Timing of Dives

[scubadada]

We do 4 dives per day all the time in Boynton Beach, FL. One hour dive, 45 min surface interval, 2nd hour dive in the morning. A couple hour lunch surface interval and repeat the schedule from the morning. We often dive 36% on the reefs.

 

[arkstorm]

That used to be the conventional wisdom but it turns out there is no actual data to support it.

Yes and no. It's a fact that diving deep first allows more total bottom time for a given SI and depths. This was historically misconstrued into a DCS issue, which is what has zero data to support (as you mentioned).

To put a finer point on it, see attached video at 49:05...

"No reason for the diving community to prohibit reverse dive profiles for no decompression dives less than 130 feet with depth differential of less than 40 feet."

 

[Seaweed Doc]

Four dives in a day is quite a bit of diving and you will want a decent surface interval between the two deep dives. Three dive in a day and an option on night dive would be what I would do. Nitrox will be good for the shallow dives. If you are in good shape I think it will be fine, but if you aren't, I would avoid making it too demanding.

I'd caution on the night dive if it's the night before doing two deep dives the next morning. Make sure you've got a good 12 hours between dive "days."

I also think it's a good plan to do the deeper dives early, shallow dives later. However, I'm not sure I like the idea of doing four dives in a day with one of them being 40 m or two going to 30 m....

 

[dmaziuk]

I've been diving DSAT since 2002, 2300 dives. I have never taken a dive day off during any series of dives. I have often done 4 dives per day, 5 only a few times. These dives were not up against the NDL. So far so good. I have had a Shearwater since 2019, do not surface with a GF greater than 80.

I think their recommendation was "extrapolated" from them bending their test diver (one of 4) on the 3rd day of 6 dives/day schedule. They continued testing on 4 dives/day for 6 days with no incidents.

PS and they were air dives, too.

 

[tursiops]

To put a finer point on it, see attached video at 49:05...

"No reason for the diving community to prohibit reverse dive profiles for no decompression dives less than 130 feet with depth differential of less than 40 feet."

You appear to have missed the point. Yes, so long as you manage your N2 load, you can do the dives in any order you want. But shallow first then deep is poor gas management....your NDL for the deep dive is greatly restricted. Sure, if you want to do that, but it is poor dive planning.

 

[arkstorm]

your NDL for the deep dive is greatly restricted. Sure, if you want to do that, but it is poor dive planning.

I don't think I missed the point at all...

The NDL restriction is only due to a quirk in the operation of dive tables (remember, they do not actually track N2 loading in real time), but as Mark Powell explains, in a no deco dive, there is no evidence of any increased safety margin.

Let's put it this way, if using a dive computer, you do your deep dive first for x minutes, followed by a surface interval of y minutes, followed by a shallow dive for z minutes vs. shallow dive first for z minutes, followed by a surface interval of y minutes, followed by the deep dive for x minutes... the computer should show an identical N2 load at the end of the second dive regardless of which profile you dove first. Hence, no greater safety for one vs. the other.

 

[inquis]

Let's put it this way, if using a dive computer, you do your deep dive first for x minutes, followed by a surface interval of y minutes, followed by a shallow dive for z minutes vs. shallow dive first for z minutes, followed by a surface interval of y minutes, followed by the deep dive for x minutes...

Try running this through a multi-dive planner and report back. You will be surprised. The difference in tissue loading between 90 ft/25 min + 1 hr SI + 30 ft/45 min vs the reverse is huge.

The NDL restriction is only due to a quirk in the dive tables

It's hardly a quirk, it's simply the mechanics of gas transfer.

as Mark Powell explains, in a no deco dive, there is not evidence of any increased safety margin

As I said, it has been historically MISconstrued as DCS-related and Powell debunks that notion.

That does not mean there are not other advantages/disadvantages to consider.

 

[tursiops]

Let's put it this way, if using a dive computer, you do your deep dive first for x minutes, followed by a surface interval of y minutes, followed by a shallow dive for z minutes vs. shallow dive first for z minutes, followed by a surface interval of y minutes, followed by the deep dive for x minutes... the computer should show an identical N2 load at the end of the second dive regardless of which profile you dove first. Hence, no greater safety for one vs. the other.

I suggest you go into subsurface or multideco and try this; you may be surprised.

 

[arkstorm]

Maybe I need to explain this better...

What Powell says is that as long as you do not exceed the NDL for any of the given dives, there is no evidence of increased risk whether you do the deeper dive first or second.

What everyone else has said here is that if you do the deeper dive first and the shallower dive second, you will have more total NDL time available.

These two points are not mutually exclusive.

 

[tursiops]

Maybe I need to explain this better...

What Powell says is that as long as you do not exceed the NDL for any of the given dives, there is no evidence of increased risk whether you do the deeper dive first or second.

What everyone else has said here is that if you do the deeper dive first and the shallower dive second, you will have more total NDL time available.

These two points are not mutually exclusive.

I agree with both points.