scjoe
Contributor
Why do teams not use the same computer or table or agree upon the controlling one in advance? What happens when members come up with different computations using RD?
average depth and tables?
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Why do teams not use the same computer or table or agree upon the controlling one in advance? What happens when members come up with different computations using RD?
Rainer
Contributor
Starting first with your question about why RD and not computers/tables.
First, you're taught tables to start (many hours spent comparing RD profiles to VPM and Buhlmann + GFs). You're also always encouraged to check planned profiles outside your RD comfort zone against software. Limitations of RD are of course discussed in detail in classes.
Second, as limeyx noted, RD is all about taking control of your diving. You're not merely reacting to the profile that some black box in a computer spits out in the water. Having to track your time and depth (for averaging purposes) keeps you engaged. The subsequent "math" to compute the shape of the profile is really trivial. Spending 100s of hours calculating average depth on recreational profiles before venturing deeper builds up the skill so it is second nature (you can compare the averages you generate in the water with later downloads while you learn this skill). Nor are you blindly following your buddy. Everyone tracks the dive.
Third, computers aren't providing the exact shaped profiles we want to dive. For example, they aren't providing the same deep stop pattern we choose, and don't emphasize high PPO2s at gas switches.
Fourth, as you note, the entire team would need the same computer (ignoring the third point for a moment). What happens when that model/algorithm is discontinued, or you travel somewhere new and people have a different computer? Nevermind the expense of a trimix computer compared to a BT. A similar issue of course arises for tables as well. At a minimum, the team (and the agency, if you want to make global unified diving a goal) must decide on a single set of tables for various depths/BTs/gases (both back and deco gases). RD is one such unified approach.
Fifth, RD allows for more flexibility than tables (unless you plan to cut a lot of tables...). Of course you enter the dive with a plan ahead of time, but RD allows for modification of that plan once in the water (within confined and understood parameters). Shorter/longer BTs, shallower/deeper depths are all easily accounted for (again, with understood parameters guiding these choices). This is particularly useful for exploration dives. Lost deco gases are also easily accommodated in this system, where new profiles can instantly be generated on the fly uniformly in the team. All of this can be done without having to pull out a set of tables.
As what to do if teammates disagree about the required deco, first, you have all of the deep stops and all the time at the gas switch to try to sort this out (here, wetnotes can help). If you're still unable to come to a consensus, just deco out on the more conservative schedule. Absolutely worst case, you can always deco on the max depth and total BT. In my dives, it's never been an issue.
First, you're taught tables to start (many hours spent comparing RD profiles to VPM and Buhlmann + GFs). You're also always encouraged to check planned profiles outside your RD comfort zone against software. Limitations of RD are of course discussed in detail in classes.
Second, as limeyx noted, RD is all about taking control of your diving. You're not merely reacting to the profile that some black box in a computer spits out in the water. Having to track your time and depth (for averaging purposes) keeps you engaged. The subsequent "math" to compute the shape of the profile is really trivial. Spending 100s of hours calculating average depth on recreational profiles before venturing deeper builds up the skill so it is second nature (you can compare the averages you generate in the water with later downloads while you learn this skill). Nor are you blindly following your buddy. Everyone tracks the dive.
Third, computers aren't providing the exact shaped profiles we want to dive. For example, they aren't providing the same deep stop pattern we choose, and don't emphasize high PPO2s at gas switches.
Fourth, as you note, the entire team would need the same computer (ignoring the third point for a moment). What happens when that model/algorithm is discontinued, or you travel somewhere new and people have a different computer? Nevermind the expense of a trimix computer compared to a BT. A similar issue of course arises for tables as well. At a minimum, the team (and the agency, if you want to make global unified diving a goal) must decide on a single set of tables for various depths/BTs/gases (both back and deco gases). RD is one such unified approach.
Fifth, RD allows for more flexibility than tables (unless you plan to cut a lot of tables...). Of course you enter the dive with a plan ahead of time, but RD allows for modification of that plan once in the water (within confined and understood parameters). Shorter/longer BTs, shallower/deeper depths are all easily accounted for (again, with understood parameters guiding these choices). This is particularly useful for exploration dives. Lost deco gases are also easily accommodated in this system, where new profiles can instantly be generated on the fly uniformly in the team. All of this can be done without having to pull out a set of tables.
As what to do if teammates disagree about the required deco, first, you have all of the deep stops and all the time at the gas switch to try to sort this out (here, wetnotes can help). If you're still unable to come to a consensus, just deco out on the more conservative schedule. Absolutely worst case, you can always deco on the max depth and total BT. In my dives, it's never been an issue.
Mr Carcharodon
Contributor
First it makes gas planning difficult. Since generally it is hard to know what deco time will be called for by a computer running a unknown proprietary algorithm how you do not know how long you will be down? If you do not know how long you will be down how do plan to insure that you have enough gas?
The ascent plan prescribed by many computers is less than optimal to minimize risk of DCS. Yount et. al. did in vitro experiments that showed that the number of bubbles created by traditional ascents are significantly greater than for exponentially shaped ascents. That is the shape of ascents produced by VPM or RD on backgas. This is a deep stop approach which reduces the pressure gradients seen on ascent which are the driving force behind bubble formation. Pyle and many others have observed that deep stops reduce post dive fatigue, which is what would be expected from the work of Yount since fewer bubbles are expected to be in circulation.
Finally if you agree to just follow one computer out of team how do you decide which one to follow? If your computer allows 20 minutes less bottom time than your buddys computer, but you have never shown signs or symptoms of DCS, were all your previous dives unsafe, or was your computer giving unreasonable answers? Knowing which is the case is important.
You should be planning your dive before hand, and using gas calculations to make sure it is ok. You don't need to hit the Deco limit every time, so you can decide to only do a set time at the bottom, regardless of whether the computer is saying you can stay longer.
Can you link to that research please?
My understanding is you use the most conservative both on depth and time, as this provides the least possible problems.
For example if my computer reads 38.1 M and yours reads 39.9M then we wouldn't decend any further if the plan was for a 40M dive (yes the values are intentionally more than what you should be seeing). The same would apply for doing a no-decompression dive. If one computer is showing it is time to ascend then you ascend. It may not mean you have been unsafe. It may mean that your buddy has set their computer more conservatively (e.g. if they feel they are less fit), or is using a different gas mix, or one of a number of other factors, that affect the bottom time.
The short answer is that they don't. At the end of the dive, you agree on the depth on which you are basing your deco. Once the depth is agreed upon, the deco schedule is known. One person is deco captain; that person is calling the depth and time of the stops. If there is some major disagreement along the way, you can have a discussion, but unless the deco captain is having some kind of arithmetic problem, there wouldn't be a major disagreement.
If you are doing dives with similar profiles a lot (which you are likely to do) you'll get to the point where you don't even have to do the RD calculations, because you know what the profile for 150 for 20 is, because you have done it before.
Mr Carcharodon
Contributor
franny,
You can run up to dive plan limits either due to gas supply or decompression considerations. Planning in advance requires that you know what factor is going to drive the limits. If you just follow the computer you do not what your deco limits are because in most cases the manufacturers do not let you know how the plan is derived. There are exceptions.
Diving the most conservative computer is a reasonable approach if you have no other basis to come up with a plan. But it is not much of a plan. Attaining the ultimate decompression safety is really very simple -- just do not get in the water. But most divers would think that would be an unbalanced and unreasonable approach. At least with VPM, RD and GF we know what the plan is and can adjust it for risk factors that the computer does not know about.
Yount’s paper from 1976 was the start of the reducing bubble count by modifying ascent profiles: Yount, D.E. and Strauss, R.H. 1976. Bubble formation in gelatin: A model for decompression sickness. J. Appl. Phys. 47:5081-5089.
If that one is not readily available to you a really good place to start is at decompression.org. The papers by Erik Baker tend to be particularly lucid.
You can run up to dive plan limits either due to gas supply or decompression considerations. Planning in advance requires that you know what factor is going to drive the limits. If you just follow the computer you do not what your deco limits are because in most cases the manufacturers do not let you know how the plan is derived. There are exceptions.
Diving the most conservative computer is a reasonable approach if you have no other basis to come up with a plan. But it is not much of a plan. Attaining the ultimate decompression safety is really very simple -- just do not get in the water. But most divers would think that would be an unbalanced and unreasonable approach. At least with VPM, RD and GF we know what the plan is and can adjust it for risk factors that the computer does not know about.
Yount’s paper from 1976 was the start of the reducing bubble count by modifying ascent profiles: Yount, D.E. and Strauss, R.H. 1976. Bubble formation in gelatin: A model for decompression sickness. J. Appl. Phys. 47:5081-5089.
If that one is not readily available to you a really good place to start is at decompression.org. The papers by Erik Baker tend to be particularly lucid.
OP
I have enjoyed all of the responses and learned some stuff.
My original question was really much simpler than has been discussed but I appreciate the further depth it has gone.
I was not intending to use any combination of computer average and tables to determine any deco for a given dive. in fact I was really only considering Rec, dives or I would have posted this in the tech section.
a simple scenario, I have all my dives downloaded to my pc from my dive computer the night before and at breakfast I smack my dive computer on the railing to the dining hall
. now I have to go buy another, or get out my tables.
I am willing to take some risk as I am not going to wait 24 hours to dive. so my choice is to go back to my room, get out my PC and recreate the last 24 hours.
Obviously I will have average depth, and a graph of how it was done. so maybe i take the average depth for the first 3 dives and then spend some time fitting the rest using multilevel techniques. to get a reasonable approximation for what PG I am at.
Now some people are going to have an embolism with that, please know, I have been using tables awhile and computers awhile, I'm OK with the math and the theory.
seems to me, this is not an impossible task and would not result in instant death, or at least not any more than is already likely using dive theory to start with.
My original question was really much simpler than has been discussed but I appreciate the further depth it has gone.
I was not intending to use any combination of computer average and tables to determine any deco for a given dive. in fact I was really only considering Rec, dives or I would have posted this in the tech section.
a simple scenario, I have all my dives downloaded to my pc from my dive computer the night before and at breakfast I smack my dive computer on the railing to the dining hall
. now I have to go buy another, or get out my tables.I am willing to take some risk as I am not going to wait 24 hours to dive. so my choice is to go back to my room, get out my PC and recreate the last 24 hours.
Obviously I will have average depth, and a graph of how it was done. so maybe i take the average depth for the first 3 dives and then spend some time fitting the rest using multilevel techniques. to get a reasonable approximation for what PG I am at.
Now some people are going to have an embolism with that, please know, I have been using tables awhile and computers awhile, I'm OK with the math and the theory.
seems to me, this is not an impossible task and would not result in instant death, or at least not any more than is already likely using dive theory to start with.
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