 feet (12m). You could go beyond your tank capacity without
needing decompression. Your computer would correctly tell you you
need no decompression. However, your group on a stand-alone dive to
40 feet could get as high as D. And this, per the tables, is without
any exposure at 100 feet. Clearly, if done on the same dive, your
group would be higher. The tables also assume at least 10 minute
surface interval between depths in order to work. So, you know your
group is higher than D, but not how much higher. Once you're beyond
the computer's plan for the next dive, you're looking at 24 hours to
degas so that the tables or a new computer can correctly handle your
gas accumulation.

Best solution for more than three dives is to rent the computer for
multiple days and dive with it every time so that it can monitor
and handle your particular gas accumulation.

--
Ron

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