This is true, of course;
for a given dive of X depth and Y time, the diver absorbs less nitrogen using nitrox compared to air.
If the dive parameters were different,
i.e. a given dive of X depth, but time was NDL-5 min, then the nitrogen absorption would be equal.
That, however, only covers one issue - the amount of nitrogen absorbed. On-gassing isn't the problem. Off-gassing is the problem.
Simplistically, decompression stress is a product of 3 factors;
1. The amount of nitrogen absorbed (volume on-gassing).
2. The amount of nitrogen released (volume off-gassing).
3. The efficiency of nitrogen release (bubble formation).
It is wrong to consider nitrox use and decompression stress only in relation to #1, but ignore #2 and #3.
The amount of nitrogen released (#2) is primarily effected by the time taken to ascend to the surface. On no-stop (NDL) dives, our tables or computers keep us within saturation parameters that allow sufficient nitrogen to be released providing a maximum ascent speed limit is not broken. That is, sufficient nitrogen release to prevent Decompression Sickness (DCS)...
i.e. ambulances, hospitals, calls to DAN and chamber rides... That is all.
Historically, the calculations used by our decompression tools were formulated to reduce incidence of decompression
sickness, not decompression
stress.
The calculations were refined over time against a statistical sample of dives, of which X didn't result in DCS and Y did result in DCS. Once 'Y' was within reasonably acceptable statistical parameters, they were deemed 'safe'. Only more recently did doppler ultrasound and 'bubble scoring' (
counting bubble frequency and size inside test divers) become a refinement to these calculations. this led to more conservative dives and also to strategies like deep stops.
In the light of this, let's consider factor #3.... the 'efficiency' of nitrogen release. Think back on how we are taught to ascend from dives.
Most of us are taught to ascend directly to the surface at a single speed no faster than 18m/60ft (
PADI) or 9m/30ft (
the rest) per min. We are recommended to complete a single 'safety' stop at 5m/15ft for 3min.... simplistic, but it helps provide a cushion against decompression
sickness.
If we stay within those parameters and don't end up in a recompression chamber it is satisfactory. If do end up in a chamber, assuming no other causal factors, then it is deemed 'non-blameworthy' or 'unattributed' DCS.
The incidence of divers suffering post-dive lethargy, malaise, fatigue, sleepiness and reduced vitality is totally ignored.
However, not everyone teaches so simplistically. Some agencies/instructors provide more training and, consequently, deem their students more capable of achieving more complex and refined ascent strategies. They specify varying ascent speeds at different phases of the ascent, multiple 'safety' stops..... even the use of optimal ascent gasses.
For instance; ANDI teaches multiple 'safety' stops at 9m/30ft (
1 min), 6m/20ft (
2 min) and 3m/10ft (
2 min). Ascent speeds are 9m per min until 9m, then 3m per min thereafter. For advanced recreational (
i.e. 'deep') divers, they also teach the use of 50% nitrox as an ascent gas for safety and 'conservatism'.
Example:
View attachment 221113
These refinements go well beyond simply 'avoiding DCS'. They can be considered from two perspectives. Firstly, they provide a much wider safety margin against clinical DCS incidence. Secondly, and important for this discussion, they dramatically reduce microbubble incidence; lowering decompression stress. The result: divers aren't unnaturally fatigued post-dive!
So... back to the original issue... the benefits of nitrox.
Nitrox is beneficial for TWO reasons. We are generally only taught one of them.
We know that using nitrox reduces nitrogen
absorption compared to air. This can be used to allow either longer no-stop (NDL) dives or to achieve a more conservative dive (
less nitrogen absorption) given identical depth/time parameters.
What we aren't generally educated about (
at least, until technical diving levels) is that nitrox also maximizes the efficiency of our off-gassing. It 'accelerates' our decompression. It helps remove nitrogen from our bodies at a faster rate. There's no magical barrier between recreational 'nitrox' and technical 'deco gasses'. Simply put, the higher the % O2, the more we accelerate the process of expelling nitrogen from our bodies. Nitrox 32% is higher than Nitrox 21% (air).
Here is a simplistic table showing off-gassing efficiency ratios:
View attachment 221122
We can see that the ratios for 'recreational nitrox' mixes compared to air:
EANx32 is 16% more efficient in off-gassing nitrogen.
EANx36 is 21% more efficient in off-gassing nitrogen.
EANx40 is 26% more efficient in off-gassing nitrogen.
Just because we don't 'gas switch' onto a different 'deco gas' doesn't mean that the nitrox we breathed throughout the dive is less efficient in accelerating off-gassing.
So.... we can see that for a
given ascent and safety stop, nitrox will also remove more nitrogen than air does.
Using nitrox isn't just about absorbing
less nitrogen for a given time/depth; it is also about releasing
more nitrogen for a given ascent time/speed.
Combined with slower ascent speeds and increased time/frequency of ascent stops, this plays a major role in eradicating nitrogen microbubbles from the body. It lowers decompression stress - reducing (
or preventing) the symptoms of post-dive fatigue, sleepiness and lethargy
Back to my initial point.... considering all three factors effecting decompression stress; nitrox use can provide a significant benefit in all three aspects.
Of course, you'd only notice that benefit if you'd otherwise subject yourself to decompression stress. If you don't, then you won't.
