One question

[Frosty]

Question for instructors.
Lets say an OW student or maybe a DM candidate was working through a question sheet from one of the Dive agencies to pass a test.
One question is using the RPD to calculate an end pressure group after some bottom time at a depth.
The calculation means the diver is into the black area on the RPD after the dive.

As a follow up question to the first.
On the basis of the above I have two questions.
1)must you stop for 3 minutes at 5.0m/15 feet at the end of that dive?
2)Allowing you DID your 3.0m safety stop must you take a surface stop of 1 hour or more if a shorter stop appears to be available as an option ?
Clearly Im stupid because its that word MUST im hooked up on.

 

[Freewillow]

The right answer is in your question. You should make a stop, but it is not mandatory in case of emergency

 

[Frosty]

The right answer is in your question. You should make a stop, but it is not mandatory in case of emergency

That's what I thought.

 

[boulderjohn]

Question for instructors.
Lets say an OW student or maybe a DM candidate was working through a question sheet
One question is a calculation to get an end pressure group after some bottom time.
You are into the black area on the RPD.
Calculation repeated to be sure.-THAT answer correct.
So then must you stop for 3 minutes at 5.0m/15 feet at the end of that dive?
Also then must you take a surface stop of 1 hour or more if a shorter stop puts you into a pressure group that offers a short /shallow dive?
Clearly Im stupid because its that word MUST im hooked up on.

I'm not sure I understand your question correctly, so please help me if my response is not appropriate.

Whenever you are in the black/gray area of the RDP the supposedly optional safety stop becomes mandatory. I am pretty sure I understood that part.

It is the second question I am fuzzy on. From your wording, it seems to me as if you are questioning a rule about a mandatory one hour surface interval following a dive in which you did a safety stop that was less than the required 3 minutes. I am not aware of such a rule. The only rule I know about requiring a minimum one hour surface interval comes if you are doing 3 or more dives in a day and you end one of them in either the W or X pressure group.

 

[Frosty]

sorry John I rephrased the questions to make em a bit vague. -Ive edited the OP again now
Im hopeing that instructors will realise the question I mean and correct my thinking.
Its just bugging me

 

[boulderjohn]

OK, new answers:

1) Yes

2) You must do whatever surface interval is required for the next dive. If you did a 130 foot dive for 10 minutes, you would be in the black and have a required 3 minute stop, which is also required because it is beyond 100 feet. You will finish that dive in the H group. If you do a 38 minute surface interval, you will be in the C group. If you then do a dive to 35 feet, you can stay there for 180 minutes, which is a fairly substantial dive. As a C diver, you can in fact go back down to 130 feet, but only for 4 minutes. If you didn't want to wait 38 minutes and were able to switch tanks in only 6 minutes, you would be in the G group and only be allowed to stay at 35 feet for 165 minutes.

The only time you are required to do a one hour surface interval is if you are doing at least 3 dives and end one of them in the W or X pressure groups. That is because the research that led to the RDP (not RPD) led to the conclusion that the 60 minute compartment was the proper one to use for the kind of dives that fall within the range of almost all dives within the limits of the RDP, but dives at that extreme would involve slower compartments and thus call for longer surface intervals.

 

[Frosty]

John--Thank you-2 with your explaination it becomes obvious.
But still 1- why?
The word I'm hooked up on is MUST. If the phrase was strongly recommended or should I have no issue. Its MUST
The implication I'm reading is that the safety stop has become a mandatory deco stop.


For anyone reading this -NO Im not suggesting/implying that in real life I'd recommend blowing a safety stop ever except in an emergency.

 

[boulderjohn]

The word I'm hooked up on is MUST. If the phrase was strongly recommended or should I have no issue. Its MUST
The implication I'm reading is that the safety stop has become a mandatory deco stop.

That is pretty much correct. I assume that they just did not want to use that language. It really is a language problem. They are really saying something like, "OK, it is not technically a required decompression stop, but we really, really, really think you should do it, so we are telling you that you must do it."

That research was done more than 30 years ago. Back then the standard ascent rate was 60 FPM, so that is the ascent rate they used for their research. Every number on that RDP is based on the assumption that you will ascend at 60 FPM. If you ascended at that rate, a safety stop was really not needed, but they put them in there to deal with the problem of some people ascending too quickly. Today we believe 30 FPM is better, but the RDP research conclusions were based on 60 FPM ascent rate, so they really can't change them without redoing all that research. The other important change since then is that research on safety stops has shown that they provide more benefit than was originally believed. Consequently, they are emphasized much more than before. If you read the PADI course that teaches computers, you will see that it tells students to use whatever ascent rate their computer advocates (usually 30 FPM), and it tells them to do safety stops on pretty much all their dives.

 

[lessthanjoey]

Keep in mind that the PADI tables are fairly aggressive though. If I run to the "black" on air @ 100 ft (20 minutes), then multideco using Bulhmann 30/85 and 60fpm ascent gives me the following deco schedule:
40' 1min
30' 1min
20' 1min
10' 3min

If I use Buhlman 100/100, it gives me 1:30 at 10'.

So, if you skip the safety stop on a 100'/20min dive on air and do a 60fpm ascent, you're above >100GF upon surfacing. Bad news.

 

[boulderjohn]

Keep in mind that the PADI tables are fairly aggressive though. If I run to the "black" on air @ 100 ft (20 minutes), then multideco using Bulhmann 30/85 and 60fpm ascent gives me the following deco schedule:
40' 1min
30' 1min
20' 1min
10' 3min

If I use Buhlman 100/100, it gives me 1:30 at 10'.

So, if you skip the safety stop on a 100'/20min dive on air and do a 60fpm ascent, you're above >100GF upon surfacing. Bad news.

So, what does multideco give you for the Navy table NDL limit of 25 minutes for a dive to 100 feet?