Help with RDP Question please !

[triswebb]

Hi all,

Im currently studying the RDP and have been getting along ok, but have been stumped by this question.

"A Diver plans to do three dives using minimum surface intervals. The planned profiles are: 10meters for 90 mins, 30 meters for 19 mins and 16 meters for 42 mins. Arrange the dives in appropiate order and calculate how many total minutes it will take to complete from the descent of the first dive to surfacing on the last dive (you may ignore ascent time, but not required safety stop time)"

The options are :
237 mins
240 mins
192 mins
189mins

If someone could briefly go through how I solve this problem I would be extremely grateful as its really frustrating me at the momment !! Thanks all.

tris

 

[Diver0001]

Hi all,

Im currently studying the RDP and have been getting along ok, but have been stumped by this question.

"A Diver plans to do three dives using minimum surface intervals. The planned profiles are: 10meters for 90 mins, 30 meters for 19 mins and 16 meters for 42 mins. Arrange the dives in appropiate order and calculate how many total minutes it will take to complete from the descent of the first dive to surfacing on the last dive (you may ignore ascent time, but not required safety stop time)"

The options are :
237 mins
240 mins
192 mins
189mins

If someone could briefly go through how I solve this problem I would be extremely grateful as its really frustrating me at the momment !! Thanks all.

tris

Three dives. First of all the right order..... Deepest first right? so you get

#1 is 30m/19min
#2 is 16m/42min
#3 is 10m/90min

Assuming we have the same version of the RDP I get group "M" After dive #1

Now work backwards.

What do you need to be in order to make dive #2 within the NDL's?

That would be group "I" right? And you come out after 19 min as group "M".

so your surface interval between dive #1 and dive #2 has to be enough to go from M to I. Thats 20 min minimum right?

So now dive #3. Again work backwards. Where do you need to start? I get group "T". What do you need to go from X to T? That's 12 min. Right? Not if you follow the W to Z rules. That would require a 60 min SI.

So now you have it all.

dive #1 is 19 min plus a mandatory safety stop of 3 min (you're in the grey) plus a SI of 20min. That's 42 in total.

dive #2 is 42 min plus a mandatory safety stop of 3 min (you're in the grey) plus a SI of 60 min because you came out as group X. That's 105 in total.

and dive #3 is 90 min. In this case you're not in the grey because you did a 60 min SI after dive #2. That's 90.

so 42 + 105 + 90 gives me 237 .... answer A. /*edited*/

I deliberately didn't show you how to find the groups I just listed them. The solution to that part is up to you. Most people forget about the safety stops.

R..

 

[Don Burke]

What do they teach for descent rate? 18 meters per minute?

 

[Diver0001]

What do they teach for descent rate? 18 meters per minute?

The assumption with these kinds of questions is that your descent time is included in your bottom time. Ascent time is not included in bottom time so the safety stops are added on after the fact.

R..

 

[triswebb]

So now you have it all.

dive #1 is 19 min plus a mandatory safety stop of 3 min (you're in the grey) plus a SI of 20min. That's 42 in total.
dive #2 is 42 min plus a mandatory safety stop of 3 min (you're in the grey again) plus a SI of 12 min. That's 57 in total.
and dive #3 is 90 min plus 3 min safety stop (in the grey again). That's 93.

so 42 + 57 + 93 gives me 192 .... answer C.

Exellent, thanks for that. An anoying thing is that on my RDP the surface interval between the X and the T is 13:15 not the 12 that you mentioned, which was putting me out each time (I now discover). Is this common ? To be honest im using a RDP printed on the back of my log book so it may not be that reliable !! Anyhow - thanks so much for you time and effort.

tristan

 

[WVMike]

The assumption with these kinds of questions is that your descent time is included in your bottom time. Ascent time is not included in bottom time so the safety stops are added on after the fact.

R..

If this is a PADI test question it could be tricky. PADI's definition is that bottom time for calculation purposes stops as soon as the ascent starts and you do not include safety stop time.

I always count my time from descent to surface, no need to cut corners here. But if it is a test question watch out for that little trick.

Mike

 

[triswebb]

Am I right in thinking here that if the residual nitrogen level added to the bottom time puts you in the black when you calculate the next depth then you should use a more conservative figure? As that obviously makes the answer above wrong. As the second dive puts you in the black...

Thanks for everyones help.

tris

p.s this question comes from the instructor candidate workbook. There are no answers accompanying it.

 

[Diver0001]

Exellent, thanks for that. An anoying thing is that on my RDP the surface interval between the X and the T is 13:15 not the 12 that you mentioned, which was putting me out each time (I now discover). Is this common ? To be honest im using a RDP printed on the back of my log book so it may not be that reliable !! Anyhow - thanks so much for you time and effort.

tristan

13:15 is what you need to go from W to S. Are you sure you're not reading one row beside where you need to be?

R..

 

[Diver0001]

Am I right in thinking here that if the residual nitrogen level added to the bottom time puts you in the black when you calculate the next depth then you should use a more conservative figure? As that obviously makes the answer above wrong. As the second dive puts you in the black...

Thanks for everyones help.

tris

p.s this question comes from the instructor candidate workbook. There are no answers accompanying it.

Black is just dark grey .... you need to put a 3 min safety stop in for black too. As long as you don't go over the NDL you're on the table for calculation purposes. You wouldn't tend to dive like this in reality.

R..

 

[triswebb]

13:15 is what you need to go from W to S. Are you sure you're not reading one row beside where you need to be?

R..

No, I have cross referenced it with another RDP that I have found online and it just seems the surface interval here is printed wrong. :-(