Minimum surface interval question ...

[AfroBoy]

Hi folks,

Just about to complete my OW certification and am doing the eLearning course. There is one question there which I just can't figure out why the answer is what it is.*

First dive of 22 metres for 26 minutes.
Second dive of 16 metres for 24 minutes.
What is the minimum surface interval required between the two dives?

The first dive gives you a pressure group of M. The second dive gives you a pressure group of P. When I join the two lines together, they meet an area without any times. Acceptable answers however are either a surface interval of 0 minutes or 4 minutes. How can this be so?

Would be grateful if someone could explain this particular scenario for me.

Cheers,
Af.


* To prevent any copyright cheating stuff, I've changed the figures but the result is still the same.

 

[ekremer]

What is the minimum pressure group you need to reach to allow for a 24 minute dive at 16 meters?

Then how long do you need to wait to get from M to that new pressure group?

 

[chrpai]

The question isn't asking what your pressure group is after the second dive. It's asking how long it'll take to get to a pressure group that allows the second dive.

Using the PADI metric RDP, 22m rounds to 22m and 26 minutes rounds to 27 minutes. That makes you an "M" diver.

For the second dive, 16m round to 16m and as an "M" diver you already has a greater allowable bottom time so the answer is 0 minutes.

 

[AfroBoy]

I think I understand now, because my second dive will leave me in a higher pressure group, then I don't need to wait after my first dive to take my second. Would that be a correct way of thinking about it?

 

[ekremer]

Your first dive leaves you at a pressure group that has an adjusted No Decompression time that is greater than your desired bottom time at your new depth.

We aren't looking at the pressure group of your second dive at all, just your new NDL.

 

[dfx]

You first use table #3 (back side) to figure that out. Start with your desired depth on the left (16 m) and go across until you find an adjusted NDL time (bottom number) that's equal or just above the desired bottom time. You'll find one that has 24 in it. Go up and check the pressure group (P). So P is the maximum allowable pressure group you need, meaning that P or anything lower than P (O, N, M, ...) will work. Then go back to the first table (table #2 technically) and find the minimal required surface interval after your first dive (M) that puts you in the maximum allowable pressure group or better. Since M is already better than P, you don't need any SI.

 

[ekremer]

You assume he is using the plastic tables. I figure he is using whatever calculator padi is trying to sell this week. or maybe the wheel

 

[TSandM]

Think of it this way: You have a bucket. Each time you go diving, you put some nitrogen in the bucket. Every minute that you are on land, you are taking some nitrogen out of the bucket. Your goal is never to fill the bucket . . . so if you put a lot of nitrogen in it during the first dive, you have to wait a while on land to empty the bucket, before you can put more nitrogen in. (If you know the depth and length of your proposed dive, you know how MUCH nitrogen you want to put in, so you can figure out if there's enough room in the bucket.)

In the case of your problem, if you didn't wait at all, you'd still have room in the bucket after the second dive. So you have no need to spend surface interval time emptying your bucket before the second dive.

 

[HIGHwing]

Shearwater Petrel. Oops. Too early! (Mods, please move my comment to page three when we get there).

 

[XS-NRG]

Think of it this way: You have a bucket. Each time you go diving, you put some nitrogen in the bucket. Every minute that you are on land, you are taking some nitrogen out of the bucket. Your goal is never to fill the bucket . . . so if you put a lot of nitrogen in it during the first dive, you have to wait a while on land to empty the bucket, before you can put more nitrogen in. (If you know the depth and length of your proposed dive, you know how MUCH nitrogen you want to put in, so you can figure out if there's enough room in the bucket.)

In the case of your problem, if you didn't wait at all, you'd still have room in the bucket after the second dive. So you have no need to spend surface interval time emptying your bucket before the second dive.

That is probably the best analogy I've ever heard and such a simple and perfect way to teach and explain it! Love it!