Deepest Dive First, no exceptions?

[txgoose]

In our training we have been taught that a diver should always do their deepest dive first with subsequent dives shallower. How critical is this in shallower depths?

Context: My family will all be newly certified by the end of the weekend. As of Sunday evening we will have each dove to a max of about 30'. We are planning a family dive trip Monday to a local dive park. We're taught "deepest dive first." Which implies as newbies we can only make progress on successive trips if we don't want our first dive to possibly stretch our comfort levels.

Main question: At these depths is it going to wreck us to do a couple of 32' dives (to several training platforms) to verify everyone's comfort levels and then a final 40/45' dive? By the charts we'd still be within the limits for our dive times (that was a good practice exercise, but then I am a data geek). We'll have i300 computers in our gear as well to watch deco limits.

While I understand best practices, I can't imagine we're pushing the envelope. Nomex suit acquired.

 

[nolatom]

Not that critical in shallow dives, especially if you are new and consume air at a faster rate, and therefore your limiting factor is going to be remaining tank air, not how many nitrogen ticks on your computer/in your body.

Later on, when you tend to get into deeper stuff, and your gas consumption rate has gradually improved through experience, then "deepest first" is a good general rule, because your limiting factor is likely to have become those nitrogen ticks, because you're using air much slower. You don't want to be borderline on built-up nitrogen absorption in a deep, last dive. Better to get that over with on the first dive.

 

[tbone1004]

it is about optimizing nitrogen loading and minimizing risk of DCS. My non-instructor, non-medical, VERY personal opinion is at those depths it doesn't matter.

Ideally to maximize your NDL times, and minimize your risk of DCS, you would make your deepest dive first. That is for more exaggerated depth changes, i.e. don't do a 40ft dive then a 90ft dive if you can help it. At up to 50ft, in my opinion it doesn't really matter. You are going to hit your gas limits long before you hit NDL, especially if diving nitrox *which I think is a waste of money at 50ft*

 

[shurite7]

In the past the main reason for the deep dive first had to do with dive tables. If one conducts reverse profiles (shallow dive first then deeper afterward) the table doesn't allow for much time at depth on the deeper dives, if any at all.

With the introduction to dive computers the rule of deep dives first began to be questioned. Another aspect that questioned reverse profiles were resorts, especially staff, who were conducting reverse profiles while guiding divers. An example of this would be a 100 foot dive followed by a 50 or 60 foot dive, after an hour surface interval, conducted in the morning. In the afternoon the same staff would repeat the above profiles.

Back in 2000 (give or take a couple of years - I can't remember the exact date) the Smithsonian Institute sponsored a study on this. It invited the recreational community (PADI, NAUI, etc.), the scientific community (universities), and the Navy. At the end of the three hundred and seventy some off pages of the report it stated they could fin no reason why reverse profiles could not be done. The report can be found on Rubicon's website.

With all that being stated, the general practice is to conduct the deepest dive first followed by shallower subsequent dives throughout the day. After all, our knowledge of No Decompression Limits is still theory.

 

[Lorenzoid]

Some prior discussions:

Deepest dive first?

Deepest dive first, or not?

Reverse profile dangerous?

reverse profiles??

Reverse Dive Profiles

 

[txgoose]

it is about optimizing nitrogen loading and minimizing risk of DCS. My non-instructor, non-medical, VERY personal opinion is at those depths it doesn't matter.

That is how I have been reading it. Here and elsewhere on the webs.

You are going to hit your gas limits long before you hit NDL, especially if diving nitrox *which I think is a waste of money at 50ft*

Hopefully short lived but, my oldest has a bad case of the "sprints" instead of the "cruises." So he/we burn a fair amount of air. I would be very surprised if we could push the limits. And for now, I am okay with our dives being short-ish. Gives me a chance to check in and review a bit more often (more detail than the okay question). Once I am more comfortable with the brood's skills I'll start asking those leading questions about his speed to consumption ratio.

Gracias!

 

[txgoose]

Some prior discussions:

Apologies. My search had multiple of hits in the "lies" thread and I got distracted from it. Thank you!

 

[northernone]

Personal experience and opinion would be it doesn't mattmatter in any significant way for the profiles described. Dive your computer and any way you'll be well within the limits. Enjoy.

Here's a little controversy if the deeper dive first suggestion is even valid in recreational diving:

Deepest Dive First? Not Anymore

Regards,
Cameron

 

[TMHeimer]

So I see there has been one study saying reverse profiles don't make a difference. I would figure since it's all mathematics it shouldn't. It's interesting there haven't been more studies. Of course, the logical reason of not being close to the NDL on a deep one following a shallow one makes sense, but you can also be near the NDL if the deep one is first. Either way I agree that at such shallow depths it doesn't matter.

 

[Rred]

The advice to make your deepest diver first is probably based on two things:
1-Maximizing bottom time on that deep dive (since you'll start with the least nitrogen loading in your body)
2-Making it easier to figure subsequent nitrogen loading and bottom time.

There's absolutely no scientific "need" to make the deep dive first, as long as you can grasp the concepts of nitrogen loading and understanding the tables, or your computer, to keep within safe limits on subsequent dives. Now, that can become complex and most of the studies and data aren't targeted at it, so you may have to learn and do more math.

But it isn't calculus, and some groupthink with the rest of the family may even be a fun exercise for this. As others have said, at those depths? You are SO well within safety limits that there's unlikely to be any problem. In the worst case, treat the two dives as 45 feet (50 feet for tables) with a repetitive dive the second day. Run the numbers, you'll still have plenty of time at fifty feet.