New divers with question on calculating min sufrace intervals!

[RikRaeder]

Perhaps herein lies the problem. 70 feet = 21.336 meters
20m for 29 min rounds up to 30 minutes and results in M (I'm using the same table)
70'=21.3m rounds up to 22m for 29 min = N

Are you using the same measurement system in your book and on your RDP?

 

[Black Water]

Four minutes.

70 feet for 29 minutes puts you in a pressure group "N."

In order to do another dive to 50 feet for 39 minutes, you must be in pressure group "M" prior to making your second dive (see "Table 3 - Repetitive Time Table" at bottom).

To go from pressure group "N" to pressure group "M" requires a surface interval of at least four minutes (and the question specifically asked for the "minimum" amount of time).

Practically speaking, you'd be hard-pressed to get out of the water, change tanks, talk about the first dive, get something to drink, return to the water and begin diving again in four minutes... So the question is really a theoretical question, not a practical one.

Practically speaking, make all of your surface intervals about an hour. It's simple, it's safe, and it gives you time to breathe, talk about the dive, make a new plan for the next dive, fix whatever issue you had on the first dive, etc. You can also change out tanks, get something to drink, warm up, etc...

Also practically speaking, there is no reason whatsoever to run all of your dives so close to the ragged edge of ANY dive table, even one so conservative as the one you're using. You should never need to know whether or not four minutes is going to make or break you. 70 feet for 29 minutes is fine... So is 50 feet for 39 minutes - if you just take the time between the dives to chill and talk and plan the next dive.

Besides, at the end of the day, if you're not running so close to the "ragged edge" - that is, if you take a 1 hour surface interval rather than descending again in a record-breaking four minutes - then you'll end up a lot less tired at the end of the day and have a lot less possibility of dive-related fatigue, even if you didn't actually get bent.

...So the answer to the question is "4 minutes." The answer to the real question is "one hour" - 'cause if you do it that way every time, it's easier to remember, easier to repeat, easier on the body, and physiologically safer. It also gives you time to rest, talk about the first dive, plan the second, tend to your body and gear, and that kind of stuff... Which leads to a much safer dive, too.

 

[ricster80]

Thanks for the responses Guys.

It appears that the problem is in the conversion from metric (I'm in Europe) to Imperial depth figures. The question states 20m/70ft and 14m/50ft; this is actually more like 65ft and 45ft.

Even with that taken into acount you can encounter the same problem with the imperial RDP as you get with the metric version:

First dive 80ft for 25mins = PG of N, followed by 40ft for 35mins = PG of V. N and V don't intersect either so what's the SI?

I think I'm in danger of becoming very confused indeed so I'll take your advice and speak to my instructor. Unless one is lurking here somewhere.....

In any case I like the idea of playing it safe and staying up for an hour at least.

PS. Thanks to the Moderator for moving my thread to Basic Scuba, I did try to put it there originally but it said I didn't have access permissions...

 

[ricster80]

Also helps if I read the whole thread before I post!

Much clearer now but guess I just need to practice in order to become familiar with things like this.

BIG thanks for all the responses people, much obliged.

Happy Diving.

 

[SteveAD]

It can be a confusing question, but it does illustrate the fact that some combinations can be done with little or no surface interval. I jokingly tell my students that if you just did 30 feet for 45 min you could have an indy pit crew pull you out, swap tanks, and throw you back to do it again... but since I don't dive with an indy pit crew, and am never in a hurry when I dive, I usually take an hour whether I need it or not.

Personally I think the minimum surface interval questions have the least real world application. I am much less likely to say "when can I do this dive?" than "I'm ready to dive now...what can I do?"

 

[RJP]

Personally I think the minimum surface interval questions have the least real world application. I am much less likely to say "when can I do this dive?" than "I'm ready to dive now...what can I do?"

That approach works for most "pretty fishy" dives or anything where you can sort of wing depth and time to make the dive work given whatever SI you're dealing with at that moment.

However on some dives where min/max depth is a given (eg: a wreck for instance, where you can't just drift along a wall at any depth you like in order to make the dive work) or where there is a time-specific/intensive mission (eg: search and recovery of an item, surveying a site, etc) you may well need to calculate a minimum surface interval in order to be able to accomplish your objectives for the second dive.

In short, there are many "real world" applications for understanding how to do this.

 

[boulderjohn]

The correct answer is scattered in several places in this thread, so perhaps it will be helpful for a quick summary.

If the problem is worked out correctly, the answer is 4 minutes (imperial) or 0 minutes (metric).

Yes, that means that in some cases it is possible to return to the water immediately after the first dive is over.

That is what makes multilevel dive planning possible. If you were using the eRDPml instead of the tables, you would see it clearly. After you enter your first level, you are told what your pressure group is, even though you are still under the water. Then you enter the second depth and are told a new pressure group. Then you enter a third and are given a third group. Each time you are given a new no decompression limit. Thus, your three levels in one dive are really the same as three consecutive dives with no surface intervals.

 

[MattJ]

Ricster: Just for fun, go ahead and run through the rest of the table and calculate your ending pressure group at the end of that set of dives. If you dive 70 feet for 29 minutes, have a four minute surface interval, then dive 50 feet for 39 minutes, how close are you to the no-decompression limit?

In classes, I've started doing that on the white board for the minimum surface interval questions -reinforcing that when you have a minimum surface interval, you're really pushing the NDL limits on the tables. Then try it again with a 60 minute surface interval, a 90 minute surface interval, and so on.

 

[Garrobo]

Should have taken the SDI course so as to avoud all that crap.