PADI RDP question

[hdtran]

Tim,

Yup, the Pentapus is the Aquaholic tutor. Shoot Larry an e-mail, and he'll set you up.

No purchase necessary. (Although I did buy stuff )

 

[scubatoys]

Hey hdtran, have you confirmed that? I heard a rumor about it several weeks ago on another thread, but no one could confirm it with Larry...

I

Not a rumor... it's the truth. Anyone on the board can download my table tutor, then just go to buy it on our site... and in the notes section on check out write "Larry was dumb enough to offer this software for free as I'm a scubaboard member." Or something to that effect.... And we'll shoot you the unlock code for no charge.

Or heck... you could just PM me here. Either way.

 

[Marek K]

Boy, we're getting all sorts of different answers to Ross567's original question.

I assume Ross567 is doing Question 7 on page 250 of the OWD manual.

As someone pointed out, there is a slight difference with this particular problem because of various rounding, depending on whether you're using imperial or metric tables:

At the end of the first dive, with imperial tables, you've going to be in Group N. According to metric tables, you're going to be in Group M.

To do the second dive, with imperial tables, you have to be in Group M or better after the SI. With metric, you have to be in Group P or better -- which you are already within.

So with imperial, you'd need an SI of 0:04 to 0:08. And with metric, you wouldn't need any surface interval at all.

And since Ross567 is using metric tables, the latter would be "the correct" answer for him.

This is an unusual problem, because I'd noticed that most of the PADI exercise and test questions come out with the same answers, whether you're using imperial or metric tables.

For two separate dives using these particular profiles, it's a moot point; you'd be out of the water anyway for the worst-case 0:08, while changing tanks etc. But would this rounding difference be significant for other profile combinations, or if doing a multi-level dive?

--Marek

 

[rakay]

OK, first contact - lots to see here - I'm a novice, 3rd week of PADI OWD course, with aims of warm water diving rather than diving in UK .... I've a query on the RDP from the quiz - 'What is the minimum surface interval required betwen a dive to 20metres/ 70 feet for 29 minutes followed by a dive to 14 metres/ 50 feet for 39 minutes?' - This doesn't compute on the RDP so I am guessing that the safe option is at least 3hours - but is there a way to compute this? I'll tell you more about me on next visit .... Ross567

I got the same answer. Minimum of 4 minute surface interval is needed.

 

[GrierHPharmD]

Sorry, Marek is right above. I had written the profile on a notepad and thought the first dive was to 79 ft (couldn't read my own handwriting!), so I looked at the 80 ft column (Imperial) on dive 1. For 70 feet, the ending pressure group should have been N. Thus, when you make N intersect with M, the surface interval should have been 0:04 to 0:08.

I didn't work it using the metric version, so I can't speak to the rounding situation.

Thanks for the correction.

-Grier

 

[WileEDiver]

Yes, for metric, the answer is 0 minutes.

 

[David P]

hey Ill throw my two cents in... I tained with NAUI instead of Padi and the difference here surprises me! Ive heard that NAUI is more conservative with their SIT, but wow! After a 21m/70' dive for 30 minutes, and before a 15m 50' dive for 40 mintes you need a minimum of 46 minutes sittin on the deck soakin up the rays drinking water. With a SIT of 10 to 45 minutes or less the second dive would be limited to 33 mintes

 

[Marek K]

And yet another Agency-based variant...

Our daughter is doing an SSI OWD course... the first of four family members not doing it with PADI. So I'm checking out her SSI tables...

The pressure groups, surprisingly, are much, much different... A through K, but there's no similarity to PADI's pressure groups. Besides that, I'd already noticed that SSI's tables are significantly more conservative than PADI's.

OK, as I'm reading these SSI tables, the first of Ross567's dives would result in SSI Pressure Group F.

In order to do the second dive, you'd have to be SSI Group D.

Which would require a surface interval of 1:30 to 2:28!!! That's hours!

I see that the SSI tables are labelled "DOPPLER NO-DECOMPRESSION LIMITS BASED ON U.S. NAVY DIVE TABLES." But I understood the old 1980s PADI Dive Tables were based on U.S. Navy tables too... and those PADI Dive Tables were more liberal than the current PADI RDT! God... It's a wonder we didn't get killed several times over...

This can really open up several cans of worms... conservatism of various agencies' tables... folks assuming that pressure groups mean the same thing between agencies... potential of error when using tables... dare I mention the advantages of a computer?

--Marek

 

[kidspot]

I've got six tables in front of me (some from the table tutor program, 3 on cards)... 1st dive 70ft for 30min 2nd dive 50ft for 40min...

PADI - requires 13-17min. SI
NAUI - requires :46-1:29 SI
SSI - 1:30-2:28 SI
old SSI - 10-45min. SI
Navy - 10-45min SI
GUE - 1:30 SI (same as all surface intervals)

2 of the tables (Navy and the one above it) put you in the same letter group ending and beginning...

btw - Ross567's original question the dives were only 29min and 39min. that brings things 1 pressure group closer on either end on the PADI RDP only requiring a SI of 4-8 min. All others remained the same.

I'm assuming it's because PADI designed tables specifically for NDL and all the others are roughly based on Navy tables which were designed around the idea of the previous dive being a deco dive (not that it always was, just the conservatism was pre-built in) Also PADI used more pressure groups to give additional credit at the lower end of the scale. If I remember correctly it's because they only used a maximum 1/2 life tissue compartment of 60 min. whereas the old navy tables were based on a maximum 1/2 life tissue compartment of 120 min. - due to decompression... which the PADI RDP does not allow.

You'll also notice that initial BT's on their tables are different...

D-fsw..PADI..NAUI..Navy..SSI...GUE
40......140....130...200...130....170
50.......80.....80....100....70.....60
60.......55.....55.....60.....50.....50
70.......40.....45.....50.....40.....35
80.......30.....35.....40.....30.....30
90.......25.....25.....30.....25.....25
100......20.....22.....25.....20.....20
110......16.....15.....20.....15.....15
120......13.....12.....15.....10.....10
130......10.....8.......10......5.....n/a


Tim

P.S. someone correct me if I'm wrong...

 

[Marek K]

1st dive 70ft for 30min 2nd dive 50ft for 40min... [...]

Navy - 10-45min SI
[...]

D-fsw..PADI..NAUI..Navy..SSI...GUE
40......140....130...200...130....170
50.......80.....80....100....70.....60
60.......55.....55.....60.....50.....50
70.......40.....45.....50.....40.....35
80.......30.....35.....40.....30.....30
90.......25.....25.....30.....25.....25
100......20.....22.....25.....20.....20
110......16.....15.....20.....15.....15
120......13.....12.....15.....10.....10
130......10.....8.......10......5.....n/a
[...]
P.S. someone correct me if I'm wrong...

Hey Tim--

I always understood that my old 1980s PADI Dive Tables were in fact the U.S. Navy tables... and that's supported by the fact that the max ND bottom times you listed for Navy are identical with those on my old PADI tables. (Somebody remind me not to ever mistakenly dive with these tables!!)

But my calculations for the two-dive profile you gave, using my old PADI tables, don't agree with your Navy figures:

First dive, 70' for 30 min gives me Pressure Group F. The second dive, 50' for 40 min (rounding up to the 44-min window) would require at least a beginning Group G... therefore no SI required. Check your Navy tables again... are they different in that way, or which one of us is making a mistake?

--Marek