OW RDP Confusion!

[knotical]

Partir, as some have already said, you are correct.

The imperial version of the question uses 70 and 50 feet instead of 20 and 14 meters. The imperial question produces a minimum surface interval of 4 minutes. The metric version (probably derived from the imperial version) is indeed zero minutes as you calculated.

I think it’s a good question because it often gets students thinking, as has happened in this case.

(And I re-confirmed the answers by checking my Open Water Diver Course Knowledge Review Answer Keys.)

 

[jhelmuth]

...
What is the minimum surface interval required between a dive to 20 metres for 29 minutes, followed by a dive to 14 metres for 39 minutes?

...
3) Therefore, this suggests that I would have an interval time of zero if I wanted, as I do not have to wait for the pressure group to lower from M to P. If I'm right, I assume this is a theoretical time only, and some form of minimum time should apply?

...

sorry. didn't read all responses but here is my take. i am an ssi diver but have the padi rdp in imperial

20mtrs = 66 feet, round up to 70 feet, 70 feet for 29 minutes = "N", need to be "M" to then dive 14mtrs (46 feet rounded to 50) for 39 minutes, therefore a 4 to 8 min SI would be required.

two things.
1st - with SSI we are required to take a minimum 10 min SI or the second dive is considered to be part of the first ie: it becomes one continuous dive. so i think this example is not a good one.
2nd - if you were planing a dive for 39 min you would never do it if it was the max time allowed. you would take a longer SI to give you more time as a safety margin. so in this example i would take "at least" a 14 min SI to be "K" which gives me a 44 min ndl (5min safety margin)

just for kicks i looked at the ssi rdp and here is how it works out using them
20mtrs (round up to 21mtrs) for 29min = F, to go to 14mtrs (round up to 15mtrs) for 39 min you must be "D", therefore you would have to take a 90min SI. and this would only give you a 2min safety margin (41 min ndl). so in the real world you would need a 150min SI giving you a 49min ndl (10min safety margin).

wow.thats a huge difference to the padi tables

Rick,

Yes... AFAIK, ALL agencies use the 10 min. minimum rule. I do believe most (if not all) of us were taking that as a given - esp. since the OP also cited the minimum would apply.


WRT... Imperial conversion. While practical to do so (mathematically anyway), this is a fine example of why one should not make such conversions. A step function (which is the main construct for the PADI, NAUI, SSI, etc. dive tables) is NEVER considered accurate when converting units (albeit there is always a possibility that it will work, it should never be relied upon). Since the problem was stated in Meters, the problem should be worked in meters using a metric table.


Lastly... Obviously PADI's step functions are more atomic than say NAUI (and I dare say SSI too). I can't say PADI is wrong, but one cannot help but think that the end game for PADI was with accuracy, not safety for the OW diver. That is NOT to say PADI air tables are not safe, but they would require a greater degree of safety margin (as evident from NAUI/SSI contra examples given). You may decide what your own opinion is of the PADI "RDP", but I personally would have a slight problem in teaching them (and maybe that is why I'm not a big fan of PADI - among other reasons)...

 

[EFX]

.... The imperial version of the question uses 70 and 50 feet instead of 20 and 14 meters. The imperial question produces a minimum surface interval of 4 minutes. The metric version (probably derived from the imperial version) is indeed zero minutes as you calculated.

I think it’s a good question because it often gets students thinking, as has happened in this case. ....

I think it's a good question if you already know how to work exactly this kind of problem (see my procedure as an example). In this case it created more confusion (the OP) than thinking. Working a problem that requires a change in groups provides students with a more general case rather than the oddball case presented here.

 

[knotical]

EFX, I'd tend to agree with you if this were the only minimum surface interval question to which the student was exposed. But it's not. They are exposed to other minimum surface interval questions both in the text and the Knowledge Review. This is simply one in a series of them. And since it is a Knowledge Review question, the instructor would review it if any student got it wrong. (I admit I don't know how that last bit works if they do e-learning.)

 

[gfaith]

I am certainly not an expert. But I believe we are missing something here RNT after the first dive. If you end the 1st dive and start the second immediately in the M group you have a RNT of 47 which must be added to the ABT of the second dive 39. TBT = 86

Or maybe not ??

That 86 would be is a problem. if you begin the dive as a G diver with 29 RNT add ABT of 39. TBT= 68

Now that would work but require a SI between the dive of 33 to 39 minutes.

 

[knotical]

gfaith, you are correct.
Since the maximum allowable bottom time for a dive to 14m is 98 minutes, the dive is acceptable.
And after that second dive they will be in the W pressure group.

As an aside, there is no need to check the RNT for problems of this sort (minimum surface interval) since the same thing was accomplished by finding the highest pressure group with sufficient ABT. However, doing that check can add confidence in your answer. Note that all the pairs of RNT/ABT in a give row on table 3 add up to the same number (in this case 98) and it is the same as the maximum allowable bottom time for a first dive to that depth.

 

[rick00001967]

Rick,
Yes... AFAIK, ALL agencies use the 10 min. minimum rule

you may be correct about that. i do however find it interesting that the padi tables show "0" as an option for a minimum SI. the ssi tables show "10" as a minimum. there is no option below 10 minutes

 

[jhelmuth]

you may be correct about that. i do however find it interesting that the padi tables show "0" as an option for a minimum SI. the ssi tables show "10" as a minimum. there is no option below 10 minutes

Well... I can't say I am (all indications to the contrary).

Interestingly... I used to "joke" to the students about the "10 min. minimum" rule (and we do that while emphasizing the NAUI RECOMENDED minimum is one hour!) and how one has to count the 2 dives as a single dive if there is less than 10 min. before getting back into the water. I'd tell the that, "...which should be no problem for you all since I've seen how long it takes you to change-over your gear" (implying that they are so new and slow that by the time they got out of the water, changed over and got back in, it would take them more than 10 mins.)