Decompression Illness and Denial- Part 2

Demystifying Dive Computers

Words and Photography by Jill Heinerth

Diver Pam Wooten explores the San Francisco Maru in Truk Lagoon. With some of the worldas most impressive wrecks, itas an easy place to rack up deco; an understanding of how your computer works could help keep you safe in remote locations. Photo: Jill Heinerth

In my previous article I looked into incident reports that fell outside of the data gathered by organizations such as DAN. In this article, Iam going to dive into the physiology of decompression, gradient factors, and conservatism strategies. When you understand how your diving computer works to model decompression, you will better understand how to improve your personal safety.A


A review of fundamental underwater physics and physiology is useful. Almost every scuba diver has considered the same demonstration in class. Take two bottles of carbonated soda: open the first bottle promptly to watch the formation of big bubbles, then open the second bottle slowly and hear only a small burp or fizz. It is an excellent analogy for what happens in the human body when ascending from a dive.

Oxygen molecules are bound to blood by hemoglobin, and as we dive they also dissolve in our blood plasma. Nitrogen, which serves no purpose to the body, is dissolved into our tissues as we descend. As the partial pressure of gases in our lungs increases, more of this gas dissolves in answer. Descending increases the ambient pressure surrounding our bodies. If you descend to 100 feet( 30 m/ 4ATA) and stay there, eventually your tissues will saturate uniformly at this depth.

Our bodies contain different tissue forms, such as blood, brain, ligaments, scalp, cartilage, bone, fat, and other organs. These tissues are perfused with blood at different levels, assimilating and releasing dissolved gas at differing rates.

These different tissue forms are grouped into acompartmentsaaa theoretical group of tissues that share similar gas saturation properties. In computations, each compartment utilizes a specific half-time. A afast tissue compartmenta such as blood will on-gas quickly. A slow tissue with less blood perfusionasuch as bonealoads and releases gas slower. Applying the concept of half-times means that the most significant degree of on-gassing results early, and the rate of absorption slows over day. For instance, a tissue with a 60 -minute half-time fills up halfway after 60 minutes and then half of the remaining in another 60 minutes. In other terms, that 60 -minute tissue is 75% full after 120 minutes. The same theory applies in the opposite direction when the diver swims up. Off-gassing continues well after surfacing, much like that soda pop, that eventually goes flat over day if left exposed to ambient pressure.

If you stay down, all your tissues eventually fill with inert gas, becoming fully saturated. As soon as you start to ascend, the reduced ambient pressure triggers off-gassing due to supersaturation. As you swim up, fully saturated tissues are not capable of holding onto the gas. The reduction in pressure causes gas to diffuse from the tissues back to the lungs. If you keep breathing, youall be off-gassing, and if you ascend at a suitable rate, then that should give your body a chance to release the additional inert gas without incident. But this is a mathematical algorithm applied to a great diversity of divers. The math does not account for your age, shape, size, hydration, traumata, thermal factors, or activity level. It is predictive and useful but not a guarantee that you will never get bent. So what can you do to increase your safety margin when diving?

A Be physically and mentally prepared for diving A Be well-rested A Ascend within safe ascent rates A Use a tested algorithm conservatively, accounting for increased risk factors A Avoid tight equipment that constricts circulation A Do a 3-5 minute safety stop at 15 feet( 6m) on every dive A Increase the conservatism level on your computer A Stay properly hydrated A Avoid post-dive exertion( such as lugging the tanks out of the water right after the dive) A Carefully monitor exertion levels and thermal factors on dives so that they match during the working( on-gassing) and decompression( off-gassing) phases of your dive A Use oxygen-rich mixes to aid in off-gassing( if qualified) A Do some asurface decoa day, relaxing before leaving the water if thermal factors allow you to do so A 3-5 minute security stop at 15 feet( 6m) should be practised on every dive, and using of oxygen-rich mixtures to aid in off-gassing is recommended for those with appropriate qualifications. Take advantage of deco bars and enclosures to help reduce stress and exertion. Photo: Jill Heinerth Being mentally was developed for your dives can increase your safety margin, especially in challenging environments such as wrecks. Photo: Jill Heinerth Your dive computer can open up a new world of escapades, but they canat be blamed for DCS; itas down to each individual diver to know the risks and minimise at every opportunity. Photo: Jill Heinerth

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