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Tables and Diving time
- Thread starter Manogr
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hphobbie@aol.com
Guest
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Large: this is Actual Bottom Time. The time from start of descent until start of direct ascent.
Total Bottom Time is the sum of the Actual plus the Residual Nitrogen Time, and is used on any repetitive dive.
Residual Nitrogen Time is the time in minutes a diver is to consider he has already spent on the bottom when planning a repetitive dive.
On the PADI Recreational Dive Planner, RNT is the numbers in white background on Table 3, while the ANDL (adjusted no-deco limits) are the blue background. I tell people to remember that blue means water and these tell you how long you can stay in the water.
AS it was already said, that when diving with table you keep it strict, I won't talk about THAT. What I AM going to explain is the reasoning behind it all-
First of all, a healthy Diving profile, when diving by tables, is a dive where you reach the deepest point fast, and then ascend slowly, when the time between the start of the ascent untill you reach 6 meters, is the ABT.
Reasons- It all has to do with the mathematical models the time table is based on. The table is based on the assumption that you first go to the deepest point. Furthermore- it also assumes that you do the deepest dive first. The reason is in the calculations behind the table, there where several calculations used, using several theoretical tissues (4 to 9 i think, depends on the table). To understand that better, you got to "dig" into this subject dip, but to shorten it up- the build-up of niterogen is different between tissues. there can be a situations (actualy, it happens in almost every dive deeper than 9 meters, that takes some time) in which some of your body tissues are releasing niterogen, while others are still absorbing niterogen. When you use a computer, it calculates all of it in reall time, so than it dosen't almost matter (though people may argue about it) if you do a reverse profile (go shalow than deep) during the dive, or from dive to dive.
P.S-
I know I might have explained it badly, but hey- I just tried.
First of all, a healthy Diving profile, when diving by tables, is a dive where you reach the deepest point fast, and then ascend slowly, when the time between the start of the ascent untill you reach 6 meters, is the ABT.
Reasons- It all has to do with the mathematical models the time table is based on. The table is based on the assumption that you first go to the deepest point. Furthermore- it also assumes that you do the deepest dive first. The reason is in the calculations behind the table, there where several calculations used, using several theoretical tissues (4 to 9 i think, depends on the table). To understand that better, you got to "dig" into this subject dip, but to shorten it up- the build-up of niterogen is different between tissues. there can be a situations (actualy, it happens in almost every dive deeper than 9 meters, that takes some time) in which some of your body tissues are releasing niterogen, while others are still absorbing niterogen. When you use a computer, it calculates all of it in reall time, so than it dosen't almost matter (though people may argue about it) if you do a reverse profile (go shalow than deep) during the dive, or from dive to dive.
P.S-
I know I might have explained it badly, but hey- I just tried.
large_diver
Contributor
Very good point. Since the scenario did not mention multiple dives, I was presenting a single dive scenario (Table 1) to keep the example simple
Warhammer
Guest
Just thought I'd add the DIR method to the mix. I'm in no way condoning it nor do I use it myself, but I thought it would be interesting in this discussion. Anyway, DIR divers use tables and calculate their BT based on average depth, instead of max depth. They track their depth every few minutes, then average those depths to obtain the number they use in figuring the NDL. The also use tables that are more aggressive than the PADI RDP. I believe the above is correct, but welcome any DIR divers that are with us to correct me if I'm wrong. Anyway, what do you guys think of this method?
And further more, do you think it could be safely used to continue repetitive dives after a computer failure? My Suunto computers keep up with average depths, so awesome that I can obtain that info from the computer, could I safely continue to dive without the computer using the above method? Please note that, if my computer dies, I'm done diving for 24hrs unless I have backup. I would just like to know your opinion is all.
And further more, do you think it could be safely used to continue repetitive dives after a computer failure? My Suunto computers keep up with average depths, so awesome that I can obtain that info from the computer, could I safely continue to dive without the computer using the above method? Please note that, if my computer dies, I'm done diving for 24hrs unless I have backup. I would just like to know your opinion is all.
Warhammer:
About the average depth-I dont know exactly the ways of DIR, but though average depth may seem a useless piece of data for the average recreational diver, it isn't. It has several usses, such as determining the deep decompresion stops when using modern models. It also is used in some calculations used by computer algorithms (I got a bit into that lately). It sometimes simplifys things.
However- what you said about the way DIR uses this sounds wierd, I am going to find out more about DIR pretty soon, heard about them to much lately
.
About the average depth-I dont know exactly the ways of DIR, but though average depth may seem a useless piece of data for the average recreational diver, it isn't. It has several usses, such as determining the deep decompresion stops when using modern models. It also is used in some calculations used by computer algorithms (I got a bit into that lately). It sometimes simplifys things.
However- what you said about the way DIR uses this sounds wierd, I am going to find out more about DIR pretty soon, heard about them to much lately
.
large_diver
Contributor
Hey Warhammer,
I'm no DIR expert, but I've been diving with a few DIR techies recently. For deeper tech dives (these guys are diving in the 130-175 FSW range), they are cutting custom tables using software from GUE (decoplanner). While these dives are square profile, deep wreck dives, they are not just cutting one table -- they are cutting multiple tables to allow for deviations in their dive plan.
I should also mention that these are obviously decompression dives.
So I guess in a sense they are giving themselves "credit" for being at shallower depths vs. strict adherence to tables as discussed above. BUT -- they are doing this with the use of sophisticated software and multiple personalized tables...not simply rough estimations.
I don't have any knowledge of their usage of "average" depth.
Also -- DIR-purists do not use computers -- they use timers only along with these customized tables.
Hopefully someone with more in depth DIR knowledge will step in here........
-LD
I'm no DIR expert, but I've been diving with a few DIR techies recently. For deeper tech dives (these guys are diving in the 130-175 FSW range), they are cutting custom tables using software from GUE (decoplanner). While these dives are square profile, deep wreck dives, they are not just cutting one table -- they are cutting multiple tables to allow for deviations in their dive plan.
I should also mention that these are obviously decompression dives.
So I guess in a sense they are giving themselves "credit" for being at shallower depths vs. strict adherence to tables as discussed above. BUT -- they are doing this with the use of sophisticated software and multiple personalized tables...not simply rough estimations.
I don't have any knowledge of their usage of "average" depth.
Also -- DIR-purists do not use computers -- they use timers only along with these customized tables.
Hopefully someone with more in depth DIR knowledge will step in here........
-LD
The software they are using probobly uses newer decompression models (such as RGBM or VPM)that allow for several things that older, Haldanian based models, will not allow. It suports also "Deep stops", which are decompression stops done deeper than than the traditional 3' 6' 9' etc dives, as deep as 100' , even more, depends on the scenario. Those methods are originaly used by comercial divers, but it is slowly getting into to tech divers, and will, probobly end up with recreational divers as well. The traditional tables, are simply not enough.
IF you read LDs post closely he mentioned that they are using Decoplanner from GUE. Decoplanner is a modified haldanian deco program. It incorporates Eric Bakers methods for deep stops and does not use VPM or RGBM.
STAY AWAY FROM VPM DECOS. VPM has been tested on Jell-O, Rats, and Salmon. If you'd like to be the next guinea pig, then step up and pet the deco pony. An implementation of RGBM is available as part of Abyss dive planning software. You will find that when using RGBM that the deep stops are pretty good while it seems to add a long time to the shallow stops.
When using decoplanner I modify the stops based on experience. I start the deep stops at about 80% from the bottom. The minimum deep stop is 20 seconds at each ten feet, which is effectively 30 feet per minute plus the moving time. I also set the helium content in decoplanner about 10% less than actual (if using 20/40 I plan with 20/30). I throw out the 10 foot stop, and just use the same amount of bottom time for the stop and ascent from 20 to the surface. (i.e. if the bottom time was 25 minutes than the stop at 20 feet and the ascent from 20 to 0 would be 25 min).
STAY AWAY FROM VPM DECOS. VPM has been tested on Jell-O, Rats, and Salmon. If you'd like to be the next guinea pig, then step up and pet the deco pony. An implementation of RGBM is available as part of Abyss dive planning software. You will find that when using RGBM that the deep stops are pretty good while it seems to add a long time to the shallow stops.
When using decoplanner I modify the stops based on experience. I start the deep stops at about 80% from the bottom. The minimum deep stop is 20 seconds at each ten feet, which is effectively 30 feet per minute plus the moving time. I also set the helium content in decoplanner about 10% less than actual (if using 20/40 I plan with 20/30). I throw out the 10 foot stop, and just use the same amount of bottom time for the stop and ascent from 20 to the surface. (i.e. if the bottom time was 25 minutes than the stop at 20 feet and the ascent from 20 to 0 would be 25 min).
Omar-
I suport what you said about VPM. I think it hasn't been tested throughly. However, I did some testing with it, and ran some scenarios with several vpm programs against Hldanian algorithms, RGBM (in my vyper) and a new algorithm I am working on, and the vpm's results were quite good. I went also through the formulas they use and those seem to be quite sensible. However, I wouldn't dive according to VPM alone. Only with backup tested software. Also, I belive that in comercial diving, VPM based programs were used, for a time, but as they are very restrickted (a lot of factors that go into RGBM wont go in there) It wasnt very usefull, and i think they were all abandoned.
I do belive though, that it is better than most pure Haldanian models.
I suport what you said about VPM. I think it hasn't been tested throughly. However, I did some testing with it, and ran some scenarios with several vpm programs against Hldanian algorithms, RGBM (in my vyper) and a new algorithm I am working on, and the vpm's results were quite good. I went also through the formulas they use and those seem to be quite sensible. However, I wouldn't dive according to VPM alone. Only with backup tested software. Also, I belive that in comercial diving, VPM based programs were used, for a time, but as they are very restrickted (a lot of factors that go into RGBM wont go in there) It wasnt very usefull, and i think they were all abandoned.
I do belive though, that it is better than most pure Haldanian models.
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