Minimum Surface Intervals When Not Using Tables

[scubadada]

doesn't it (your PDC) scroll NDL's for subsequent dives? Every computer I have owned shows this in one way or another during the SI after dive each dive....

We have watched them to make sure we are "in a good place" for dive #2....

Exactly, use your computer's dive planner to check your NDL for the next dive, if it's adequate, you're good to go. I often do 2 or 4 dives per day when home in Boynton Beach. The west facing reef is relatively shallow and we are often diving richer mixes of nitrox. As an example, here is a 71 minute dive and a 75 minute dive on EAN34 with a 29 minute SI. I had a minimum of 52 minutes of NDL on the 1st dive and a minimum of 13 minutes of NDL on the 2nd dive. The 2nd dive was limited by gas. These dives were done on DSAT.

You can use your dive planner to choose an appropriate SI for whatever depth and time your next dive will be. Try it, you'll like it.

 

[Addicted2H2O]

Couple of things.

First Shortbus got it backwards DSAT is one of the most liberal recreational algorithms while Z+ is their version of Buhlmann ZHL-16C which is moderately conservative.

Second. There is no “safe” surface interval. It is generally recommended to wait an hour but there is nothing magical about that number. However, the longer you wait between dives, the longer your NDL time for the next dive. Theoretically you could do your second dive 10 minutes after the first but it could be a very short dive. Waiting longer would give you more time at depth.

As others have said, at any time after your dive you should be able to use the dive plan feature to see what NDL you would have at a particular depth if you jumped it at that moment during your SI.

Thanks for that catch! When I was diving my ProPlus 3 I typically left it in conservative just to ensure I was withing my limits. I haven't dove that computer in a while, though so I got it mixed up. I dive a Cobalt II now.

 

[ResortDiver850]

Okay, thanks everyone. So I need to check to see if the future NDL time would be appropriate for my next dive.

 

[PatW]

I have been diving the cobalt 1. Before a dive, I check the no deco limits for the depth I am diving. I also look at the no deco time for 60’. That is just to give me an idea of my nitrogen load. Most of the time, a 45 min surface interval is fine. But I rarely dive deep. Most of my diving is 60’ or less and I am quite happy at 30’ or less. There is one area I really like where the top of the wall is 70’ to 90’, but I never dive that two dives in a row.

 

[boulderjohn]

I have read the manual a couple times. I am going to sign up for the DIV Nav class when I get a chance. I did not see the topic addressed. The manual is very detailed.

The fact that the manual is so detailed (and all of them are) is a key part of the problem. A key concept in information theory is that too much information is the same as too little information. You don't need to know the overwhelming majority of the information in that manual, but they have to put it all in there to cover their buns. The information you need is lost amid all that other stuff, which is classified in information theory as "noise."

In the computer version of the PADI OW course, students are taught that all computers have several key functions--really only a handful that you truly need to know. Once you get your computer, you should go to the manual and learn how that model works with those specific functions. You can safely disregard pretty much all the rest. Dive planning is one of those functions, and a new computer owner should pick up the manual, turn to the table of contents, and look for the section that explains how that works.

As long as I am on that topic, another one of those key functions is emergency decompression. Your computer will tell you how to get to the surface safely if you violate NDLs, but you have to understand what it is telling you to do. I was once working with some students, and several other divers from my group were diving on their own at the same time. They ended their dives when we did, and as soon as one of them reached the surface, he started talking about the weird thing his computer did. When he was at the bottom, he noticed that it was giving the some signal he did not recognize, and it was counting minutes up instead of down. I did not know his computer, but I immediately recognized that he must have passed NDLs, and the computer was telling him how long his decompression stop needed to be. I checked his computer, and it was in error mode. He did not remember what numbers it was showing when he surfaced. I took him back down to 20 feet and we did a very, very long omitted decompression stop. When we went back to the surface, I put him on oxygen just to be safe.

 

[scubadada]

...As long as I am on that topic, another one of those key functions is emergency decompression. Your computer will tell you how to get to the surface safely if you violate NDLs, but you have to understand what it is telling you to do. I was once working with some students, and several other divers from my group were diving on their own at the same time. They ended their dives when we did, and as soon as one of them reached the surface, he started talking about the weird thing his computer did. When he was at the bottom, he noticed that it was giving the some signal he did not recognize, and it was counting minutes up instead of down. I did not know his computer, but I immediately recognized that he must have passed NDLs, and the computer was telling him how long his decompression stop needed to be. I checked his computer, and it was in error mode. He did not remember what numbers it was showing when he surfaced. I took him back down to 20 feet and we did a very, very long omitted decompression stop. When we went back to the surface, I put him on oxygen just to be safe.

Hi John,

I wonder why the diver was in violation gauge mode? It sounds like it was not from the current dive, unless others also went into and cleared deco, or he was diving a very conservative computer. If it was the latter, the extra deco stop and oxygen were probably not necessary. If he would have told you within the conditional violation period, usually 5 minutes, going back down for the omitted stop would have prevented him from entering VGM and the associated lock out (I assume he was not diving a Shearwater).

Alternatively, perhaps he had a much longer exposure on a previous dive that day and had a more significant deco obligation. In that case, your action might have saved him from more serious outcome.

On many occasions over the years, I have been asked by others on the boat why their computers have quit working. The most common reason is that their computers have entered VGM. Almost always the computers have been running one of the more conservative RGBM variants and has followed a dive with a significant exposure. One morning recently, I boarded a boat for a day of diving and noticed that a rental computer next to me was in VGM, likely from the previous day. I brought it to the attention of the crew and they switched it out. It would have been quite a surprise had it been discovered after we were out on the water.

You are absolutely correct, divers really do need to know how their computers work, particularly, the functions that are important for their diving.

 

[boulderjohn]

I wonder why the diver was in violation gauge mode? It sounds like it was not from the current dive, unless others also went into and cleared deco, or he was diving a very conservative computer....Alternatively, perhaps he had a much longer exposure on a previous dive that day and had a more significant deco obligation. In that case, your action might have saved him from more serious outcome.

It was probably his third dive that day, and, yes, it was a Suunto.

 

[Steve_C]

OP did not say what kind of diving he was doing (or I missed it). If he is doing 3 shallow reef dives to 20 ft the SI does not really matter. Now 3 dives to 110 fsw and he is good on air and dives big tanks, well different story.

 

[dmaziuk]

Okay, thanks everyone. So I need to check to see if the future NDL time would be appropriate for my next dive.

Most dive computers' "plan" mode will tell you what your NDL is going to be if you dive to planned depth now. You need to keep checking until you see your NDL's where you want it to be.

Tables will tell you right away how long you need to wait. With the caveat that your computer will not match the tables exactly, it's only ballpark.

1 hour SI will most likely keep your computer happy. That's ballpark too, you can decide which ballpark's better than the other.

 

[Fastmarc]

After your 1st dive (or subsequent dives) when on your SI, go into the Plan (planning) mode. It will show you how much NDL you have for the depth you are planning for next dive at that specific time you checked. The longer your SI, the more the NDL increases. With every check you'll see this.

Generally 1 hour SI is a common practice, but you can use your computer planning mode to help make that final determination.