Determining surface interval between dives when at multiple depths during the dive?

[Finneli Felwitch]

I take a scuba trip with my family every year, we usually dive the same shore location every time and we follow the same routine every dive. Recently I have been reviewing the dive tables and I am a bit confused as to how I should determine the appropriate surface interval between dives.

Besides using the dive tables, I have an oceanic data plus computer and an Aries Manta computer.

For our routine dive, we stay between 55-60 feet and swim against the current. My father uses up gas faster than the rest of us and so we always turn around when he reaches 1500 psi. Upon turning around, we ascend to 35-30 feet and ride the current back. We take our 3-5 minute safety stop at 15 feet.

We usually eat lunch during the surface interval and always make sure to stay up for 1 hour and 30 minutes or more.

On the second dive, our max depth is at 45-50 feet and again when turning around, we ascend to 35-30 feet, ending with the 3-5 minute safety stop.

Each dive lasts 45-50 minutes (dive watch starts to record the time at 5 feet and ends at 3 feet).


Is there a way to determine the surface interval when you change depth like my family does?

 

[dfx]

Using tables (going by the PADI RDP) the easiest way is to calculate for the "worst case" scenario, meaning you assume you stay at the maximum depth for the entire dive. However, for your first dive profile, this would mean 50 minutes at 60 feet, which already puts you very close to the NDL. You'd end up in pressure group U, but for your second dive (50 minutes at 50 feet) you must be an H at most. Minumum SI for that would be 57 minutes, which actually isn't too bad.

A more complicated approach is to calculate your dives are multi-level dives. You'd have 25 minutes at 60 feet, putting you in PG I. Then you ascend to 40 feet for another 25 minutes, putting you in PG P. Going from P to H you only need 39 minutes of SI, however this still assumes "worst case" for your second dive.

Taking the multi-level nature of your second dive into account, you work out the NDLs remaining (for each level/part of the dive) as if it were a singular dive. 25 min at 50 ft puts you in PG G (with 55 minutes of NDL left), ascending to 30 ft for another 25 mins puts you in PG M, equivalent to 65 minutes BT. NDL is 205 minutes, so you have 140 minutes NDL left. 55 is the lower number of the two, meaning you can disregard the second half of the dive. You look for a PG that has 55 or just less RNT at 50 feet, which is still H.

So your theoretical minumum SI is 39 minutes, assuming you absolutely stick to the profile you're describing. Keep in mind that your computer is probably going to disagree in some way, and longer SI is always better in any case.

 

[aprivetera]

Dfx has done a nice job describing the tables answer to your question. I will handle the dive computer answer.

The instrument on your wrist (or HP hose) does an excellent job of doing all those wonderful calculations for you (as long as it is properly serviced). You need to know the tables to understand how it works, but the beauty of the computer is understanding the planning feature it offers.

A quick view of the Oceanic Data Plus manual online mentions a "pre dive planning sequence". This will allow you to see exactly where your NDL limits are based on your current surface time and plan your dive accordingly before you enter the water. Find the depth you plan to reach and verify your NDL time. It "should" be something similar if not exact to the tables (your model may vary).

My computer is different than yours but they all have similiar features. Read the manual, do a few dives and verify what the manual says, read the manual again. Repeat until you fully understand what it is telling you.

Always follow your computer. It helps to practice with the table as well in case your computer craps out on you and you have to manually calculate your SI.

Final thought: There also looks like their is a interesting tissue loading graph that will help you estimate your current nitrogen loading level. Not included on my computer but definately would be something I would figure out if I owned one.

 

[Gachnar]

Always follow your computer. It helps to practice with the table as well in case your computer craps out on you and you have to manually calculate your SI.

I would say it's the other way. Always plan your dive, then know that generally your computer will be similar but probably give you a little more bottom time. Computers, like any other machines, can fail. If you look at the tables (or RDP, or wheel, or whatever) you start to learn what your bottom times will be like. While I've never personally heard of a computer screwing up bottom times, I'd hate for someone to blindly follow one if it told them they had 40 minutes at 110 feet. By planning ahead, you'll know if something seems wildly off with the computer mid dive.

 

[emoreira]

I take a scuba trip with my family every year, we usually dive the same shore location every time and we follow the same routine every dive. Recently I have been reviewing the dive tables and I am a bit confused as to how I should determine the appropriate surface interval between dives.

Besides using the dive tables, I have an oceanic data plus computer and an Aries Manta computer.

For our routine dive, we stay between 55-60 feet and swim against the current. My father uses up gas faster than the rest of us and so we always turn around when he reaches 1500 psi. Upon turning around, we ascend to 35-30 feet and ride the current back. We take our 3-5 minute safety stop at 15 feet.

We usually eat lunch during the surface interval and always make sure to stay up for 1 hour and 30 minutes or more.

On the second dive, our max depth is at 45-50 feet and again when turning around, we ascend to 35-30 feet, ending with the 3-5 minute safety stop.

Each dive lasts 45-50 minutes (dive watch starts to record the time at 5 feet and ends at 3 feet).


Is there a way to determine the surface interval when you change depth like my family does?

If you are familiar with planning with tables, you could consider 4 dives :
1st dive : 60 feet - 25 minutes
2nd dive : 35 feet - 25 minutes - 0 minutes SI
3rd dive : 50 feet - 25 minutes - 90 minutes SI
4th dive : 35 feet - 25 minutes - 0 minutes SI

I did the excersise with SSI tables and it´s possible, ending with PG = "J"

 

[Steve_C]

I would say it's the other way. Always plan your dive, then know that generally your computer will be similar but probably give you a little more bottom time. Computers, like any other machines, can fail. If you look at the tables (or RDP, or wheel, or whatever) you start to learn what your bottom times will be like. While I've never personally heard of a computer screwing up bottom times, I'd hate for someone to blindly follow one if it told them they had 40 minutes at 110 feet. By planning ahead, you'll know if something seems wildly off with the computer mid dive.

It is good practice to do a plan. But if it is a multilevel plan there is an excellent chance that you will go off plan. You plan to turn on thirds. That is different times on different days. You drift back but it is at 35 instead of 30. Somebody sucks more air, so you finish up doing 20 instead of 30. You find a neat fish and stop a bit longer at 50 not using much air. The point is that with a group of folks on a recreational multilevel dive it is very easy to deviate from the plan. If my computer is telling me I have 15 min of NDL left and my plan said I should have 20 I will believe the computer (acctually one of two computers). If you spend less time at depth and more shallower than you planned your computer can give you a lot more that the plan. Especially if the dive turned into a 3 or 4 level dive and you planned for 2 levels. Had that happen more than once. Was murky below, clear above and interesting things to watch from the anchor line. Trading off a few minutes at 90 ft can add a lot at 30.

 

[iamjblevine]

If you are familiar with planning with tables, you could consider 4 dives :
1st dive : 60 feet - 25 minutes
2nd dive : 35 feet - 25 minutes - 0 minutes SI
3rd dive : 50 feet - 25 minutes - 90 minutes SI
4th dive : 35 feet - 25 minutes - 0 minutes SI

I did the excersise with SSI tables and it´s possible, ending with PG = "J"

I'm not at home to check this, but I think if you do it this way you run the risk of under estimating pressure group at the end of the dive series. I thought I remembered a fraction you needed to multiply the depth by to make this work. Possibly read it in the old divemaster study guide, but can't remember right now.

 

[nimoh]

PADI doesn't recommend using tables for multi-level dives, they used to have the wheel for this but discontinued it in favor of the eRDPML.

not sure about other agencies.

 

[boulderjohn]

PADI doesn't recommend using tables for multi-level dives, they used to have the wheel for this but discontinued it in favor of the eRDPML.

not sure about other agencies.

If you take the PADI computer-based OW course, it tells you to use the computer's planning function as described above. If you check for your NDL at your planned maximum depth just before the dive, then you have the same information you would have if you had planned the dive with tables and had taken the number based on the pressure group after the surface interval. If the computer has a problem and malfunctions during the dive, you do what the course also says to do--either use your backup computer or abort the dive. If you have a watch or bottom timer as a backup, you can use that as well, since you are once again in the same boat you would have been with the tables.

 

[DiveNav]

Try divePAL. You can use it to plan sequences of multilevel dives.

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