This is the first in a series of comparisons between SAUL and other dive planners.   For obvious reasons, I can’t do a direct NDL to NDL comparison.  (SAUL, being a probability-based model, doesn’t actually have NDLs.)    Instead, single NDL profiles from the PADI tables will be paired with their expected probability of DCS according to SAUL.


Dives with air, including 3 min safety stop at 15 fsw


PADI  NDL                                        SAUL                                       SAUL  at 75% PADI NDL BT

Depth(fsw)    BT(min)                    Prob. of DCS (as a %)                   Prob. of DCS (as a %)     

 35                  205                               0.1750                                        0.0637

 40                  140                               0.1560                                        0.0645

 50                    80                               0.2090                                        0.1020

 60                    55                               0.3210                                        0.1510

 70                    40                               0.4030                                        0.1720

 80                    30                               0.4140                                        0.1490

 90                    25                               0.4740                                        0.1670

100                   20                               0.4080                                        0.1130

110                   16                               0.3000                                        0.0640

120                   13                               0.2020                                        0.0300

130                   10                               0.0796                                        0.0014

140                     8                               0.0250                                        0.0000

To put it another way, your likelihood of getting bent, if you dive profiles right at the PADI no-decompression limits, averages out at just over 1 in 400.   But your likelihood of getting bent on any particular NDL profile ranges from a low of about 1 in 4000 (140 fsw, 8 min) to a high of almost 1 in 200 (90 fsw, 25 min).   Of course, most of us don’t usually dive right at the limits.  If you limit your bottom time to three-quarters of the PADI no-decompression limits, your  likelihood of getting bent (shown in the right-hand column) averages out at just over 1 in 700 and ranges between a low of pretty near zero (less than 1 in 1,000,000 for 140 fsw, 6 min) to a high of about 1 in 550 (70 fsw, 30 min).   If you were diving with SAUL, your own personal “NDL” would depend on what you chose as an acceptable level of risk.  If, for example, you input 0.5000 (1 in 200) as your risk level – not really advisable – you could dive any of the profiles in the PADI NDLs and even increase your bottom times on many of them.  If you input a more sensible 0.2500 (1 in 400), you could dive the PADI NDLs at 35, 40, 50, 120, 130, or 140 fsw but would be held to varying shorter bottom times between 60 and 120 fsw.


Dives with “32 NITROX” (32% O2), including 3 min safety stop at 15 fsw

PADI  NDL                                                     SAUL

Depth(fsw)    BT(min)                                    Prob. of DCS (as a %)

  45                  220                                           0.0219

 50                  155                                           0.0122

 55                  110                                           0.0073

 60                    90                                           0.0194

 70                    60                                           0.0444

 80                    45                                           0.0845

 90                    35                                           0.1100

100                   30                                           0.1690

110                   25                                           0.1730

120                   20                                           0.1060

130                   18                                           0.1260


Dives with “36 NITROX” (36% O2), including 3 min safety stop at 15 fsw

PADI  NDL                                                     SAUL

Depth(fsw)    BT(min)                                    Prob. of DCS (as a %)

  50                  220                                           0.0004

 55                  155                                           0.0000

 60                  115                                           0.0000

 65                    90                                           0.0000

 70                    75                                           0.000008

 80                    55                                           0.0150

 90                    40                                           0.0198

100                   35                                           0.0688

110                   29                                           0.0822


SAUL indicates that diving with either form of Nitrox is safer than PADI NDL tables would suggest.  The “riskiest” dive in the lot – 32 NITROX at 110 fsw , 25 min –  has just slightly more than a 1 in 600 chance of resulting in DCS.  The safest for 32 NITROX – 55 fsw, 110 min – runs a DCS risk of less than 1 in 14,000.  The 36 NITROX in the PADI NDL tables, as a group, are safer still, with almost half of them bearing DCS risks of less than 1 in 1,000,000.  The “riskiest” 36 NITROX dive – 110 fsw, 29 min – still has a DCS risk of less than 1 in 1,200.  While I did calculate the probabilities of DCS for dives at 75% of the PADI NDL bottom times for both forms of Nitrox, it’s not really worth printing them out – they’re all pretty close to zero, the highest probability there being just over 1 in 10,000 (32 NITROX, 110 fsw, 18 3/4   min).    

Looking in a more general way at comparisons between SAUL and PADI, their respective conclusions on safe versus unsafe dives are not too far apart.  Nitrox is, indeed, significantly safer than air.  For air, SAUL sees the PADI NDLs as being, for the most part, of roughly equal risk and at a level of risk that is reasonable (considering that they are NDLs – i.e, limits, not necessarily preferred profiles).  SAUL diverges from PADI in finding its NDLs in the mid-depth range to be a little riskier than some divers may expect, while dives at more shallow or deeper depths are safe enough that divers who tolerate greater (but still reasonable) risks could be allowed a little more leeway.   Of course, being “allowed” to increase times at the shallowest depths means nothing on a single tank of air.  Very few, if any, divers can stretch their air to accommodate the 205 minutes PADI permits at 35 fsw or even the 140 minutes permitted at 40 fsw.  







  1. Fascinating article. How do you think that the 75% NDL and use of nitrox relates to the role of inflammation in DCS?

    • Thanks for your question and interest.

      The origin of inflammation in “Gas Bubble Disease”, which includes both DCS and the creation of arterial gas emboli (AGE), is probably independent of the rule-of-thumb you may be using to keep safe – whether its keeping within 75% of the NDL, or something else. Gas Bubble Disease arises from excess Nitrogen and/or Helium, that drains into blood from supersaturated tissues, subsequent to an overly rapid decompression.

      Inflammation is one of several biological responses to harmful stimuli, which include the presence of gas bubbles in circulation. These bubbles, through their interaction with vascular endothelium, cell membranes, and other proteins, are interpreted as foreign bodies by the immune system which, on detecting a foreign body, triggers a complex cascade of protective mechanisms.

      This is described in greater detail in Chapter 7, Section 7.7.2, of “Gas Bubble Dynamics in the Human Body”, by Saul Goldman, J. Manuel Solano-Altamirano and Kenneth M. LeDez, Elsevier /Academic Press (2017).

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