MDL using average depth, where average <60ft

[elgoog]

There's a lot of background information in here and just 1 question (which I've bolded). Apologies in advance for the wall of text. You can just look at the bolded sentence in the next paragraph and give a "yes/no" answer Reasons either way are much appreciated.

For clarification, "130 rule" -> MDL + average depth = 130 (using EAN32)

I found the GUE EAN32-30/30 MDL (image attached) table in a book I believe is used for Rec 1 and the instructions say to use this table with max depth and not average depth.
The table has the following limits - 100min for <=60ft, 170min for <=50ft, 344min for <=40ft. All of these are well above the 130 rule. Deeper than that, from 70-100', the MDL values associated with max depth match the 130 rule.
Can I use the MDL values in the GUE EAN32-30/30 MDL table with average depth for on-the-fly calculations? I would use the 130 rule for deeper than 60' and remember the 3 values noted above for anything shallower than that.

Said wall of text -
I've been using the 130 rule for the last few months and it has worked really well for me as-is for all depths in single tank dives with my HP100+EAN32 (gas and thermal tolerance being my current limiting factors). I understand that this formula works best in the middle ranges 60-100' but breaks down in both directions outside that. I'm not interested in going any deeper at the moment but keeping the average depth at or below 60' for longer dives is much more interesting.

The reason I'm going through this entire rigamarole is that I really like the idea of using the simple formula and remembering only a couple of extra numbers for MDL to cover the entire depth range I'd be targeting when I move to doubles in a year or so.
One of our best local sites (Pt Lobos) has a hefty surface swim and is much better to do as 1 long dive with doubles instead of 2 dives with single tanks. On top of extra bottom time, this also opens up some of the farther sites on the 100' depth contour - this is my personal limit in terms of how deep I want to go and how far I'm willing to kick at this location. (I can already here some of the local divers saying I should get a DPV but that's even farther away than double tanks)

Regarding deco training - If the MDL diving described here is safe to do, 2xHP100 with EAN32 and the 100' line will keep me plenty occupied for quite some time. I would delay/avoid mixed gas, staged deco, etc as I prefer to keep my diving as simple as possible. Having said that, I'm not opposed to deco training if it makes planning and executing these types of dives safer.

I ran some sample "to the limit" profiles in MultiDeco and these seem to indicate you can use the MDL values in the GUE table with average depth safely. For all profiles -
- Plan is for a single dive a day.
- I used the weighted running average method to calculate the average depth (not shown here but I did it in a spreadsheet)
- I use profiles that drop to 100' and spend at least 15-20min in the 90-100' range before ascending slowly and leveling off in 40-60' range. This is within 10min or so to the exposure at depth which would result in mandatory deco. This is also what I anticipate the diving would actually be like.
- I used ZHL16B with GF20/85 as I believe this is what the tables and the 130 rule are based on (correct me if I'm wrong)
- Gas requirements are ignored as I wanted to see what a dive going up to the limit would look like in MultiDeco and how much mandatory deco was required (if any). I would never actually be able to do any of these dives in 2xHP100 anyway nor would my thermal tolerance allow it.

Example 1 -
For ~60ft weighted average, run time 100min - This is 30min over MDL if using 130 rule and same as the max MDL in the table. No additional stops required beyond MDL ascent.

Decompression model: ZHL16-B + GF
Surface interval = 5 day 0 hr 0 min.
Elevation = 0ft
Conservatism = GF 20/85

Dec to 100ft (1) Nitrox 32 60ft/min descent.
Level 100ft 13:20 (15) Nitrox 32 1.29 ppO2, 81ft ead
Asc to 80ft (15) Nitrox 32 -30ft/min ascent.
Level 80ft 10:00 (25) Nitrox 32 1.09 ppO2, 64ft ead
Asc to 70ft (26) Nitrox 32 -30ft/min ascent.
Level 70ft 10:00 (36) Nitrox 32 1.00 ppO2, 56ft ead
Asc to 60ft (36) Nitrox 32 -30ft/min ascent.
Level 60ft 10:00 (46) Nitrox 32 0.90 ppO2, 47ft ead
Asc to 50ft (46) Nitrox 32 -30ft/min ascent.
Level 50ft 20:00 (66) Nitrox 32 0.80 ppO2, 38ft ead
Asc to 40ft (67) Nitrox 32 -30ft/min ascent.
Level 40ft 20:00 (87) Nitrox 32 0.71 ppO2, 30ft ead
Asc to 30ft (87) Nitrox 32 -30ft/min ascent.
Level 30ft 10:00 (97) Nitrox 32 0.61 ppO2, 21ft ead
Asc to 20ft (97) Nitrox 32 -30ft/min ascent.
Stop at 20ft 0:20 (98) Nitrox 32 0.51 ppO2, 13ft ead
Stop at 10ft 1:00 (99) Nitrox 32 0.42 ppO2, 4ft ead
Surface (100) Nitrox 32 -6ft/min ascent.

OTU's this dive: 78
CNS Total: 28.1%
221.0 cu ft Nitrox 32
221 cu ft TOTAL

Example 2 -
For ~50ft weighted average, run time 170min - This is 90min over MDL if using 130 rule and same as the max MDL in the table. No additional stops required beyond MDL ascent.

Decompression model: ZHL16-B + GF
Surface interval = 5 day 0 hr 0 min.
Elevation = 0ft
Conservatism = GF 20/85

Dec to 100ft (1) Nitrox 32 60ft/min descent.
Level 100ft 13:20 (15) Nitrox 32 1.29 ppO2, 81ft ead
Asc to 80ft (15) Nitrox 32 -30ft/min ascent.
Level 80ft 10:00 (25) Nitrox 32 1.09 ppO2, 64ft ead
Asc to 70ft (26) Nitrox 32 -30ft/min ascent.
Level 70ft 10:00 (36) Nitrox 32 1.00 ppO2, 56ft ead
Asc to 60ft (36) Nitrox 32 -30ft/min ascent.
Level 60ft 10:00 (46) Nitrox 32 0.90 ppO2, 47ft ead
Asc to 50ft (46) Nitrox 32 -30ft/min ascent.
Level 50ft 40:00 (86) Nitrox 32 0.80 ppO2, 38ft ead
Asc to 40ft (87) Nitrox 32 -30ft/min ascent.
Level 40ft 40:00 (127) Nitrox 32 0.71 ppO2, 30ft ead
Asc to 30ft (127) Nitrox 32 -30ft/min ascent.
Level 30ft 40:00 (167) Nitrox 32 0.61 ppO2, 21ft ead
Asc to 10ft (168) Nitrox 32 -30ft/min ascent.
Stop at 10ft 1:00 (169) Nitrox 32 0.42 ppO2, 4ft ead
Surface (170) Nitrox 32 -6ft/min ascent.

OTU's this dive: 110
CNS Total: 40.3%
342.2 cu ft Nitrox 32
342.2 cu ft TOTAL

So, uh, yeah ... can haz use of average depth with GUE table and not get bent?

Thanks in advance,
elgoog

 

[Sloth]

Yes GUE depth averaging can be used with either the tables or rule of 130. Assuming you are using GUE depth averaging methods.

Continuous ascent averaging
max depth + first MD stop /2 = average depth

Depth averaging
max depth + intermediate depth /2 = average depth

Your weighted depth averaging doesnt look like it would fall into GUEs guidelines for depth average profiling.

In regards to using rule of 130 at shallow depths it may not be to conservative in singles when properly following GUE gas planning methods. So consumption rate and thermal sensitivity may be your deciding factor over MDL at shallower depths so the conservative nature of the rule of 130 may not actually come into play. I always just used rule of 130 for anything from 50ft and deeper. Shallower then 40ft who cares what the MDLs are?

Just make sure if you use rule of 130 you are doing a minimum of an hour surface interval and are properly recalculating the MDL for the second dive and do your deepest dive first.

Was this stuff not covered in your fundies class? Might want to reach out to your instructor about it.

 

[elgoog]

Yes GUE depth averaging can be used with either the tables or rule of 130. Assuming you are using GUE depth averaging methods.

Great! This is what I wanted to know in general and understand that this is only recommended with the associated GUE averaging method.

Continuous ascent averaging
max depth + first MD stop /2 = average depth
Depth averaging
max depth + intermediate depth /2 = average depth
Your weighted depth averaging doesnt look like it would fall into GUEs guidelines for depth average profiling.

The method I'm calling weighted depth averaging is described here - Best Practices – Dive Planning Part 2 It seems like it errs on the deeper side for calculated average. I think this is similar to the 2nd method you mentioned - is this the same as the GUE method? I'll talk to my GUE instructor about the recommended method anyway.

Was this stuff not covered in your fundies class? Might want to reach out to your instructor about it.

I've only done Fundies 1 so far, part 2 will be completed this year. In part 1 we briefly went over the 130 rule and basic gas planning, the details of both are covered in the 2nd half. I imagine the detailed analysis I'm looking for will be covered there (or at least I can bring it up then).

In regards to using rule of 130 at shallow depths it may not be to conservative in singles when properly following GUE gas planning methods. So consumption rate and thermal sensitivity may be your deciding factor over MDL at shallower depths so the conservative nature of the rule of 130 may not actually come into play. I always just used rule of 130 for anything from 50ft and deeper. Shallower then 40ft who cares what the MDLs are?
Just make sure if you use rule of 130 you are doing a minimum of an hour surface interval and are properly recalculating the MDL for the second dive and do your deepest dive first.

Yeah, this is what I'm doing for my single tank diving now. My post was for the scenario when I would be in doubles and doing a single dive only. I won't be doing these dives before I finish full Fundies and probably even some doubles work with my GUE instructor so I should be more prepared by then. Right now, this is just for my knowledge and was brought about when I saw the GUE table for the first time.

 

[PfcAJ]

You *might* get bent doing depth averaging.

That said, I think it's fine.

It's certainly more aggressive than using your max depth to calculate your no deco limit.

 

[LiteHedded]

You *might* get bent doing depth averaging.

That said, I think it's fine.

It's certainly more aggressive than using your max depth to calculate your no deco limit.

YOU might

 

[Sloth]

The method I'm calling weighted depth averaging is described here - Best Practices – Dive Planning Part 2 It seems like it errs on the deeper side for calculated average. I think this is similar to the 2nd method you mentioned - is this the same as the GUE method? I'll talk to my GUE instructor about the recommended method anyway.

Looks like what he is talking about is dive tracking and not so much dive planning since he is mentally calculating his average depth on the fly during the dive.

I would think your instructor might caution you that it is a little more of an advanced method. I'm not sure if GUE would ever touch on "on the fly" averaging. I dont recall it being mentioned in fundies 6 years ago, tech 1 five years ago or Rec 1 a month ago Maybe Tech2/Cave2 and beyond it is discussed? I dont know.

For myself, I never tried to do any on the fly depth averaging as I never trusted myself to err on the side of caution. Unless it was a huge change in average depth from what was planned then what was being dived I wouldnt bother.

Maybe once you go to doubles you will want to eek out as much bottom time within MDLs as you can and if you are hell bent on using this method I would recommend using a computer - in gauge mode -that shows your average depth and check it in post dive analyze to see if your "on the fly" averaging was accurate before tweaking your MDLs.

GUE is still very much "plan your dive, dive your plan" and this method starts to deviate from that.

I think fundies 2 will really solidify all this for you and really simplify things.

 

[KevinNM]

You should be using a computer that calculates your actual average depth unless it's trivial to determine. And I was told not to do this if you are diving from shallow to deep, it works fine if you are going from deep to shallow.

 

[sea_otter]

No comment on the math, but that's a LOT of kelp to swim over to get to a spot to drop at 100' in Point Lobos. It seems to me that it would be pretty difficult to stick to that profile in the real world.

 

[elgoog]

Looks like what he is talking about is dive tracking and not so much dive planning since he is mentally calculating his average depth on the fly during the dive.
....
GUE is still very much "plan your dive, dive your plan" and this method starts to deviate from that.

Yeah, that makes sense. Planning is going to be even more important than it is now when I move to doubles and the dives get bigger (for me, anyway - maybe not in the grand scheme of things).

Maybe once you go to doubles you will want to eek out as much bottom time within MDLs as you can and if you are hell bent on using this method I would recommend using a computer - in gauge mode -that shows your average depth and check it in post dive analyze to see if your "on the fly" averaging was accurate before tweaking your MDLs

You should be using a computer that calculates your actual average depth unless it's trivial to determine. And I was told not to do this if you are diving from shallow to deep, it works fine if you are going from deep to shallow.

I've been using a Tec2G in gauge mode since Fundies 1. I'd spoken to the instructor about it and he had also suggested to reset average depth when I get to the deepest part of the dive. I noticed this doesn't really make much of a difference for the single tank dives I do now but have been doing it anyway to get used to that step.

 

[elgoog]

No comment on the math, but that's a LOT of kelp to swim over to get to a spot to drop at 100' in Point Lobos. It seems to me that it would be pretty difficult to stick to that profile in the real world.

Yeah, this is more of a "what's possible with doubles but safely within MDL" exercise than anything. To actually get to a 100' at Lobos, I *think* you need to drop near HITW or Lone Metridium and swim out from there. Not something we're going to be doing for a while