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Ascending From A Dive


Ascending From A Dive
(Ascent Rates, Deep Safety/Deco Stops, Time Limits & more)

by
Brian R. Morris


Copyright (c) 2000-2011 by Brian R. Morris.  All rights reserved.

Intro

    If you are reading this page, it is also recommended that you read Decompression Strategies to add to your understanding of how to plan and execute a safer dive.  Contrary to old school thinking that decompression sickness can be random, you do have a lot of control over the outcome of a dive.

    The articles that follow are overly simplistic and are for illustrative purposes only.  There are many different models including different models that account for bubbling.  When the term bubbling is used it generally refers to "excessive bubbling" and not minute bubbling as there is such a concept as efficient bubbling.  What also must be kept in mind is that decompression sickness and illness (DCI) can be unpredictable and can occur after any dive, even a conservative dive.   Therefore divers must take responsibility for themselves and dive conservatively to lessen (but not eliminate) their chances of getting DCI.  With this being said, let's start with . . .

Deep Safety and Deco Stops

    It has long been known that deep stops benefit divers performing decompression dives.  The same rationale behind deep decompression stops holds true for safety stops.  What technical divers know, but many recreational divers don't, is that all dives are decompression dives whether the dive requires a mandatory stop or not.  The reason all dives are decompression dives is simple - the diver is ascending from under pressure (i.e. decompressing).  What must be kept in mind is that even though a dive is within NoStop time limits, the diver is probably experiencing asymptomatic bubbles (minor bubbling that does not cause DCI).   This is not unusual and causes no harm since the body can be forgiving.  This brings us to our first topic . . .

    Why Safety Stops?

    Every diver has been told to perform safety stops, yet the vast majority of divers I see never do one.  Why?  Probably because they are not understood.  Many divers have been informed that performing a safety stop allows them to slowly eliminate nitrogen.  This is simply not true.  A safety stop assists the body in rapidly eliminating nitrogen!  Why?  The reason is very simple.  Bubbling does not occur in a diver under pressure, it only occurs when the pressure is reduced "too much".  Once bubbling occurs, gas elimination is reduced.  This is because the driving force for nitrogen elimination has diminished.   The bubble now has to be reabsorbed - which can be a time consuming process.  However, if bubbling can be minimized or prevented, then the nitrogen stays in a gaseous solution (i.e. dissolved) in the body.  If this can be maintained while surfacing, then the nitrogen pressure is maintained while the pressure surrounding the diver is reduced.  When the nitrogen pressure is allowed to stay greater than the surrounding pressure, a driving force for nitrogen elimination is created and nitrogen will "zing" out of the body.   To put it even simpler, a diver that performs a 3 minute safety stop after a dive will have less nitrogen in their body immediately upon surfacing as compared to a diver that did not perform a safety stop, but has been on the surface "off-gassing" for 3 minutes.  Therefore, one of the best things a diver can do for themselves is to perform a safety stop, no matter how short, after every dive.

    How Deep?

    Since the goal is to promote nitrogen elimination prior to surfacing so the risk of DCI is lessened, the next question is how to calculate the depth of a safety stop.  Years ago,  a safety stop was recommended at 10 feet after every dive.  This recommendation wasn't so much a depth recommendation other than a recommendation to just perform a safety stop.  The depth of 10 feet was chosen because the final stop on decompression tables was at 10 feet.   Later, the recommendation turned to 15 feet and is now currently 15 to 20 feet for 3 to 5 minutes.  A deeper depth of 15 feet was chosen for several reasons, but a driving force behind this deeper recommendation was overwhelming information that deeper stops promote greater nitrogen elimination.  My favorite example was a fairly well controlled study where two control groups were taken to the same depth for the same time and then performed safety stops for the exact same time, but at different depths.  One group performed a safety stop for 5 minutes at 10 feet while the other group performed a 1 minute stop at 20 feet and then a 4 minute stop at 10 feet.  Even though both groups did 5 minutes of safety stops, the group that started at 20 feet had significantly less bubbling not only upon surfacing, but also over the hours that followed the dive.   While this violates standard half-time decompression theory, the evidence is in . . . deeper is better.  But how deep?  Again, one of the common recommendations is 15 feet.   A rule I like to live by is "keep it simple" and picking one single depth that a diver can remember definitely keeps it simple and makes it easy to perform.  However, when it comes to a simple task such as picking a safety stop depth, I like to give divers a little more credit and believe that the main difference between sport divers and technical divers is the amount of information each has been exposed to.  Therefore, I believe that since technical divers perform decompression stops over multiple depths, so can sport divers especially considering that these stops are not mandatory, but merely safety stops.   Therefore I don't recommend a single depth.  Instead I recommend, performing safety stops over a range of depths - which is extremely easy to do.  The only calculation that needs to be done is to pick the depth of the first safety stop.  All that is needed is a rule of thumb.  The rule I use is to take your deepest depth of the dive and divide it by one-half and round to a deeper depth if necessary.  For example, after performing a dive to 90 feet, the depth of the first stop would be 45 feet - which could also be rounded to 50 feet if desired.  This calculation can be performed easily in the diver's head while underwater.  The question is . . .

    How Long?

    For the first stop, 2 minutes is recommended and has been shown to be very beneficial.  The next step is to calculate where to go from here and for how long.  The answer is come up another 10 feet and stay there for 1 minute until 15 feet is reached.  A 20 foot stop is also recommended needed no matter what the profile is.  When diving deeper than 80 feet, at least a 2 minute stop at 20 feet should be performed.  Once you reach 15 feet, stay there for 2 to 5 minutes or longer . . . whatever you desire.  A sample profile looks like this.  After a NoStop dive to 100 feet for 15 minutes, ascend to 50' for 2 min., then go to 40' for 1 min., then 30' for 1 min., then to 20' for 2 min., then 15 feet for 2 to 5 minutes.  Another example is a dive to 90 feet.  Since the first stop is to an odd depth of 45, perform a 2 minute stop at 45' then 1 min at 35', 1 min. at 25', 1 min. at 20', then 3 to 5 minutes at 15 feet.  Once you finish your desired time at 15 feet, slowly come to the surface.  Creep your way up and maybe stop at 5 to 10 feet on the way up for 30 seconds to a minute.

    More detail and information is given in the Getting More Advanced section at the end of this page, including tables, information on gas management and multilevel dives

    Nitrox and Altitude

    If you are diving on Nitrox, then you may use the Equivalent Air Depth of your dive to calculate the depth of your first stop and then follow the above procedure until you reach an actual depth of 15 feet.  If diving at altitude, while you need to plan your diving using altitude procedures and Equivalent Ocean Depths (EOD's), you do not need to think in terms of EOD's when calculating safety stops.  You need take only your actual depth (assuming you have an altitude adjustable depth gauge) and follow the same procedure until you reach the EOD of 15 feet at that altitude.   For example, if you dive to an actual depth of 80 feet at Lake Tahoe (an elevation of 6229'), your first stop would be at an actual depth of 40 feet, your second stop would be at 30, then to 20 feet and your last stop would be at 12 feet (Lake Tahoe's equivalent ocean depth of 15 feet).

    If you haven't really performed safety stops before or haven't paid much attention to them, the procedure I just described may sound boring.  But try it.   I don't think you will find it boring.  When the stops are actually broken up into different depths, the time passes very quickly.  If you also understand why safety stops are one of the best things you can do to reduce the risk of DCI, it shouldn't seem boring and a waste of time, especially when compared to the time that could be spent in a recompression chamber and knowing that you will be surfacing with less nitrogen in your body as compared to not performing a stop. 

    Tech Divers

    While there is no rule of thumb for performing decompression stops, the above rationale for deep initial decompression stops apply.  Most tech divers are familiar with Pyle Stops - the suggestions by Richard Pyle on how to add deep stops to current dive tables.  While these suggestions are applauded and were much needed, tech divers need to go beyond these suggestions.  There are different models that perform these calculations such as using a gradient factor.  However, this method may not calculate the first stop as deep as this author would prefer.  For this and other reasons, this author designed a different method of calculating decompression stops and designed a computer software program around this called Departure.  For more information on this program, please visit Departure's web site.

Ascent Rates

    Sport Diving

    Different dive tables use different ascent rates.  In the U.S., most divers are accustomed to a 60 foot per minute ascent rate.  Unfortunately a lot of divers come up much faster than this.  In fact, a 60 foot per minute ascent rate would seem slow to many divers if they actually measured and timed themselves during an ascent.  This is an important topic to discuss - which leads into advocating for an even slower standard ascent rate.  First a little history.  The 60 foot per minute ascent rate originally came from the U.S. Navy.  The original Navy Tables used a much slower ascent rate of 30 feet per minute.  However, the divers did not like this ascent rate as they found it too slow.  So they requested a much faster ascent rate of 100 feet per minute.  A compromised was then reached of 60 feet per minute and it stuck.  However, the majority of dive tables (not all) use slower ascent rates and 10 meters (33 feet) per minute is common in other countries.  The U.S. Navy has now reduced their ascent rate back to 30 feet per minute.

   So what should the ascent rate be?  If just one ascent rate is chosen, then it should be 30 feet per minute.  But for those that want to be more technical and advanced, two ascent rates can be used.  When diving deeper than 60 feet, use an ascent rate of 30' per minute.  And when diving to 60 feet or less, use an ascent rate of 20 feet per minute.

    So now that we know what the ascent should be, how can we actually perform these slower ascents.  Ascent rates can be viewed as "mini decompression stops" and can be performed in stages.  There are two ways to perform this ascent rate in a very easy manner.  The first is to come up 10 feet and wait until the time catches up.  In other words, for an ascent rate of 30 feet per minute, it should take 20 seconds to come up 10 feet.  For example, if it takes the diver 5 seconds to come up 10 feet, the diver then waits another 15 seconds before going up another 10 feet.  The ascent rate between the different 10 foot "mini stops" isn't overly critical as long as one does not ascend more than 10 feet at one time and does not "rocket" to the stop.   However a more uniform ascent is always better - which is why I prefer the next recommendation.  The same principal of ascending and waiting can be applied to breaking your ascent into 5 foot segments instead of 10.  So again for a 30 foot per minute ascent rate, simply ascend 5 feet an wait until 10 seconds elapses and for a 20 foot per minute ascent rate, simply ascend 5 feet and wait until 15 seconds has elapsed (or 30 seconds every 10 feet).  With some practice, you will be able to get very close to the desired ascent rate and then just using every 10 feet as a "check" point.

    After reading the above section on deep stops, you now can draw the analogy that the deep 1 minute stops are nothing more than drastically slowing your ascent rate down (i.e. to 10 feet per minute) during the final portion of your ascent.  Likewise, slowing your ascent down is analogous to performing very brief "mini stops" on your way up.

NoStop Time Limits

    The last topic is modeling and choosing NoStop time limits.   While there is consistency among dive tables in the "mid range to deeper depths", there can be a lack of consistency in the shallow depths.  While most divers never reach the NoStop limit on a shallow dive, the time limits can play a crucial role when it comes to repetitive diving and when modeling and calculating dives requiring mandatory decompression stops.  The reason for the inconsistency in the shallow depths is the way in which these time limits are modeled or not modeled. No-stop time limits follow a pattern. If time limits are graphed on log-log coordinates, the time limits appear to be a straight line. However, the time limits deviate off this straight line at the shallow depths. The reason for this deviation is the time limits start approaching infinity as the depths get shallower and approaches the "minimum bends depth." The minimum bends depth (MBD) is the depth at which there is no time limit - the depth at which a diver may "saturate" and immediately surface.

    Many time limits at the shallow depths have been calculated with educated guesses to take into account the shallow depth deviations.  Instead of educated guesses, I was able to come up with a model that predicts and models time limits at all depths including the shallow depths and their deviations.  This model was derived by combining three different decompression models and theories.  It calculates time limits when the following is plugged in: 1) the desired minimum bends depth, 2) the desired model slope on the log-log coordinates, and 3) a desired time limit at a known depth.  This model results in time limits that maybe more conservative at the shallow depths than divers are used to. This is because this model does not show as much of a deviation at shallow depths as others allow.  In fact, comparison of time limits at shallow depths is a good way to start evaluating tables, programs and models. While a brief safety stop can add a margin of conservatism after a deeper dive, a brief safety stop after a long shallow dive may not yield the same level of conservatism.   Therefore, when safety stops are performed, the time limits at the shallow depths may be more critical than the time limits at the deeper depths (within reason).  This also shows the importance of not deviating in too radical a manner at the shallow depths as too much of a deviation could result in a decompression profile that is not long enough. Many divers look at only the time limits at the deeper depths when evaluating tables and will notice a difference of 1 to 2 minutes at 130 feet, but do not notice a difference of 60 minutes or greater at 35 feet – which may actually be the more critical measurement. The reason the time limits used are so important is they are a product of the decompression model used and show the conservatism level of the model itself.  So don’t forget to critically evaluate the shallow time limits for two reasons 1) because a safety stop may not be as beneficial after a long shallow dive as compared to a deep short dive, and 2) because time limits at the shallow depths may be calculated by educated guesses.

    Below are some example time limits showing consistency at the middle to deeper depths, yet big differences at the shallow depths depending upon what MBD is chosen for the model.  The times in the first column were modeled with a minimum bends depth of 20 feet and a time limit of 55 minutes at 60 feet (a common table depth,  time and MBD).  The rest of the times were modeled with different minimum bends depths around a time limit of 50 minutes at 60 feet (the depth and time I like to model).  The times generated can be compared to different dive tables and computers to compare conservatism, especially at the shallow depths.   Whatever tables are being used, the time limits should be considered MAXIMUMS and should not be approached.  While reducing a time limit by 5 minutes on a deep dive might be considered conservative, reducing the time limit at a shallow depth by 5 minutes may not add much conservatism at all.  A better approach is to view reducing time limits by a percentage such as by 10 or 20 percent.  In addition to reducing the time limits, safety stops should always be performed.

  Minimum
Bends Depth:
             
    20'   20'   15'  
  Depth   No-Stop Time Limits
(in min. at sea level)
 
  (feet)    
  30   275   251   222  
  35   185   169   157  
  40   135   122   117  
  50   81   74   73  
  60   55 50 50  
  70   39   36   36  
  80   30   27   27  
  90   23   21   21  
  100   18   16   16  
  110   15   13   13  
  120   13   11   11  
  130   10   9   9  
Copyright © 2000 by Brian R. Morris
diverssupport.com

Note: A starting minimum bends depth of 20 feet was chosen because this is the general consensus of what the depth is even though DCI has occurred at this depth.  Times were also given down to a minimum bends depth of 12 feet for example purposes.  What the above times in the last column do mean is that more conservatism is reached in the shallow depths where safety stops may not be as effective as they are at the deeper depths.  But again, safety stops should always be done even with conservative dive planning.

Getting More Advanced

    The previous simple rule of dividing your diving depth in half to find the first safety stop depth can be made more exact as well as more beneficial.  Below are charts showing the first stop depths.  You will notice some rule of thumbs that can be used in calculating the first deep safety stop. 

Generally:
Air:
80' dive and deeper, the first stop is one-half depth plus 10'
Less than 80' the first stop is one-half max depth.


Nitrox 32:
Can do stops as if on air, can use an Equivalent Air Depth for your diving depth to calculate the first stop, or may do the following:
80' dive and deeper, first stop is one-half depth; and
less than 80' the first stop is one-half max depth minus 5'.

Chart 1

Air
Depth 1st Stop
up to 25'
30' to 35'
40'
45'
50' to 55'
60'
65' to 70'
75'
80''
85 to 90'
95'
100' to 105'
110'
115' to 125'
125' to 130'
10' to 15'
15'
20'
25'
30'
35'
40'
45'
50'
55'
60'
65'
70'
75'
80'
Nitrox 32
Depth 1st Stop
up to 35'
40' to 45'
50' to 55'
60'
65' to 70'
75'
80' to 85'
90' to 95'
100'
105'
115'
120'



10' to 15'
15'
20'
25'
30'
35'
40'
45'
50'
55'
60'
65'



Copyright © 2010 by Brian R. Morris
diverssupport.com

 

Air Dives to 70' or deeper:
A 20' stop is always required.

Air Dives to 80' or deeper:
Two minutes at 20' is always required.

Air Dives to 100' or deeper:
Instead of doing 2 minutes at the first stop, a diver may do a 1 min. stop at first depth and then ascend 5' to do the 2nd minute of the initial stop.
This should not be done when diving to less than 100'.

Sample safety stop profiles
A diver may customize stops any way they like especially increasing the time of the stops 30 feet and shallower and/or adding a 25' stop.

55' dive . 110' dive   110' dive
Stop Depth Time . Stop Depth Time or Stop Depth Time





30'
20'
15'
10'
5'





2 min
1 min
2 + min
30 + sec
15 + sec
.
.
.
.
.
.
.
.
.
.

70'
60'
50'
40'
30'
20'
15'
10'
5'

2 min
1 min
1 min
1 min
1 min
2 + min
2 + min
30 + sec
15 + sec
70'
65'
55'
45'
35'
25'
20'
15'
10'
5'
1 min
1 min
1 min
1 min
1 min
1 min
2 + min
2 + min
30 + sec
15 + sec
1st stop: 2 min .
.
.
.
.
.

Dive is greater than 80'
so 2 min at 20' is required
  Dive is greater than
100' so can ascend 5'
after 1 min at first stop

Dive is greater than 80'
so a 20' stop is required

 

Chart 2

Example Safety Stops (Air)
Depth 20 30 35 40 45 50 55 60 70 75 80 90 95 105 110 120 130    
(feet)                                      
10 5 3 4 5 * * * * * * * * * * * * *   10
15   2 1 - 5 4 4 5 3 4 3 3 3 3 2 2 2   15
20     1 2 - 1 1 - 2 1 2 2 2 2 2 2 2   20
25         2 - - 1 - 1 - 1 - - 1 - 1   25
30           2 2 - 1 - 1 - 1 1 - 1 -   30
35               2 - 1 - 1 - - 1 - 1   35
40                 2 - 1 - 1 1 - 1 -   40
45                   2 - 1 - - 1 - 1   45
50                     2 - 1 1 - 1 -   50
55                       2 - - 1 - 1   55
60                         2 1 - 1 -   60
65                     1 1 - 1   65
70                             1 1 -   70
75                               1 1   75
80                                 1   80
* Brief stops should be made at 10' and 5' to slowly ascend
Copyright © 2010 by Brian R. Morris, diverssupport.com

 

Example Safety Stops (Nitrox 32)
Depth 30 35 40 45 50 55 60 70 75 85 90 95 100 105 115 120    
(feet)                                    
10 5 4 3 4 5 1 * * * * * * * * * *   10
15   1 2 1 - 4 5 4 5 4 5 4 4 4 4 4   15
20       1 2 2 - 1 - 1   1 1 1 1 1   20
25             2 - 1 - 1 1 - - 1 1   25
30               2 - 1 - - 1 1 - -   30
35                 2 - 1 1 - - 1 1   35
40                   2 - - 1 1 - -   40
45                     2 2 - - 1 1   45
50                         2 1 - -   50
55                           1 1 1   55
60                             1 -   60
65                               1   65
* Brief stops should be made at 10' and 5' to slowly ascend
Copyright © 2010 by Brian R. Morris, diverssupport.com
 

  Multilevel Dives

    When performing multilevel dives and working your way from the deep portion of your dive back to shallow portions and the surface, you should not forget to do the stops along the way and use the stops as a "ceiling" while traveling to the shallow portions.

  Gas Supply

   Of course a diver needs enough gas to not only do the dive, but to also ascend performing safety stops and with a reserve in case a dive buddy runs our of gas and needs to share.  Therefore it is important for a diver to know their breathing rate often referred to as RMV (Respiratory Minute Volume) or SAC (Surface Air Consumption).  Often it is assumed that a basic diver will have an RMV of 1.0 which means they will breath 1 cubic foot of gas per minute at the surface at a normal underwater exertion rate.  However, this should not be assumed and divers, especially newer divers can breath at a greater rate.  Assuming a breathing rate of with an SAC of 1.25 cubic feet a minute (experienced divers have a much lower SAC), the following chart shows how much gas is needed to ascend from a dive and perform the above safety stops with an 8 cubic foot (very minimal) reserve.

Depth (feet):

30

40

50

60

80

100

130

SAC: 1.25

 

 

 

 

 

 

 

Gas needed for ascent (cubic ft.):
(using stops in Chart 2 above
w/8 cubic foot reserve)

18

22

22

29

34

40

49

You will notice that you need to leave a deep dive with a significant volume of air.  This amount is greatly increased when you factor in a buddy running out of air at depth - which must always be assumed and included in any dive plan.  So a diver should always plan their dive time and gas reserve (the amount you start ending your dive with) as if a dive buddy will need to share air back to the surface.  When making out of gas (OOG) calculations, the stops should be shallower and longer.  This gets the buddy team closer to the surface in case a second gas failure occurs and an out of gas swimming ascent needs to be made.  The chart below shows the amount of gas needed for an OOG situation - which is the amount a diver needs to have when starting their ascent in case it is at that moment gas sharing is needed to surface.

Depth (feet):

30

40

50

60

80

100

130

SAC: 1.25

 

 

 

 

 

 

 

Gas needed for OOG ascent (cubic ft.):
(using stops in Chart 3 below
w/8 cubic foot reserve)

22

24

30

32

45

52

63

Chart 3

Example Out Of Gas Safety Stops

Depth

30

40

50

60

80

100

130

(feet)

 

 

 

 

 

 

 

15

3

3

2

2

2

2

2

20

 

 

2

2

2

2

2

25

 

 

 

 

2

2

 

30

 

 

 

 

 

 

2

Now that it is known that a dive must be planned around an OOG possibility, the amount of gas needed for such an ascent (the gas needed to start an ascent) can be calculated.  The calculation are beyond the scope of this article, but the chart below shows that a diver with an SAC of 1.25 should start his ascent from 30 feet with at least 865 psi in a common aluminum 80 cylinder (which really only has 78 cubic feet of gas).  The chart below shows why deep diving should only be done by experienced divers with proper training whose breathing rate is at a low comfortable level.   Inexperienced divers simply do not have enough gas in an aluminum 80 tank to do deeps dives with a proper reserve.

Depth (feet):

30

40

50

60

80

100

130

SAC: 1.25

 

 

 

 

 

 

 

Gas needed for OOG ascent (cubic ft.):
(using stops in Chart 3 above w/7.8 cubic foot reserve)

22

24

30

32

45

52

63

PSI reserve needed for ascent with a 3000 psi
"80" aluminum cylinder with a 300 psi reserve

865

930

1150

1235

1745

1985

2415

An experienced diver can have a lower gas consumption rate and an SAC of .75 cubic feet per minute can be easily achieved as shown in the chart below.  In such a case, the gas needed for the ascent is less and thus more gas is available for the dive.  But it must not be forgotten that the gas needed for the ascent needs to account for a possible higher breathing rate of a dive buddy. 

Depth (feet):

30

40

50

60

80

100

130

SAC:  .75

 

 

 

 

 

 

 

Gas needed for OOG ascent (cubic ft.):
(using stops in Chart 3 above w/7.8 cubic foot reserve)

17

18

21

22

30

34

41

PSI reserve needed for ascent with a 3000 psi
"80" aluminum cylinder with a 300 psi reserve

640

680

810

860

1170

1310

1570

Whatever time limits and safety stops you choose to use, keep in mind that you must take responsibility for your dive plan.  Decompression sickness has occurred on many conservative profiles.  Therefore plan conservatively, manage your ascent rate, gas supply and always perform a safety stop.

 

       For additional information on controlling decompression risk in diving, further explanations and education can be found at Decompression Strategies since we all know that any pressure reduction and ascent is a decompression.

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