average depth and tables?

[Rainer]

On one hand you are quite correct, there's wiggle (that's what I called fuzzy at the start of but for repetitive diving (or anything outside of the aforementioned limitations), I feel that RD may be a tar-baby.

Could you *please* give an example of where it produces these super aggressive dive profiles for repetitive dives?

How many "technical" dives are you planning to do in one day?

I just ran *four* "typical T1" 150' 25 min bottom time dives through V-Planner (of course, 0 conservatism, we wouldn't want to deal with those "fudge factors" you so hate), each with a 2hr surface interval (what GUE and UTD recommend); 21/35+50%. Dive 1, on V-Planner (VPM - B), calls for 21 minutes of deco. RD would give 25 minutes between 70' and the surface, plus another 5 minutes of deep stops (so 21 vs 30). On V-Planner, dives 2-4 *all* call for 22 minutes (no extra time is added for each subsequent dive after the first). RD would give the same 25 minutes between 70' and the surface, plus an additional 5 minutes of deep stops. I'm having a hard time seeing how RD is less conservative. For shorter SIT, you should extend the deco with RD, just as you would do with tables. This is covered in the relevant courses.

Let's take some T2 examples. How about two dives to 220' for 25 minutes, with a 2hr surface interval (let's use the UTD 2:1 set point at 200'); 15/55+50%&O2. For dive 1, V-Planner gives 59 minutes of total deco, with 45 of those between 70' and the surface. RD gives 74 minutes total deco, with 60 of those between 70-0'. What happens with the second dive? V-Planner gives the exact same profile, except adds time to the final stop- an additional 7 minutes. That gives a total V-Planner deco of 66 minutes (52 between 70-0') vs 74 minutes on RD (60 between 70-0'). What happens with even *more* dives (as if anyone is really doing >2 T2 dives in a day)? Well, if you did the same dive a third time, V-Planner only adds an additional *one* minute to the final stop (still less conservative than RD). A fourth dive? Again, only an additional *one* minute (still less conservative than RD).

One of the main reasons RD is more conservative to start is that it allows, as the examples above demonstrate, for repetitive dives without making drastic changes to the schedules (though as the V-Planner profiles also demonstrate, repetitive dives hardly result in massive amounts of extra deco as long as a reasonable surface interval is observed).

In any cases, both RD and V-Planner are just generating profiles. You actually have to go out and dive to see how your body holds up. If either set proves too aggressive for your personal physiology, you *do* need to institute some type of "fudge factor" (i.e. add deco time). This is hardly a slight against RD or any other tables. Both can only give you a starting point that is conservative for most.

Hardly the "tar-baby" you make it out to be...

 

[Trace Malinowski]

Hmm, that didn't even occur to me. You could be right, though.

From the OP's follow-up post, he states:

to be honest I was not considering using it to plan a dive with, as in, plan to use the average as a means of determining deco for that dive.

I was more interested in using a series of dives downloaded from my computer and then logging (converting) them using tables and the average depth from the computer over the past say 24-36 hours to then make a table dive ... <snipped>

... I have tried to convert computer dives to tables using max depth and this is meaningless as many are deco dives on the tables and it would take hours to do a reasonable job plotting them as multilevel.

This definitely isn't how we use depth averaging during ratio decompression to determine our average depth and inert gas load in order to calculate our decompression obligation and ascent strategy.

It sounds like he is taking the average depth displayed on the computer after the dive into account rather than determining the average depth during the bottom portion of the dive.

The method Sail Naked seems to be describing is that he is using the graph from the download and the average depth from the computer post dive to guide him in taking computer profiles and turning them into tabled dives. If anything, the average depth from the computer could throw off the calculations and much too liberal pressure or letter group may result from this method. Such a pressure group would be a poor choice by which to plan a repetitive dive.

The graph might provide enough information during the bottom portion of the dive to determine the average depth for which he would incur bottom time. As he ascends, his computer could allow him to rack up a significant amount of time in the deep and intermediate portions of the ascent. These zones are problematic because he could actually push the depths and times to the point that he will on-gas rather than off-gas and his computer will simply add deco time to shallow stops. Finally, the shallow stops could be completed over a significant time period further throwing off the average depth of the dive that the computer would display. In this way, multi-level calculations are necessary with the possibility of incurring mandatory deco in the intermediate portion at depths one would assume off-gassing is taking place.

Calculating dives this way is a labor of love and risks diver error in planning repetitive dives from this method. Profiles may become too liberal to accurately and safely plan a repetitive or subsequent dive as the OP asks in his first post.

If a diver just wants to find a way of making computer dives "fit a table" to enter logbook information, that is up to the individual and there are various methods to do this. However, repetitive dives should never be planned this way. One should not switch between tables since a "J" Diver on one table may not be a "J" Diver on another table and so forth. One should also not switch between computers and tables because you may actually find yourself accumulating mandatory deco on the bottom with a large inert gas load, fail to see the computer clear you in the mid-range depths, and surface after a time with a shallow average depth. Taking that average depth to a table would allow you to make a longer and deeper repetitive dive that could result in DCS.

 

[Blackwood]

It sounds like he is taking the average depth displayed on the computer after the dive into account rather than determining the average depth during the bottom portion of the dive.

Maybe/probably, though of course if you download the data you can get the bottom portion average.

This definitely isn't how we use depth averaging during ratio decompression to determine our average depth and inert gas load in order to calculate our decompression obligation and ascent strategy.

Certainly not.

 

[Thalassamania]

Could you *please* give an example of where it produces these super aggressive dive profiles for repetitive dives?

How many "technical" dives are you planning to do in one day?

We are here trying to answer a question concerning repetitive diving (remember, there was an OP?).

I just ran *four* "typical T1" 150' 25 min bottom time dives through V-Planner (of course, 0 conservatism, we wouldn't want to deal with those "fudge factors" you so hate), each with a 2hr surface interval (what GUE and UTD recommend); 21/35+50%. Dive 1, on V-Planner (VPM - B), calls for 21 minutes of deco. RD would give 25 minutes between 70' and the surface, plus another 5 minutes of deep stops (so 21 vs 30). On V-Planner, dives 2-4 *all* call for 22 minutes (no extra time is added for each subsequent dive after the first). RD would give the same 25 minutes between 70' and the surface, plus an additional 5 minutes of deep stops. I'm having a hard time seeing how RD is less conservative. For shorter SIT, you should extend the deco with RD, just as you would do with tables. This is covered in the relevant courses.

Let's take some T2 examples. How about two dives to 220' for 25 minutes, with a 2hr surface interval (let's use the UTD 2:1 set point at 200'); 15/55+50%&O2. For dive 1, V-Planner gives 59 minutes of total deco, with 45 of those between 70' and the surface. RD gives 74 minutes total deco, with 60 of those between 70-0'. What happens with the second dive? V-Planner gives the exact same profile, except adds time to the final stop- an additional 7 minutes. That gives a total V-Planner deco of 66 minutes (52 between 70-0') vs 74 minutes on RD (60 between 70-0'). What happens with even *more* dives (as if anyone is really doing >2 T2 dives in a day)? Well, if you did the same dive a third time, V-Planner only adds an additional *one* minute to the final stop (still less conservative than RD). A fourth dive? Again, only an additional *one* minute (still less conservative than RD).

One of the main reasons RD is more conservative to start is that it allows, as the examples above demonstrate, for repetitive dives without making drastic changes to the schedules (though as the V-Planner profiles also demonstrate, repetitive dives hardly result in massive amounts of extra deco as long as a reasonable surface interval is observed).

In any cases, both RD and V-Planner are just generating profiles. You actually have to go out and dive to see how your body holds up. If either set proves too aggressive for your personal physiology, you *do* need to institute some type of "fudge factor" (i.e. add deco time). This is hardly a slight against RD or any other tables. Both can only give you a starting point that is conservative for most.

Hardly the "tar-baby" you make it out to be...

Read Edmonds' paper in the the AAUS Dive Computer Workshop proceedings for an example of the sort of problem that I am concerned about. Granted not the same issue, but an example of where something that seemed quite reasonable based on the practice for which it was designed goes off the track when used for multiple repetitive dives.

... The method Sail Naked seems to be describing is that he is using the graph from the download and the average depth from the computer post dive to guide him in taking computer profiles and turning them into tabled dives. If anything, the average depth from the computer could throw off the calculations and much too liberal pressure or letter group may result from this method. Such a pressure group would be a poor choice by which to plan a repetitive dive.

I think that what Sail Naked is suggesting is not using averages (as the OP inquires), but rather is more akin to the methods proposed by Sharkey and Emmerman.

...

If a diver just wants to find a way of making computer dives "fit a table" to enter logbook information, that is up to the individual and there are various methods to do this. However, repetitive dives should never be planned this way. One should not switch between tables since a "J" Diver on one table may not be a "J" Diver on another table and so forth.

At first blush it appears to me that the method of recording the time/depth pairs and then cross-referencing them to grab a group, and then using the worst case group indicator should work out even if the deco models for the computer and the tables are different.

One should also not switch between computers and tables because you may actually find yourself accumulating mandatory deco on the bottom with a large inert gas load, fail to see the computer clear you in the mid-range depths, and surface after a time with a shallow average depth. Taking that average depth to a table would allow you to make a longer and deeper repetitive dive that could result in DCS.

An argument against averaging, but not against the method I identified.

 

[Rainer]

Read Edmonds' paper in the the AAUS Dive Computer Workshop proceedings for an example of the sort of problem that I am concerned about. Granted not the same issue, but an example of where something that seemed quite reasonable based on the practice for which it was designed goes off the track when used for multiple repetitive dives.

Just read the paper. I can't see how it applies in this case.

Your claim is that RD is overly-aggressive (compared to what???) for repetitive dives. I've given actual examples at both the T1 and T2 level where RD is more conservative than VPM - B profiles (default conservatism since you don't like "fudge factors") for up to four repetitive technical dives (more than I could ever imagine anyone actually doing in a single day of fun diving). This is in great part due to RD being intentionally conservatively designed to deal with repetitive dives (unlike the ORCA EDGE in Edmonds' paper).

If you want to be taken seriously when it comes to RD discussions, please offer up some example profiles to substantiate your claims. Right now they come off as completely ignorant and baseless.

 

[scjoe]

Given the availability of computers and tables, what prompted the development of RD? Why do people that use RD prefer it over other methods that do not require mental calculations while diving?

 

[Mr Carcharodon]

Given the availability of computers and tables, what prompted the development of RD? Why do people that use RD prefer it over other methods that do not require mental calculations while diving?

One of the major advantages is that it gives a common decompression plan that can be shared across a team. Different dive computers can vary significantly. In one of Karl Huggins (Catalina Hyperbaric Chamber) presentations on decompression he presented a chart that showed a 17 minute spread in decompression/remaining NDL time for a set of computers on a hypothetical 100 fsw dive. That variation is enough to make planning and team diving difficult. And the dives can be planned in advance or changed on the fly based on conditions you know about like thermal stress or exertion that your computer has no insight into.

 

[lamont]

It seems the OP is trying to do multi-level diving with tables and be able to avoid treating the entire dive like it was all bottom time at max depth.

The easy way to do this is to just run multiple dives back-to-back with zero surface interval, and get the resultant pressure group. Dive #1 could be 100 feet for 20 mins, #2 could be 60 feet for 20 mins, #3 could be 30 feet for 20 mins. That approximates a single 60 minute multi-level dive within reason.

 

[Trace Malinowski]

Given the availability of computers and tables, what prompted the development of RD? Why do people that use RD prefer it over other methods that do not require mental calculations while diving?

Most technical divers generate tables using decompression software for their planned dive, for a bailout schedule and for worst case depth and time overstays. Ratio decompression allows divers the advantage of being able to keep exploring as long as they stay within gas reserves and continue a dive well past an overstay plan. It also allows the safety of being able to get out of a dive earlier than any generated plan in an emergency. Should an emergency occur at the beginning of a dive, you can potentially get off the bottom and out of deco sooner than your bailout. If an emergency occurs prior to your planned run, you can potentially get out of deco sooner.

Ultimately, it provides complete freedom within the parameters of gas and ratio deco rules and often can give you a more conservative ascent than VPM, RGBM and Buhlmann.

 

[limeyx]

Given the availability of computers and tables, what prompted the development of RD? Why do people that use RD prefer it over other methods that do not require mental calculations while diving?

It is about taking control of your dive, and not letting something or someone else run it for you.