Average depth calculation

[*Floater*]

The problem is that technique is OK for a nice easy dive like you described but if you apply depth averaging to something more agressive see what happens:
here is a dive with 2 level components using v-planner

<snip>

So you see the relationship is not linear. If you add more levels and more time the error can only increase. I am hoping to leann something here so those who reply are not contributing anything useful.

That may be. I'm not tech certified and haven't thought or been taught how to apply this concept beyond NDL other than going over the limits a few min, so I can't help you out.

 

[wedivebc]

Just to throw this in, using the gasses I would choose (non-GUE standard) for these dives results in much less deco:
Dec to 200ft (4) Trimix 17/45 50ft/min descent.
Dec to 250ft (4) Trimix 17/45 60ft/min descent.
Level 250ft 5:10 (10) Trimix 17/45 1.46 ppO2, 103ft ead, 123ft end
Asc to 200ft (11) Trimix 17/45 -30ft/min ascent.
Level 200ft 10:00 (21) Trimix 17/45 1.20 ppO2, 79ft ead, 95ft end
Asc to 130ft (24) Trimix 17/45 -30ft/min ascent.
Asc to 120ft (24) Nitrox 32 -30ft/min ascent.
Stop at 120ft 0:40 (25) Nitrox 32 1.48 ppO2, 99ft ead
Stop at 110ft 1:00 (26) Nitrox 32 1.38 ppO2, 90ft ead
Stop at 100ft 1:00 (27) Nitrox 32 1.29 ppO2, 81ft ead
Stop at 90ft 1:00 (28) Nitrox 32 1.19 ppO2, 73ft ead
Stop at 80ft 1:00 (29) Nitrox 32 1.09 ppO2, 64ft ead
Stop at 70ft 1:00 (30) Nitrox 32 1.00 ppO2, 56ft ead
Stop at 60ft 1:00 (31) Nitrox 32 0.90 ppO2, 47ft ead
Stop at 50ft 3:00 (34) Nitrox 32 0.80 ppO2, 38ft ead
Stop at 40ft 3:00 (37) Nitrox 32 0.71 ppO2, 30ft ead
Stop at 30ft 4:00 (41) Nitrox 80 1.53 ppO2, 0ft ead
Stop at 20ft 6:00 (47) Nitrox 80 1.28 ppO2, 0ft ead
Stop at 10ft 10:00 (57) Nitrox 80 1.04 ppO2, 0ft ead
Surface (57) Nitrox 80 -30ft/min ascent.

 

[Diver0001]

Just to throw this in, using the gasses I would choose (non-GUE standard) for these dives results in much less deco:
Dec to 200ft (4) Trimix 17/45 50ft/min descent.
Dec to 250ft (4) Trimix 17/45 60ft/min descent.
Level 250ft 5:10 (10) Trimix 17/45 1.46 ppO2, 103ft ead, 123ft end

Yeah, Dave's put his finger right on it sore spot. Even the best models start to break down as you go deeper and things like this depth averaging, that probably works most of the time for nitrox diving within the NDL's become unreliable out of this context.

Personally I think we need to look carefully at what serious deep divers are doing and project *those* practices onto shallow diving instead wasting our time with methods that none of them would apply for fear of their lives. Someone mentioned that the depth averaging works for a lot of people.... That's simply false. There is ample evidence to the contrary, especially in the realm of extremely deep diving.

R..

 

[Spectre]

here is a dive with 2 level components using v-planner

Level 250ft 5:10 (10) Trimix 15/55 1.28 ppO2, 74ft ead, 94ft end
Level 200ft 10:00 (21) Trimix 15/55 1.06 ppO2, 55ft ead, 72ft end
Surface (64) Oxygen -30ft/min ascent.

And here we have the dive usiing the average of the 2 depths
Level 225ft 15:35 (20) Trimix 15/55 1.17 ppO2, 65ft ead, 83ft end
Surface (70) Oxygen -30ft/min ascent.

So you see the relationship is not linear. If you add more levels and more time the error can only increase. I am hoping to leann something here so those who reply are not contributing anything useful.

...

So the average depth method comes up with a more conservative deco profile than the max depth method. And if you chose to use a ratio deco algorithm, it's even more conservative. Keep running examples until you come up with a case where the 'average' method gives you -less- deco then the 'actual' plan, and then you have a case to argue that it's not useful.

Think a realistic scenario. You plan a dive to 250 for 20 minutes. You end up doing 10 minutes at 250, 10 minutes at 200. The "plan" has you out of the water in 77 minutes, the on the fly adjusted plan has you out of the water in 70 minutes. Sure, you could have gotten away with 64 minutes, as you find out when you run the profile after the dive; but a little adjustment on the fly still gave you some credit for the change in plan.

One thing I've chosen to accept for myself is that not only does 'close' count in horseshoes and hand grenades, close also counts in deco.

 

[wedivebc]

...

So the average depth method comes up with a more conservative deco profile than the max depth method. And if you chose to use a ratio deco algorithm, it's even more conservative. Keep running examples until you come up with a case where the 'average' method gives you -less- deco then the 'actual' plan, and then you have a case to argue that it's not useful.

Think a realistic scenario. You plan a dive to 250 for 20 minutes. You end up doing 10 minutes at 250, 10 minutes at 200. The "plan" has you out of the water in 77 minutes, the on the fly adjusted plan has you out of the water in 70 minutes. Sure, you could have gotten away with 64 minutes, as you find out when you run the profile after the dive; but a little adjustment on the fly still gave you some credit for the change in plan.

One thing I've chosen to accept for myself is that not only does 'close' count in horseshoes and hand grenades, close also counts in deco.

OK but in most cases when deco starts the main event is over. This seems rather inprecise and will cause me to spend more in-water time and consume more deco gas than expected or nessasary. I can see when exploring a new cave with unknown depth it is a nessasary evil and a little extra deco is par for the course, but for open water diving what advantage does this method give you?

 

[Soggy]

I can see when exploring a new cave with unknown depth it is a nessasary evil and a little extra deco is par for the course, but for open water diving what advantage does this method give you?

For one thing, you don't have to carry a laptop or $2000 VR3 on board with you. Also, if you change your profile while you are down there...for example, you are told the wreck is at 160, but it's dead low tide, so now it's at 145 and has 15 ft of relief, your planned 160 ft dive could very well end up averaging out at 135....maybe you're spending a few extra minutes in the water compared to what vplanner tells you, but you are still getting out 15 minutes earlier than originally planned.

 

[Spectre]

OK but in most cases when deco starts the main event is over. This seems rather inprecise and will cause me to spend more in-water time and consume more deco gas than expected or nessasary. I can see when exploring a new cave with unknown depth it is a nessasary evil and a little extra deco is par for the course, but for open water diving what advantage does this method give you?

The key here is 'expected'. First let me state for the record that we are assuming you are not diving with a deco profiling computer, you are diving tables. Now if the dive plan is, say, 10 minutes at 200, 10 minutes at 250, then yes, it would make perfect sense to plan for that dive. It's more along the lines of when your plan changes.

Using the examples you've given. Let's say that you planned for 20 minutes at 250, but you end up doing 10 at 250 and 10 at 200. What would you do for deco? The deco you planned for [the deco for 20 minutes at 250?]? I don't have tables cut for all possible contingency multi-level scenarios, but I -do- have tables cut for the realistic depths and times involved. So while I planned for that 20 @ 250, I know my average was really 225 and therefore I can pull out my tables and shave off some deco and run my 20 @ 230 table instead.

Let me run another example, albeit away from the OW examples, but to demonstrate the practical use with a real example. Hole in the Wall cave, mainline downstream. There are lots of little ups and downs between 60 and 85 feet, with one window that's up around 55'. During my full cave class, we did that dive. The max depth obtained was 84 feet, but I knew the average was less than 70. At about 50 minutes runtime I pulled out my tables and did some quick calcs, figuring about 15 minutes left before the ascent, and came up with the schedule to get to my o2 bottle. When we got to the o2 bottles the instructor looked at me and asked how much deco. I said "5". He asked the other student, he looked at his Nitech and said "7". The instructor looked at his VR3, said "4" and we ok'ed on the 7. My calculations came out right on their computer calculated numbers.

Now diving a standard table algorithm, with a max depth of 84, I would have to use a 90' 32% table, or a 70' air table, or 10 minutes on oxygen. But by figuring my average depth around 70 feet and adjusting for EAD, I referred to my 60 foot air table to come up with 4 1/2 minutes on oxygen for 70 minutes of BT, rounding to 5, I effectively shaved 5 minutes off the dive by being aware of my average depth.

After downloading my data recorder I saw my actual average depth was 68 feet, and running it with 32% through vplanner it basically comes out as a no deco dive. So yes, I did more deco then necessary, but I did less than if I just assumed a square profile.

I can give a similiar, and much more extreme, story for Little River... max 116, average 95, 70 minutes BT; tracking average depth took my time on oxygen from 29 minutes to 10 and my buddy and I settled on 11.

 

[JeffG]

OK but in most cases when deco starts the main event is over. This seems rather inprecise and will cause me to spend more in-water time and consume more deco gas than expected or nessasary. I can see when exploring a new cave with unknown depth it is a nessasary evil and a little extra deco is par for the course, but for open water diving what advantage does this method give you?

Another example is we planned a 150ft for 20 mins. Due to an Octopus attack (which was so cool), we went and hung out with the octopus for 8 mins at 160 ft. The next 12 mins we did at 140 so we didn't have to change the ascent profile that we planned.

 

[wedivebc]

Thanks guys, it's starting to make sense.

 

[Diver0001]

Let me run another example, albeit away from the OW examples, but to demonstrate the practical use with a real example. Hole in the Wall cave, mainline downstream. There are lots of little ups and downs between 60 and 85 feet, with one window that's up around 55'. During my full cave class, we did that dive. The max depth obtained was 84 feet, but I knew the average was less than 70. At about 50 minutes runtime I pulled out my tables and did some quick calcs, figuring about 15 minutes left before the ascent, and came up with the schedule to get to my o2 bottle. When we got to the o2 bottles the instructor looked at me and asked how much deco. I said "5". He asked the other student, he looked at his Nitech and said "7". The instructor looked at his VR3, said "4" and we ok'ed on the 7. My calculations came out right on their computer calculated numbers.
.

Playing devil's advocate....if the results are the same what possible advantage does this technique have over the use of the computer? The computer will give you the same results every single time for a million dives..... With doing the math manually you only introduce the possibility (In fact I would say *certainty*) of human error.

R..