Minimum Surface Intervals When Not Using Tables

[scubadada]

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.

A one hour SI may or may not be the standard, depending on where you are diving, the depths, and the gas. We rarely do a 1 hour SI between the 2 morning or afternoon 2 tank dives in Southeast Florida. That is true for Jupiter, West Palm, and Boynton Beach.

 

[rsingler]

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.

This is a common frustration for many divers. From your manual, determine whether or not your computer has a "look-ahead" function. Many do not, and your original post expresses all the frustration of divers who have to scroll and wait, scroll and wait to find out if the 50 min dive that the DM briefed, that they want to do next, is yet possible. It IS frustrating, especially on boat trips that hop from site to site and plan on a 30-40 min SI.
A couple of options: keep scrolling, and as you dive during the week, you'll get a sense for how long is long enough to wait.
Alternatively, invest in a computer that lets you enter a request for NDL's as of xx minutes from now. Or finally, you can just wait until the DM says "we're anchoring in 5 minutes!", and not sweat the planning until it's time to splash. Have a sandwich instead.
But you're right - you need to know at some point in advance how close you'll come to your no stop limit before you splash. That's proper planning. It's not so good to look down and see "0" in your NDL window.

 

[dmaziuk]

A one hour SI may or may not be the standard, depending on where you are diving, the depths, and the gas. We rarely do a 1 hour SI between the 2 morning or afternoon 2 tank dives in Southeast Florida. That is true for Jupiter, West Palm, and Boynton Beach.

It was our experience in Guanacaste as well. It seems ops that cater for "one morning" or "one afternoon" public commonly do 40 minutes. Ones that work on "one week" basis (e.g. Roatan, Bonaire) usually use 1 hour.

 

[KenGordon]

Which computers have a plan ahead function?

Another approach is to use the planning software which comes with your computer...

Or, assume the DMs have being doing this at least once a week for 6 months and will,pick a site that get shallower towards then end.

Or, get trained properly, do the dive and do the stops.

 

[rsingler]

Which computers have a plan ahead function?

The Scubapro Galileo Luna/Sol/G2:

The Atomic Cobalt/Cobalt 2:

Of course, the magnificent Shearwater Perdix/Teric:
(See attached file below)
I am sure there are others, but they don't include such common computers as the Oceanic Pro or Veo series, nor the Aqualung i550, nor the Suunto Zoop.

Keep in mind, that in estimating whether you have enough no stop time to satisfactorily complete your dive, you should consider both max depth and likely average depth.
Consider the example below. If you asked your computer whether you had enough No Stop Time to dive the Hilma Hooker in Bonaire for 45 min at 96', your computer would tell you, "No!".

But of course, the dive went just fine, because the average depth for the whole period was only 53 feet. So it's important to assess whether you will be following a square profile, with a deeper average depth, or going deep and then meandering back up the reef for the latter part of the dive.

 

[Aloha Joe]

Last weekend I did a typical Deep then shallow dive. Deep was to 90-125ft and we were limited by NDL (diving air). After only 30min surface interval we went for dive 2 to 35-45. It was a 60+ min dive and we were gas limited, so 30min surface interval was plenty for that particular dive sequence.

I don't know the calculated average depths unfortunately.

 

[KenGordon]

But of course, the dive went just fine, because the average depth for the whole period was only 53 feet. So it's important to assess whether you will be following a square profile, with a deeper average depth, or going deep and then meandering back up the reef for the latter part of the dive.

As you say, it is the profile that matters, had your dive ended deep it would have been different, and different again to a square dive. The ending shallow bit is how DMs subtly control divers to try to avoid bends.

There was a thread in the Shearwater sub forum with someone asking why his 100ft NDL was almost unchanged before and after a 100ft dive. The reason was he was shallow for most of the dive and had effectively done a load of stops such that the deep bit made no difference once he got out.

BTW, nobody giving a 45 minute SI is getting my money a second time.

Remember too that all this business about disolved gas is just the model. Nobody has done manned testing to see whether the behaviours of reef divers are actually effective.