Decompression Stops: What We Don’t Know

If you want to know precisely how long and how deep to make your decompression stop to absolutely avoid the bends, you'll be disappointed. Decompression theory is not exact, if for no other reason than there is no one exactly like you: your body is unique.

In 1908 John Scott Haldane observed that goats, saturated to depths of 165 feet of seawater, did not develop decompression sickness (DCS) if subsequent decompression was limited to half the ambient pressure. That is to say, he could safely bring them back to 60 feet deep without apparent symptoms. Goats were sacrificed for decompression studies for several more decades, and the military used human volunteers in tightly controlled studies, presumably without harm. After all, it would be unethical to intentionally give humans DCS.

Divers who actually get bent are few. Dr. Maurice Cross, a leading UK researcher of diving diseases, went to the Philippines in the 1980s because insufficient numbers of divers were getting bent in his home country to produce any meaningful research results. Poor Filipino hookah divers provided a better supply of subjects, such were their ill-disciplined diving practices.

Doppler testing of bubbles in the bloodstreams of bent divers has never been conclusive. Such research, mainly done by medical experts accompanying deep diving expeditions, gives clues rather than conclusions.

Today, divers who go deeper than 100 feet tend to take staged decompression stops during an ascent. Most recreational divers try to stay within no-decompression-stop limits and end their dives keeping an eye on their remaining no-stop computer time when ascending.

This slow ascent is the key to simplified deco diving, and divers must know how to recognize when their computers switch to a staged deco display. The first mandated stop shown will be around 15 feet, and this might well cease to be displayed before you get there if time is spent ascending to slightly deeper than this. Instead, your computer will revert to some no-deco-stop time remaining.

Since the '80s, a 15 foot stop was considered the first stop. That is, until Richard Pyle, a deep-diving ichthyologist, noticed that he felt better after a dive if he stopped for a short while at half the maximum depth he'd achieved during a dive and began to make that a habit. It was anecdotal, but by the late '90s, Pyle Stops became all the rage, and such short deep stops were incorporated in many computer algorithms as an option.

In 2010, Peter Bennett, founder and former president of the Divers Alert Network (DAN) and recognized as a leading authority on the effects of high pressure on human physiology, stated, "The deep stop at half the depth for 2.5 minutes significantly reduces not only bubbles but also the critical gas supersaturation in the 'fast' tissue compartments (like the spinal-cord's 13.5 minutes) without increasing the 'slow' compartments usually related to limb pain."

Since then, others have argued that these deep stops allow slow tissues to accumulate more nitrogen, thereby undoing the good of letting the faster tissues off-gas more slowly. Nobody really knows because the evidence has been anecdotal.

Meanwhile, the accepted procedure for recreational divers is to ascend at the computer-recommended safe no-deco [stop] maximum rate (usually about 30-45 feet/minute), then make a three-minute pause at around 15 feet deep for added safety.

Taking a safety stop is all about reducing risk. So, what is the risk, and how much should be reduced? Nobody really knows because until symptoms of DCS are actually present, one doesn't know how close you have come to being bent.

Everyone has an individual tolerance for DCS. Divers who in the past did not bother with stops either got away with it or didn't. Decompression procedures are very much an act of faith. Dive computer manufacturers tend to use algorithms that may be more cautious than presumed necessary, but occasionally someone gets bent even following tables. So, computer manufacturers are always eager to state that the correct use of their products does not confer immunity to DCS.

How can one manage to stay at a prescribed stop depth? First, you can't always depend on a line or a bar to grasp, so you must be a master of buoyancy control. The shallower you get, the more difficult it is to maintain buoyancy because pressure changes are greater. Take a course if you have any problem whatsoever controlling your buoyancy.

Using a late-deployment surface marker buoy can help you maintain a specific depth by supplying a fixed visual datum, but first practice deploying it in a benign environment. And avoid hanging from its line. If you're in a strong current, you have an important reason to use a DSMB. You'll be moving in the water and will only notice the current and whether you're rising if you are trying to stay alongside a fixed object or topography.

As you age, you apparently have an increased risk of DCS, and if you take a hit, it can be tougher on your body. Dive cautiously.

- John Bantin