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.