The Editor says: Over the past few days, I noticed a lot of heated debate in our groups about whether the 5-metre safety stop is truly necessary. Brad, co-founder of Deepblu, shared his insights based on his experience developing the Deepblu dive computer, hoping to help everyone dive more safely — and to debunk a few long-circulating myths in the diving community.
Deepblu co-founder Brad Chen
The following article was posted by Brad Chen on his personal Facebook page (2017.7.12).
Yesterday I responded to some questions about decompression theory and stops on a diving forum. I'd like to compile those thoughts here and hopefully spark a broader discussion. On one hand, I want instructors to have an accurate way of explaining these concepts so students can learn through the right channels. On the other, I want divers to develop a deeper understanding of the underlying reasons behind these ideas.
Does Recreational Diving Require a Safety Stop?
Recreational diving is also known as no-decompression diving. The debate many people are having centres on whether the 5-metre, 3-minute safety stop is absolutely mandatory. The purpose of a safety stop is to give the faster half-time theoretical tissues a bit more time to off-gas nitrogen. Doing it is certainly beneficial — theoretically, the fastest half-time tissues will have off-gassed about 70% of their nitrogen within those three minutes. However, if completing a 5-metre, 3-minute stop poses difficulty or risk on a given dive (strong current, heavy surge, hazardous marine life, equipment issues, physical discomfort, etc.), you should not — and do not need to — force yourself to hold that stop. Breathing enriched air nitrox (EANx) or extending your surface interval by half an hour can compensate for the accumulated nitrogen just as well.
For most recreational dives conducted within a plan where all compartment tissues are calculated to be within the M-Value or GF limits (the green zone in the chart below), honestly speaking, the pressure difference between being at the surface and being at 5 metres is only 0.5 bar — and nitrogen actually off-gasses faster at the surface. But does staying underwater always help? Not necessarily. For deep stops, slower half-time tissues are actually still absorbing nitrogen. In the short term, what affects diving over several days are the fast half-time tissues. Looking at the ZH-L16C tables, it would take over 3 days at your M0 depth to saturate the 635-minute half-time tissue — but for the 5-minute fast half-time tissue, spending just 5 minutes beyond 38 metres already requires a decompression stop (not merely a safety stop).
Understanding Safety Stops Through Theory
Based on my personal experience with VPM, Bühlmann ZH-L16C, and the Deepblu algorithm, decompression theory is ultimately just that — theory. It's not an immutable law like F=MA or H=½GT². Only by understanding the reasons and purposes behind decompression stops and safety stops can you truly convey those concepts to students. Diving techniques, practices, and procedures are constantly evolving; there's no need to rigidly follow any single approach. Take the deep stop example mentioned above — it's only been in recent years that some cases of decompression sickness (DCS) have been traced back to divers forcing deep stops where they weren't appropriate.
Furthermore, the single greatest risk factor in diving is ascent rate — ascent rate — ascent rate. It's important enough to say three times. No matter how deep you go or how long you stay down, as long as you have sufficient time and gas to complete the off-gassing process, you won't get DCS. However, each person's nitrogen tolerance varies at any given time and under any given set of conditions. That is precisely why Haldane and Bühlmann used theoretical tissues to calculate nitrogen uptake and release in the body, rather than directly measuring your actual organs.
So let's revisit the cause of DCS. Everyone knows it's caused by the body accumulating too much nitrogen — but strictly speaking, that's only half the answer. The more accurate statement is that the nitrogen pressure accumulated in the body's tissues becomes too high. Because we breathe continuously underwater, it's as though we are constantly pumping high-pressure nitrogen into our bodies, and the gas dissolves into our tissues under elevated pressure. Once the nitrogen pressure within the tissues reaches a specific differential compared to the ambient pressure — known as the gradient — nitrogen comes out of solution directly as bubbles, causing the various symptoms of DCS. For this condition to occur — an excessive gradient between the nitrogen pressure in the body and the ambient pressure — ascent must be too fast, leaving no time for nitrogen to be eliminated through metabolic pathways (breathing, excretion, etc.), forcing it to emerge as bubbles instead. Therefore, regardless of your dive depth or duration, a slow ascent and necessary decompression stops are always essential.
Looking at the chart above: for typical recreational diving, my algorithm doesn't allow any tissue compartment to come anywhere near the Safety Line — so ascending directly to the surface theoretically requires no decompression stop whatsoever. However, the moment your ascent rate is too fast, the slope of your M-Value changes. In other words, the gradient between your body's internal nitrogen pressure and the ambient pressure widens rapidly. Once any single half-time tissue compartment reaches its gradient limit, you'll need to do a safety stop — or else. This ascent rate limitation applies equally across all certification levels and all algorithms: ascend too fast and you're heading to the hyperbaric chamber.
In summary:
- A 5-metre, 3-minute safety stop helps fast tissues off-gas additional nitrogen. Under risky conditions, it can be skipped — but your surface interval should be extended accordingly.
- Regardless of depth or bottom time, a slow ascent is the golden rule. DCS after a dive to 15 metres is not unheard of. The shallower the water, the more critical it is to watch your ascent rate. Even after completing a 5-metre safety stop, ascending slowly all the way to the surface is what prevents headaches.
- Should you do hyperbaric oxygen therapy after a lot of diving? Opinions are divided. From what I understand of dive medicine, unless you're a saturation diving engineer spending all day in saturation conditions — where it has been proven that prolonged pressure saturation impairs circulation in hip bone tissue and can even lead to avascular necrosis, with many saturation divers overseas having had hip replacements — there's little basis for recreational divers. As for it "feeling better," well sure — breathe pure oxygen for three or four hours straight and you'd feel great even lying at home doing nothing.
- A dive computer is important — 1,000,000 times more important than your camera. I'll leave the product promotion at that.
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