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Sample Decompression Comparisons


Decompression Comparison between Departure, VPM and RGBM

Both Departure, VPM and RGBM use bubble models and algorithms to calculate their decompression stops.  This article was written to show both some similarities and some differences between these different models.  Departure has the ability to run some profiles similar to both VPM and RGBM models as well as allowing a diver to make the model more conservative (with both times and ascent methods) while still maintaining a bubble model approach to the modeling.

As is discussed in the Ascending From a Dive article, the first step in comparing dive tables, a decompression approach or software is to first determine the no-stop time limits to determine their conservatism.  To determine how conservative (or aggressive) a dive table or software is, one should investigate not the no-stop time limits at the deeper depths, but at the middle to shallower depths.  The reason no-stop time limits need to be compared is this is the starting point to get an idea of the allowable gas tensions allowed in the body upon surfacing from a dive.  After evaluating the no-stop time limits, the next step is to evaluate how a model allows a diver to ascend through the decompression stops - which is presented below.  Below are some comparisons between the time limits given by both Departure, RGBM and VPM.  The VPM comparisons were chosen with V-Planner and the RGBM models were generated by GAP.

V-Planner is modeled after the VPM model with their VPM-B model having some more conservative adjustments to the model.  While some individuals may disagree, the VPM model is very similar to the RGBM model since the RGBM model is simply an extension of the VPM model with some different factors.  As can be seen, starting at 60 feet and deeper, Departure offers the flexibility of having either more conservative or more aggressive time limits than V-Planner, thus giving the diver a choice in how to plan their dive.  The interesting thing to note are the shallow time limits of V-Planner.  The shallower the time limits, the more they deviate from "accepted" time limits.  For example, it is well known that a saturation depth (minimum bends depth) is shallower than 30 feet (and is closer to 20 feet), but V-Planner allows for such a dive to occur.  This is not limited to V-Planner and it's VPM model.  The same thing occurs with the RGBM model.  While a diver may never do such a dive, it shows that the model is extremely aggressive and allows for some very high nitrogen/gas tissue tensions upon surfacing, especially in the "slower compartments".  Departure is able to stay within the more accepted no-stop time limits and still have a bubble algorithm (as shown below).

 

 

V-Planner

Departure

 
  VPM-B
Nominal
VPM-B
+ Four

1.00

1.20

1.30

1.50

2.00

DCIEM
(1992)

Bassett

Depth
(feet)
MBD:30' 23' 20' 19' 18' 17' 15'    
25
30
35
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
99999
99999
1291
322
99
53
33
24
18
15
12
10
8
6
5
5
4
4
14549
556
207
118
58
36
25
18
14
11
9
7
6
5
5
4
3
3
521
300
201
146
89
60
43
32
25
20
16
13
11
9
8
7
6
5
437
264
178
131
80
55
40
30
23
18
15
12
10
8
7
6
5
4
409
244
167
123
74
50
36
27
21
17
14
11
9
8
7
6
5
4
342
211
146
108
66
45
32
24
19
15
12
10
8
7
6
5
4
4
226
146
103
77
47
32
23
17
13
10
8
7
5
4
4
3
3
3

300

150
75
50
35
25
20
15
12
10
8
7
6
6
5
5

220

120
70
50
40
30
25
20
15
12





 
Note:
V-Planner has five different sets of time limits when using the VPM-B model. The nominal setting is the most aggressive and the + Four setting is the most conservative setting of V-Planner.  The above V-Planner time limits are based upon the following: Version 3.22, dive altitude is sea level, post dive altitude is 0 feet, the water is sea water, the descent rate is 150 feet per minute, the ascent rates are 30 feet per minute.

Departure offers 100 different time limit settings in the form of a sliding scale in addition to being able to completely customize the time limits you wish to model.  The 1.00 setting gives the least conservative times available. The 1.50 is the middle set of time limits available and the 2.00 is the most conservative set of time limits available. The time limits are based upon: a sea level dive, a post dive altitude of 0 feet, a descent rate of 150 feet per minute, and sea water for the dive.

While there are more modern time limits, Dr. Bruce Bassett’s are presented as they are consistent with other time limits and because we wished to give recognition to Dr. Bassett’s early involvement in sport diving.

The next step in evaluating a model is to see how the model brings a diver through the decompression stops to the surface.  Any model that does not have a true bubble algorithm should not be considered.  Both VPM and RGBM have true bubble algorithms.  Such a bubble model requires "deep stops".  For some information on why deep stops are important, visit the Ascending From a Dive article.  While the article is overly simplistic, it is a good start to understanding the importance of deep stops and a bubble model.  While V-Planner comparisons are being shown below, the discussion here is really more about the VPM model as a whole, and also about the RGBM model.  To it's credit, V-Planner uses a new concept of incorporating  Boyle's law to the VPM model (hence the term VPM-B ... for Boyle).  V-Planner also has the original VPM model without the Boyle's law adaptation.  Departure was the first commercially available decompression software with a bubble model that calculated deep stops.  Departure uses a different approach than the VPM model, but still generates profiles similar to VPM or RGBM.  Departure can also be used to generate some more conservative profiles and keep a diver a little longer at decompression depths.  The VPM and RGBM models attempt to decompress a diver to keep bubbles at an "efficient maximum".  In other words, bubbles occur, but do not get "too big" by being kept at a size which does not let the bubbles grow or expand.  The big question though is whether the VPM and RGBM models do what they intend to do or whether bubbles do in fact expand.  This is one reason Departure has decided to stay with it's proprietary bubble model, since it's concept is to keep bubbles smaller verses an "efficient" or an "allowable" maximum.  It is well known that a diver can have "silent" or asymptomatic bubbles after a dive, whether it is within :no-stop" times or a mandatory decompression dive.  The goal is not to just decompress a diver to where they are asymptomatic from the bends or DCS/DCI, but instead to not let a diver get too many or too large of bubbles, even if the diver doesn't feel bent.  A hard lesson learned are those from older commercial divers where they are now experiencing dysbaric osteonecrosis or bone degeneration.  There have also been studies showing that divers have lesions on the brain that non-divers do not have and that divers short-term memory may be affected or compromised as compared to non-divers.  So the goal is to lessen the impact of bubbles so that long term health consequences are lessened and hopefully prevented.  So if the goal is to maintain health as a diver ages, then a model that attempts to minimize bubbles should be taken.  If on the other hand a diver wants to throw caution to the wind and just not get bent, there are models out there that will decompress a diver quickly with the added chance of the diver getting bent.

The main difference between the VPM and RGBM models and that of the Haldanian/Buhlmann type models (in addition to having deep stops) is that M-values (a set maximum allowed gas tension per tissue compartment) are not used.  The M-values are constantly changing with these models.  One way to describe the process is to state that the M-values are relaxed when a diver does deep stops.  This is brought up since Departure allows a diver to take advantage of a relaxed M-value approach to get "credit" for doing deep stops, or the diver may stick to the fixed M-value approach (which is more conservative).

The best way to understand the differences between the VPM and RGBM models and Departure is just to do some comparisons.  VPM is discussed first with some RGBM examples being given later.  Many examples are given so that one can understand the differences between the VPM model and Departure.  The examples below are sea level dives.

There are many options available in Departure.  In addition to being able to truly customize your no-stop time limits desired, a diver can also select how conservative or aggressive they want their ascent through the decompression stops to be.  The most conservative approach is the minimized bubble model.

Departure vs. RGBM and VPM-B's Nominal modeling

The below example uses a no-stop time limit setting of 1.00 in Departure (the most aggressive) compared to the VPM-B and the RGBM models in the Nominal setting.  For a 200 foot dive, the following ascent times are seen (which include the time to the first stop):  Of course, all dives with all the programs can be made more conservative.  The nominal settings were just chosen for comparison purposes.

200 ft. 18/45
Deco:    EAN 50
            O2

TBT:

10

15

20

25

30

RGBM:

18

27

36

45

54

VPM-B:

16

25

33

43

53

Departure:

17

28

38

46

55

For the 200' profiles, two examples are shown.  The first one is a  for a bottom time of 15 minutes.  Here it easy to see the ascent patterns for all models.

the diver longer at 20 feet as compared to the 10 foot stop to keep the diver under more pressure - which keeps any bubbles smaller to increase off-gassing.  The trend of Departure keeping the diver under more pressure at the shallower stops will be noticed in all profiles.  You will also notice the trend with RGBM in that it makes its stops a little deeper.  Later you will see that RGBM also starts surfacing the diver sooner.

250' Comparisons

In this profile, Departure's trend of keeping a diver under more pressure at the shallower stops is still seen.

250 ft. 15/55
      Deco:   35/25
                 EAN 50
                 O2

TBT:

10

15

20

25

30

RGBM:

26

39

51

65

77

VPM-B

26

39

54

66

78

Departure:

29

43

55

69

82

300' Comparisons

In the following examples, it is again sen that all models yield similar profiles with Departure keeping more pressure on the diver which helps lessen any bubbles for more efficient off-gassing.

300 ft. 10/70
      Deco:   35/25
                 EAN 50
                 O2

TBT:

10

15

20

25

30

RGBM:

50

77

97

122

156

VPM-B

50

79

103

124

150

Departure:

52

78

102

130

159

In this profile, Departure's trend of keeping a diver under more pressure at the shallower stops is still seen.

350' Comparisons

350 ft. 10/70
      Deco:   21/35
                 35/25
                 EAN 50
                 O2

TBT:

10

15

20

25

RGBM:

51

76

100

126

VPM-B

59

89

115

140

Departure:

58

87

118

148

400' Comparison

400 ft. 10/70
      Deco:   18/35
                 21/35
                 35/25
                 EAN 50
                 O2

Changing the Ascent

Departure gives many options.  One of them is the ability to make the ascent through the decompression stops less aggressive.  Again, this is to keep bubbles smaller during the decompression.  An example of this is shown below.  This is the same 300 foot profile above, but with a reduced ascent of Departure being shown.  In this example, reducing the ascent only added 4 more minutes of decompression.  In addition to being able to modify the ascent, the amount of allowable gas load can also be reduced to make profiles more conservative.

Wrapping It Up

What the above shows is that Departure can generate profiles similar to either the VPM or RGBM models or allow the diver to stay a little longer at depth for their decompression if desired.  Additionally, as shown at the beginning of this article, the VPM and RGBM models allow some extreme no-stop time limits for the shallow depths which may allow "too much" of a gas load in the "slower" compartments.  Departure's decompression modeling assumes more realistic time limits for the shallow depths and thus acknowledges that the "slower" compartments are not allowed to have as much of a gas load/tension upon surfacing.  While any model can result in decompression issues, making it slightly more conservative may lessen the chance of a diver getting bent.  However, just adding some conservatism may not adequately address other issues (such as dysbaric osteonecrosis) that can arise from significant asymptomatic bubbling.  Departure requires a diver to decompress slightly deeper than other models in an attempt to lessen bubbling.  It is also encouraged that a diver choose a more moderate approach when planning their decompression and not just take the approach of surfacing rapidly.  It is not enough to be symptomatic free from the bends, it is important to reduce bubbling as well as the amount of gas upon surfacing from a dive to lessen other health issues ... so dive conservatively, perform deep stops, and don't rush your decompression.



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