Dive planning question

[parrothead600]

In our Open Water classes, we are all taught that we should plan our deepest dive for the day as our first dive and make all subsequent dives progressively shallower. Just how important is this rule? :huh:
If we are diving with a computer rather than using the charts, how closely should this rule be followed?

 

[BarryNL]

I think the main issue is that most deco data gathered is from "deepest first" profiles. Mathematically, it doesn't really matter, but OTOH you're talking about maths that have been tuned to match empirical data gathered from "deepest first" diving.

Doing it the other way round puts you in largely unexplored territory.

 

[spankey]

it is a general rule and should be kept to, and you should always check your tables before a dive even if on a computer, you never know if it's going to bomb out on you and then you have no idea what you should do. Also if you are using RGBM tables, you are unable to calculate anything if you don’t do your deepest dives first, and if you don’t know what RGBM is, then you should absolutely be adhering to ALL “Rules” you were taught in your open water class.

But at the end of the day they are just tables made by someone 10x more intelligent than you or I, so it’s up to you what you do

 

[rongoodman]

It seems to have become more controversial in recent years, as more information about diving medicine and illness have accumulated.

 

[redacted]

Just from the standpoint of maximizing your total bottom time, minimizing SI requirements, or minimizing residual nitrogen at the end of the planned series of dives; doing the deeper dive first seems to be the way to go.

 

[Meng_Tze]

Just from the standpoint of maximizing your total bottom time, minimizing SI requirements, or minimizing residual nitrogen at the end of the planned series of dives; doing the deeper dive first seems to be the way to go.

As long as you 'clean up' on the later dives.

But overall to answer the original question. Yes, deep first has been done for years and has good results. But as mentioned here as well, there is a trend to do it the other way around. But there is not enough data/experience with this to make it into the mainstream of the diving industry. When starting out I would suggest to stick with what has been taught, if and only if knowledge of decompression increases, one can look at alternatives. Experimenting with this without background information and understanding thereof has its risks...............

 

[nolatom]

I agree that for nitrogen/bottom time reasons, deepest-first makes sense.

But aside form nitrogen management, isn't it counterintuitive to do the deepest, and presumably most difficult, dive first? If I haven't been wet in a while, and want to make sure my gear, and me, still do okay underwater, I'd appreciate doing a shallow, no-challenge dive first just as a checkout, then have more confidence and less anxiety for the deeper dive.

That's how most activities which involve some safety risk are typically done (mountain climbing, swimming, skydiving, skiing, whatever). For perfectly good reasons, in diving we generally have to do this "backwards", hardest first rather than easiest first.

 

[Meng_Tze]

... If I haven't been wet in a while, and want to make sure my gear, and me, still do okay underwater, I'd appreciate doing a shallow, no-challenge dive first just as a checkout....

You are in the right mimdset there. I would do a check out dive first (and see it as a completely seperate dive), maybe in a confined and controled environment. Then go for the trip/boat.

This way you have an opportunity to address issues (skills, equipment, planning etc) before going for a dive series. You dont want to find out on the first boat dive (doing the check out dive as the first dive) for instance that your BC is not working as you need.

If you have not been wet for a while, check everything first.

 

[loosebits]

I agree that for nitrogen/bottom time reasons, deepest-first makes sense.

But aside form nitrogen management, isn't it counterintuitive to do the deepest, and presumably most difficult, dive first? If I haven't been wet in a while, and want to make sure my gear, and me, still do okay underwater, I'd appreciate doing a shallow, no-challenge dive first just as a checkout, then have more confidence and less anxiety for the deeper dive.

That's how most activities which involve some safety risk are typically done (mountain climbing, swimming, skydiving, skiing, whatever). For perfectly good reasons, in diving we generally have to do this "backwards", hardest first rather than easiest first.

Obviously other considerations such as comfort and safety overrule trying to maximize total bottom time by doing the deepest first but many of us are diving as often as every weekend or more and it isn't necessary to start off with an "easy one".

 

[Mako Mark]

there is no physical reason to do the deepest dive first, it happens because of the way the early dive tables were modeled and the limitations of the mathematics and computing power at the time. That said, it has worked pretty well for quite a while.

OTOH, there is quite a lot of discussion which is hotly debated on both sides that perhaps we actually should be doing out deepest dives last.

Without getting too technical (so no flaming me please) the early dive tables were based on gas disolved in our tissues, now, this gives a pretty good estimate of how the gas is entering and leaving the body, but it does have a fundimental flaw. Disolved gas does not give us decompression sicknes, only bubbles do.

So, there are new decompression that use this information that are perhaps more accurate than, and have less assumptions than the older models.

the other thing is the theory (well accepted) that we have bubbles in our body after every dive, but they are pretty small and dont really have any great effect on us. They generally make their way to the lungs where they get stuck in the capillaries there and gas out.

However, there is a theory that when those bubbles stuck in the lungs (which is the best place ofr them really) get compressed by a shallower dive, they can escape into the arterial system and then head to the head (which is probably the worst place for them). Some radicals have even sugested that it is imperative that we do our deeper dive later in order to crush these bubbles back into a disolved state to stop this happening. They have even gone out and done some extraordinary dives with profiles that the critics and cynics direly predicted would end in tradgedy, and they suffered no ill effects.

Decompression is still a very inexact science, more of an art really and it is in constant evolution.