Manual calculation for accelerated deco

[lowviz]

@Dr Simon Mitchell

Could I trouble you to comment on this (below) to the current post?

Manual calculation for accelerated deco

 

[Dr Simon Mitchell]

@Dr Simon Mitchell

Could I trouble you to comment on this (below) to the current post?

Manual calculation for accelerated deco

Hello Lowviz,

Sorry, I think I understand what you are getting at.

Scaling stop depths, and stop time (by calculating a sea-level equivalent dive depth, and then decompressing according to the sea level table for that new depth) is the crux of typical standard approaches to altitude diving - such as the guidelines published in the US Navy Diving Manual. The formulae for that approach are similar to the ones Kev has presented. For example:

Equivalent depth (fsw) = Actual depth (fsw) x (1 ATA / Atmospheric pressure at altitude)

Altitude stop depth (fsw) = Sea level stop depth (fsw) x (Atmospheric pressure at altitude / 1 ATA)

These approaches are promulgated in the US Navy Diving Manual.

Simon M

 

[rossh]

Remember that decompression models are based on pressure ratios, rather than on absolute pressures. ....

Kevin, Not so....


The ZHL coefficients are based on zero ATA absolute (the A values). According to Erik Baker's Understanding M-values reporting of Buhlmanns model, the ZHL A/B values are usable down to 0.5 ATA absolute.

In VPM, all its internal calculations are based on 0 ATA absolute. VPM has the advantage here in elevation diving limits, as its core concept is to calculate microbubble growth measured against ambient pressures. i.e. real calculations for the changes that elevation brings.


Hence for ZHL / VPM-B, with proper elevation information entered, then no external model adjustments is needed, with the condition that the divers depth instrument knows the correct surface start pressure, and displays proper elevation adjusted depth.

Further more, some modern computers are able to measure and track diver tissue state before the dive, and compensate favorably for time spent at elevation and acclimatization.


The older elevation planning methods, involve what Simon described - manual adjustments to achieve sea level equivalents. This was necessary as older burbon tube depth meters (needle type) usually have no ability to correct for elevation, and the diver will end up diving that extra 0.2 ATA (2m) deeper. Also, older table methods have to assume the worst case where the diver only ascended to dive elevation recently, with the extra deco penalty that involves.

.

 

[KenGordon]

Also, older table methods have to assume the worst case where the diver only ascended to dive elevation recently, with the extra deco penalty that involves.

The BSAC88 tables have transfer tables that give a tissue code based on a change of altitude, then you apply a “surface interval” to that tissue code to arrive at the tissue code you will start the dive with.

So you might start the day as an A, drive up a big hill becoming a C, wait a while and become a B, go diving, surface as a F, wait a while etc....

 

[lowviz]

Honestly, you could check if Buhlmann and VPM compensate for altitude in a similar way. You could create a dive profile at sea level, compensate it for altitude (like 30m@SL and 24m@2000m altitude) and validate it that way yourself. ...//...

I'm getting to that but it isn't like I NEED that information, I have BSAC tables that will take me higher, deeper, and longer than I will ever dive.

I used to work in a research support team and all successes were shared events. I have recently become fascinated in how to 'grasp' altitude diving.

I don't have the least concern when it comes to being proven wrong or shown up as thinking about something the wrong way. In short, being wrong. I have a life full of that and have become rather good at not 'covering' but rather continuing to probe for the causes behind the effects.

Thank you for having the courage to express your positive support of my emerging ideas as to this approach. I would be most happy if you or anyone were to run those comparisons, it would be enlightening. I'm no stranger to that sort of comparison, that is how I came up with 222,222/(Depth in feet)^2 as the simplest reasonable approximation to the USN air table NDL's. I know myself too well, I would become wrapped up in the mechanics of the comparison. And I will someday, probably soon. Thank you.

Recent posts in this thread bear out the reason I'm taking this approach. (Basic principles first) Some intensely interesting material has been posted and I have some reading and thinking to do before I respond.

 

[Dr Simon Mitchell]

The older elevation planning methods, involve what Simon described - manual adjustments to achieve sea level equivalents. This was necessary as older burbon tube depth meters (needle type) usually have no ability to correct for elevation...

Hello Ross,

You raise a valuable point about the potential inaccuracy of bourdon tube depth gauges. But as I explained in earlier posts, it is the need for more decompression time at altitude that drove the use of equivalent depth conversions; not concerns over inaccuracy of depth gauges. There are separate guidelines for dealing with that. But as you imply, depth gauge inaccuracy is less of an issue these days because of the use of electronic devices that compensate accurately.

Simon M

 

[Kevrumbo]

Kevin, Not so....


The ZHL coefficients are based on zero ATA absolute (the A values). According to Erik Baker's Understanding M-values reporting of Buhlmanns model, the ZHL A/B values are usable down to 0.5 ATA absolute. . .

Ross, the Buhlmann ZHL-16 A/B/C M-Values (NOT coefficients!) are mathematical constructs which can be expressed as a "linear relationship between ambient pressure and tolerated inert gas pressure in the hypothetical "tissue compartments". (Read again and comprehend the Historical Background section in the link above with regard to the dissolved gas or "Haldanian" decompression model).

From a historical pre-digital dive computer perspective, these M-values are simply just an easier way of presenting the original empirically determined or exponentially derived N2 Tissue Compartment Supersaturation Values from a "unitless" ratio, to a more practical number in terms of either N2 pressure absolute (in mswa or fswa), or total absolute pressure (ATA), or in terms of gauge pressure in meters or feet representing a No Decompression Depth Limit -all as an exercise for manually generating a usable Dive Table.

For example, the 5min Tissue Compartment as originally determined empirically under the US Navy Model can theoretically undergo six half-times and tolerate a Surfacing Supersaturation Ratio of 3.17:1 or less, and not require any decompression to directly return to the surface. Well, how do we go about practically using a ratio of 3.17:1 ??? Hence the expression and utilization of normalized Workman/Buhlmann M-values derived in meaningful units.

In the link given above on p.5 (Erik Baker's Understanding M-values), divide all the Table 2 Workman 1965 M0 values in msw by ten, and you will get the original classic empirical Surfacing Supersaturation Ratios for each of the representative Half-time Tissue Compartments of the Navy Model. Divide each of these ratios by 0.79 (the %age of Nitrogen in Air), and you get the No Decompression Depth Limits for each Half-time Tissue Compartment in ATA. Convert to gauge msw or fsw as needed to form a dive table matrix.

 

[sigxbill]

with regard to RD, the bottom line is that what we are discussing here are deep dives requiring deco - not shallow dives. The support of all the anti-RD comments are essentially based on the NEDU deep stops study. BUT - the 170' NEDU study was on AIR - not trimix! RD doesn't support air past 100' / 30m, so it is possible that the NEDU study does not apply to RD - because the RD prescribed gas for 170' is 18/45. So one could argue that the results obtained from the NEDU study, that seem to indicate shallower stop emphasis, do not apply to RD. It is possible that RD works better with 18/45 at 170' than it would with air ... and no one has won the prize from Shell Oil for knowing for sure ...

 

[Kevrumbo]

with regard to RD, the bottom line is that what we are discussing here are deep dives requiring deco - not shallow dives. The support of all the anti-RD comments are essentially based on the NEDU deep stops study. BUT - the 170' NEDU study was on AIR - not trimix! RD doesn't support air past 100' / 30m, so it is possible that the NEDU study does not apply to RD - because the RD prescribed gas for 170' is 18/45. So one could argue that the results obtained from the NEDU study, that seem to indicate shallower stop emphasis, do not apply to RD. It is possible that RD works better with 18/45 at 170' than it would with air ... and no one has won the prize from Shell Oil for knowing for sure ...

In summary, the implications of the NEDU Deep Stops Study shows that deep stop strategy may now in fact supersaturate the Slow Tissues later in the deco profile along with an increased risk of DCS upon surfacing. In other words, the Slow Tissues are still on-gassing while you are decompressing the Fast Tissues with Deep Stops at 66% and 50% max depth per the Ratio Deco 2.0 method for example. The real practical and takeaway point of the NEDU Study is not that the deco profiles tested experimentally in the study's paradigm are non-representative of "real world" deco profiles as performed by sport technical divers, but that the same disadvantageous pattern of Slow Tissue Supersaturation is inherent to all bubble model deco algorithms which prescribe deep stop profiles.

To compensate, and still choosing to use a Dual Phase/Bubble Model Deco Profile with DeepStops like VPM, RGBM, or Ratio Deco method, you might have to extend the shallower stop times on Oxygen to effectively offgas & clear the Slow Tissues' surfacing supersaturation along with any potential pathological DCS causing micro or proto-bubble formation.

At present, exactly how much to extend this O2 stop time is arbitrary. For practical consideration, take into account the FN2 of the bottom mix and how many consecutive days of decompression diving you plan to do along with the slow tissue loading of residual inert gas over that time.

The real essence of the NEDU Study and the vital parameters to consider are the bottom time and depth of fast & slow tissues' on-gassing exposure, and effective final decompression relief of surfacing Slow Tissue supersaturation to prevent pathological DCS bubble formation. The Fast Tissues are a lot more robust & tolerant than we once thought (because of their greater blood vessel vasculature & perfusion?), and therefore prescribed deep stops to protect them from inert gas supersaturation are not as critical as preventing further Slow Tissues on-gassing and later supersaturation upon surfacing.

 

[Dan_P]

@Dr Simon Mitchell
Thank you for your insights. Loads of good input to the discussion here!

One thing I'm pondering, in terms perhaps more semantical:
To me, it makes sense to address first the difference in relative pressure drop, rather than the difference in surface pressure, under the observation that the gradient is what's driving dissolution.

To illustrate, using an example of a 30m dive at sea level and 2000m altitude:
Sea level: 4,0 bar to 1,0 bar = 75% pressure drop
2000m: 3,8 bar to 0,8 bar = 79% pressure drop
Hence, 4% difference in pressure drop as opposed to 20% difference in ambient pressure.

Expanding further through two points in a fictive ascend

30m to first stop, proposition 1, 50% of depth
Sea level: 4,0 to 2,5 = 38% pressure reduction
2000m: 3,8 to 2,3 = 39% pressure reduction
1% difference

30m to first stop, proposition 2, 25% of depth
Sea level: 4,0 to 1,75 = 56% pressure reduction
2000m: 3,8 to 1,55 = 59% pressure reduction
3% difference

Finally, an example as the ascend nears the surface

3m to surface
Sea level: 1,3 to 1,0 = 30% reduction
2000m: 1,1 to 0,8 = 38% reduction
8% difference

As we see from these examples, the difference (across sea level and 2000m) in pressure drop gradient is steeper with approximation to the surface.
Hence, it would stand to reason that even from a pure dissolution perspective, deeper stops would be sensible compared to sea level diving on the same algorithm or ascend approach, as the gradient would reach a theoretical tissue group's M-value earlier in the ascend.

NOTE: I believe the above to be a different matter from a discussion on deep stops, or emphasis ratio between dissolved gas and bubble mechanics, per se. If using an algorithm that had little or no emphasis on deep stops, I would still expect the first stop to be deeper on an altitude dive.