Average Depth for planning a dive?

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DiveNav

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Hi,
a while ago I was asked to add the average depth profile to divePAL becuase "some tech diving organizations use average depth for planning purposes"

So, we did it.

And now that we have average depth in divePAL, I wanted to see for myself if - and why - it makes sense to plan a dive using the average depth instead of the actual profile.

So I did an experiment: first I did a plan for a deep multilevel dive, then I obtained the average depth and use it to plan an almost square profile using that average depth.

And the results are quite interesting: for both dives the bottom time is 35 minutes and the average depth is 72ft, but the second dive is way more penalizing.

Dive 1 divepal_average_depth1.jpg


Dive 2 divepal_average_depth2.jpg

So, if I am not mistaken, if you plan a dive using average depth, most likely your real multilevel dive profile will be more conservative.

Did I get this right?

Alberto (aka eDiver)
 
I suspect the average may be used for gas planning, not nitrogen loading.

If it's a multi-gas plan then a number averaged legs would be needed for each resource.

As for your observation, the average eliminates most of the shallower than average time when some off gassing gets to happen. It also fails to penalize the diver for the deeper time.

Pete
 
The fun of learning "technical diving" on the interwebs...

Using average depth as a planning / execution tool requires more than simply being able to calculate an average depth for the total dive. You can start here: DIR-diver.com - Average depth for deco?

Some points of note:

(1) For conservatism, start timers leaving the surface (or some other agreed upon shallow depth) and start depth averaging once on the bottom. The depth average does not include the deco ascent.

(2) Understand what depths are relevant/irrelevant for the purpose of on-gassing and off-gassing (e.g. spending an additional 10 minutes at 10' at the start of the dive isn't going to hurt you when you go on to do 30 minutes at 200'; doing 10 minutes at 10' at the end of that dive on O2 is going to help substantially).

(3) Plan dives deep-to-shallow, not the other way (or the average will be misleading; see the linked article above).

(4) Learning to (a) calculate a RELEVANT average depth, and (b) using that average to generate decompression profiles is something that takes intimate understanding of how to safely apply various rules, and practice. One builds that experience base during hundreds of recreational profiles before attempting to do so on much less forgiving deeper dives.

Simply relying on a global average for the entire dive (what some BT/gauges now give), especially without understanding the limitations and associated risks, is a fool's errand.
 
Practical Dive Planning & Execution Using On-the-Fly Depth Averaging Function of the UWATEC 330m Bottom Timer:

Been successfully using the following profile range & gases for the past ten days in Chuuk Lagoon:

20/20 trimix with Eanx50 and Oxygen for deco;
Sidemount 11L tanks with 4.2L deco bottles.

The deep wrecks of the Fourth Fleet Anchorage (San Francisco Maru; Aikoku Maru; Nagano Maru; Shotan Maru; Nippo Maru; Hoki Maru etc), all fall in the depth/BT ranges below. I just pick the average depth and the corresponding time on pre-dive planning (& on-the-fly as needed:wink:):

Depth: BT:
45m 35min
48m 30min
51m 25min
54m 20min
57m 15min

. . . And then the following same deco profile for any one chosen depth/BT combination above:

42-39m short deep stops 5 to 10sec
36m 30sec
33m 1min
30m 1min
27m 2min
24m 3min
21m 4min Eanx50 switch
18m 3min
15m 3min
12m 3min
9m 2:2min Eanx50:backgas break
6m 10min Oxygen switch
-->ascend 1m/min for 6min time to surface on O2

(based on Buhlmann GF 30/85, and Ratio Deco 1:1)
SCR: 10 to 12L/min; Deco SCR: 12 to 15L/min
Minimum Gas Reserve Rock Bottom from 45m: 1600 litres total, or 70bar reading in BOTH BOTTOM MIX TANKS.

Eanx50 used: 640 litres
Oxygen used: 350 litres
20/20 used: 2700 litres

Water temp 29deg C (skinsuit, no added lead weights & Z-systyem sidemount harness).
 
Oops . . . looking at your two examples, what you have done on the first one is to come up with an average depth that includes all of the decompression time. That's not how we do it at all. On your first dive, the average depth for planning purposes is going to be about 125, from reading your chart; I'd probably call it 150, myself, excluding the descent time, because I start my clock at 70 feet. So I'd call that 150 for 8, which would give me a small amount of mandatory deco, which I would execute much as the diagram indicates.

When you depth-average a multi-leveled dive, you have to set some limits on the portion of the dive you're going to average. Some part of the dive is going to be ascent/decompression, and you have to exclude that from your averaging, for deco determination purposes (you DO use it for figuring SAC rate, so long as you stay on backgas for the whole dive).

To give you an absurd example: We did a dive in the Red Sea with a max depth of 167 and an average depth of 150, and a bottom time of 20 minutes. We did the required deco, and when we got to 20 feet, the wall was so beautiful with the sunlight on it, we decided to swim back to the boat, rather than surface and board the RIB. So we did about 30 minutes total at 20 feet. This would have brought the average depth of the dive WAY up -- I mean, we spent more time at 20 than we did below 100, so it would all look like a no-deco dive. But I'm quite sure those intermediate stops were not just done for fun . . .
 
mmmhhhhh ... granted that decompression is not an exact science .... it seems to me that this "depth averaging" stuff is even less exact as it appears to be open to individual interpretations.

I think that the dive profiles in my original post don't make too much sense anymore ..... and I should revisit this subject once my team gives me the tech version of divePAL with Trimix, GF and 3 OC gasses.
 
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