Flightline recently posted a list of tips for avoiding shark attacks. It’s mostly common sense, but there are a few points I’m not sure I agree with. I’ll go over them one by one:

• Swim in a group. The article claims sharks are less likely to attack a group of divers. This white shark research page claims that sharks target lone or small groups of seals, where “small” is defined is 6 or fewer. How often are divers in a group larger than 6? Besides specially trained divers, you should never be diving alone anyway.
• Do not go in the water if you are bleeding. I agree here. The article even points out that there is no research that indicates menstruating women are in additional danger, although there is probably too little data to conclude anything.
• Do not wear shiny jewelry. I’ve heard this advice often for barracuda, but never for sharks. For one, I don’t think big sharks eat small fish. Second, sharks that are attracted to small fish would quickly realize you are not an easily killed prey and leave you alone.
• If you see a shark, stay calm, stay quiet, and stay where you are. I hope everyone knows not to draw attention to yourself if approached by a shark large enough to view you as prey.

What do you think? Am I off the mark on any of these points? Let everyone know in the comments below.

The most popular post here on The Diving Blog is easily on writing an Emergency Assistance Plan. To date, this article, with the free templates, have helped hundreds of divers in one of the most unnecessarily confusing parts of the PADI Rescue Diver course.

In the process, I’ve received valuable feedback from many readers. First of all, to those of you who have commented or emailed constructive feedback on the EAP templates, thank you! Our post shows up as a top result when searching for emergency assistance plans, and your input helps to make it better, in turn helping many scuba divers in training.

There’s one thing I’ve consistently noticed: there is no standard set of requirements for an EAP to be considered complete. This isn’t necessarily a short-coming on PADI’s part, it simply reflects the vastly different requirements for different countries, cultures, and environments. Responding to an emergency in San Diego, California would be an entirely different experience than responding to one in the Maldives.

Nevertheless, I am working to coalesce all this information into a form usable by everyone. This is where you, the reader, come into play.

First, if you haven’t yet, please read the original EAP post, and take a look at the template.

Second, send me your feedback. As an instructor, would you consider the template complete and acceptable? What is missing? If you dive in a particular location, what information is necessary that the template does not provide? If you are a certified rescue diver, did your instructor note anything that the template does not give? Feel free to leave a comment here, or use the contact form linked to above.

I will incorporate your feedback and ideas into an update of the post and template. Thank you for taking the time and paying it forward, helping future generations of scuba divers! You guys rock!

Diabetes is a widespread and complicated illness. Like most diseases that aren’t completely understood, doctors often take an overly conservative stance when patients ask what they are and are not allowed to do.

Such has been the case for the past 20 years in scuba diving, with doctors flat out denying the privilege to insulin-requiring diabetics. In the past five years, however, that’s starting to change.

Data has come out that some diabetics are still scuba diving, and not dying. This has caused the diving medicine community, in particular, the Divers Alert Network (DAN), to revisit their stance on diabetics and recreational scuba diving.

If you are a DAN member, you have access to an online seminar concerning recent policy changes on diabetes and scuba diving. When logged in to the site, access it through Training & Education, Online Seminars.

Included in the approximately hour-long seminar is a PDF summary of guidelines for scuba diving diabetics. Here is an incomplete sample:

• Delay diving after starting or changing medication.
• No episodes in the previous year.
• No significant secondary complications.
• No depths greater than 100 ft (30 m) or 60 minutes.
• No decompression stops.
• Both you and your buddy should not be diabetic.

There are more, and you should consult the seminar if this applies to you, your friends, or if you are an instructor curious how to handle diabetics interested in diving.

I think it’s great that DAN has revisited long-standing policies to come up with an intelligent, yet simple set of guidelines to increase the ranks of potential scuba divers.

Even in my limited experience, I’ve found the medical community to be annoyingly conservative about anything related to scuba diving. It basically comes down to the fact that we understand so little about our bodies and their response to the underwater environment. This is true with a healthy body, so change one variable and the doctors default to a “no”.

I understand this point of view, but it is reassuring to see changes enacted once data becomes available.

Photo by .:[ Melissa ]:.

FayObserver.com published an article yesterday on sharks and scuba divers. The article is a part of a common attempt to dispel the myth of man-hunting sharks.

One diver interviewed for the article, Hank Parfitt, frequently swims alongside shark, and even acts as a shark wrangler for underwater photo shoots. The most practical bit of the article is Parfitt’s tip on giving an angry shark its space.

When is a shark angry? Parfitt gives three signs:

1. Pectoral fins angled down. A neutral to happy shark will swim with its pectoral (side) fins straight out, like an airplane. If you see them angled down, the shark is upset, hungry, defensive, or some non-friendly emotion. Give it some distance.
2. Arched back. My guess is that an arched back is like a spring, getting the shark ready to dart at potential food or something it feels may harm it.
3. Shaking its head back and forth. In conjunction with the previous two, a shark shaking its head like it’s saying “no” is not happy and may be aggressive.

Calm sharks are easy to observe. A few brave like Parfitt will even touch the shark, but always in front of the dorsal fin. Like all animals, the rear is particularly vulnerable, so avoid sneaking up on a shark (even if it’s unintentional). When treated with the respect they deserve, scuba diving with sharks can be a great experience.

The first place I ever dove after certification was Bermuda. On one boat dive we were briefed by the divemaster in preparation for our first dive. He asked if there were any questions.

“Yeah, should we make a safety stop”, I asked. Fresh out of my checkout dives, I couldn’t remember the rules for when to make a safety stop.

The divemaster scoffed at me. “We’ll hardly be going deep enough to require a safety stop.”

Fast forward a few years. I’ve read a mountain of material for fun and as part of my professional development. Looking back on this occasion, what would I have done knowing what I know now? I most definitely would’ve made a safety stop.

In case it’s been a while since you’ve had one, a safety stop is an approximately three minute stop made at 15-20 feet (5-7 meters) at the end of a dive. For deeper dives greater than 60 feet / 20 meters, they are usually considered a requirement, and optional for anything less.

The idea is that this brief time at a relatively shallow depth will eliminate a large amount of microbubbles, an effect strongly correlated with decompression sickness. Studies have shown that a safety stop eliminates virtually all detectable bubbles, decreasing the chances of decompression sickness drastically.

I’m sold.

In fact, unless I’m short on air or in a hurry, I always make a safety stop. Even for 50 foot dives. If you have a proven way to decrease your risk, why would you not?

I don’t blame the Bermuda divemaster for what he said. If there is ever a time when it is perfectly reasonable to forego a safety stop, it is on the first dive of the day (unless it’s a deep dive). Nevertheless, I see no reason to scoff at the suggestion, as they are something everyone should take seriously and include in their dive planning.

Photo by tslane888

A year ago I was working in New Zealand. I was interested in what the local diving had to offer, so I signed up for a small trip off the local coast of Wellington. What happened on that trip turned out to be a valuable learning experience.

It was a cold morning when we pushed off from shore in a small fishing boat, six divers and two crew. I was a little nervous since I knew the water would be cold—it was winter there, after all. I wore an old, uncomfortable rented wetsuit. A farmer john that had seen more than his fair share of divers. Add in the unfamiliar equipment and all weights and cylinder measured in that odd system known as metric, and I was in an uncomfortable place to start a dive.

Once on the water the captain asked us each to introduce ourselves and briefly outline our experience. As we went around, I quickly realized I was the most unseasoned diver on the boat. Everyone else had hundreds, some thousands, of dives, most in the murky Wellington waters. Despite having recently completed my rescue diver certification, I was feeling a little intimidated when I had to announce my meager dive experience.

I was teamed up with Tom, one of the highly experienced divers, and a nice guy to boot. We did backrolls into the choppy water. With the new equipment and unfamiliar waters I was unsure of my weighting. When grabbing my weights in the shop, I had no idea how many to use. Not only were they in kilograms (although I knew the rough conversion), I had all new equipment and did not have a good starting point. I asked around what others were wearing, but I couldn’t use anyone remotely close to my size’s weights as a guide (for example, one diver had a steel plate in his BCD).

Time to descend. Instead of stopping Tom and doing a proper weight check, I didn’t want to look like an amateur and descended anyway. As soon as I slipped beneath the surface I knew I had made a mistake. Not a great way to start a dive.

Despite this ominous feeling, the dive went off without a hitch. I was cold and burning through air faster than Tom. I felt bad that I was limiting our dive time, but when my air got low, it was time to end the dive, but first, a safety stop at 5 meters.

During the safety stop I started to feel the effects of not being properly weighted. As my tank emptied, it became more buoyant, requiring more weight to stay underwater—weight that I didn’t have. Only a minute in and I was struggling to stay underwater. In fact, I was completely upside-down kicking to stay at safety stop depth. Unable to fight the buoyancy any more, I floated to the surface right before the end of the requisite three minutes, while my confused buddy watched from his safe depth.

In an attempt not to embarrass myself and look like an “amateur”, I ended up embarrassing myself far worse. Fortunately, my dive buddy was gracious and did not say anything as I found more weight for the next dive. I realized my stupid mistake and swore I would never put myself at risk again for the sake of looking more experienced. This time it wasn’t a big deal, but next time it may be in a more unforgiving environment.

No matter how little or how much you dive, there is nothing embarrassing about being safe and comfortable on all your dives. If your buddy or anyone else has a problem, then it is their problem, and you can rest assured that they’ll be the ones who end up looking inexperienced. No diver worth their salt would belittle you for being a safe and cautious scuba diver.

Photo by JennyHuang

Snorkels are considered a core piece of scuba gear. Mask, fins, and snorkel: the three items every beginner starts with. We take this for granted, and dive away with those plastic tubes strapped to our head. After a while, though, we start to question the utility of a snorkel. When diving the smooth Caribbean waters, for example, is a snorkel really necessary?

At this juncture, there are a few paths the blossoming diver can take:

• Keep the snorkel. Your first option is to heed your training and continue to dive while wearing a snorkel at all times. Sure, it can be uncomfortable in a current, but you rest easy knowing it’s always there.
• Exchange the snorkel for a pocket snorkel. Your next option is to remove the snorkel from your mask. Aaah, how liberating! There are times when you may need a snorkel, so for those situations you carry a foldable pocket snorkel in your BCD pocket. In an emergency, it’s only a zipper away.
• Ditch the snorkel. Look out, divers, this future tech diver means business! The last option is to ditch the snorkel completely. I don’t imagine anyone throws their snorkel away, but rather keeps it in their gear bag and wears it on a case-by-case basis. Choppy waters with low viz? Bring the snorkel. Bonaire shore dive? No thanks.

If you find a snorkel uncomfortable, I think carrying a pocket snorkel is your best option. I don’t own one (yet), so I wear a snorkel based on the dive conditions. However, be aware that some dive operators may not let you in the water if you don’t have a snorkel. For this reason, you should always carry a snorkel with your gear.

What do you think? Are snorkels for sissies, or would only a fool go in the water without one?

Photo by chrisada

As a scuba diver, theoretical knowledge cannot always be immediately recognized as useful. It does, however, come in handy. If you decide to move on to a professional level of certification, theoretical knowledge development is a requirement. It can also provide understanding behind practical decisions, guiding your reasoning in a more educated manner than just following a set of rules. Today we’re going to talk about the biggest theoretical area there is for scuba divers, decompression theory.

The need for decompression theory arises from decompression sickness (DCS). DCS encompasses the illnesses that may occur from the body’s exposure to varying pressures. This is not strictly limited to scuba divers, but clearly we have a highly vested interest in the development of sound theory to describe the causes, effects, and preventions of DCS.

I say this, because the discovery of DCS predates recreational scuba diving by about 100 years. There is documentation of DCS symptoms as early as the 1840s, where workers in pressurized French mines fell ill with the now-recognizable effects of “the bends.”

It was well over 60 years before enough progress was made for any practical advantage. In 1906, the British Royal Navy commissioned physiologist John Scott Haldane to study DCS. He built on the work of Paul Bert who, years earlier, made progress in identifying the cause of DCS. In particular, it was Bert who named dissolved nitrogen as the culprit in DCS. However, it was Haldane who built the first complete theoretical model.

Haldane and his team experimented with goats in pressure chambers. This research led him to describe a theoretical decompression model and build the first dive tables that could be verified experimentally. Today, over 100 years later, practically all dive tables and dive computers are built upon this original Haldanean decompression model.

Decompression models

A decompression model is some theory you can follow and apply in order to decrease your risk of DCS. A model is only as good as it has been verified to prevent DCS. There are too many factors involved to currently guarantee prevention. As they say, the only way to 100% prevent DCS is to not dive. Aside from that, there are models that have been in use for a very long time (like the Haldanean model) and have been shown to decrease your risk of DCS drastically.

Ideally, a model is developed through scientific means—by studying the physics and physiology of the human body. It doesn’t have to be, though. For example, a model followed by early divers was “the 50 rule.” This “model” dictates that the depth (in meters) and time of your profile should add up to no more than 50. A 10 meter dive for 40 minutes, 20 meters for 30 minutes, and so on. This actually wasn’t a terrible model, although there is no real theory behind it. You’ll notice that it is overly conservative, though. This guides continual research into model development—getting you the maximum dive time in the safest way possible.

Haldanean model

Building on the observation that dissolved nitrogen triggers DCS, the Haldanean model is build around a few principles:

• Nitrogen dissolves into tissues. After enough time, the tissue becomes completely saturated. This is Henry’s law.
• The tissue will reach saturation determined by the ambient pressure. So a given tissue under higher pressure contains more nitrogen than the same tissue at the earth’s surface.
• The difference between the ambient pressure of nitrogen and a tissue’s partial pressure of nitrogen is called the pressure gradient.
• When ascending, the dissolved nitrogen’s partial pressure may be higher than the ambient pressure. The body can tolerate some amount of pressure gradient without DCS.
• If the pressure gradient becomes too high, the dissolved nitrogen cannot be eliminated quickly enough. Nitrogen bubbles form, leading to DCS. Thus, the risk of DCS can be reduced by keeping the body’s pressure gradient within acceptable limits.

To understand these ideas better, we need to review a few concepts.

Partial pressure

Recall from our article on enriched air / nitrox the idea of partial pressure. Total ambient pressure at sea level is 1 atm. Therefore, the air we are breathing is also at 1 atm. This air is comprised of mainly two components: 21% oxygen and 79% nitrogen. We can say, then, that the partial pressure of oxygen at the surface is 21% of 1 atm, or .21 atm (some people write this as .21 PPO, for partial pressure oxygen).

Likewise, the partial pressure of nitrogen at the surface is .79 atm. This principle is captured by Dalton’s law, which states that the 1 atm of pressure at the surface can be written as the sum of the partial pressures, .79 atm nitrogen + .21 atm oxygen = 1 atm total.

This is all at sea level. The deeper we go, the higher the pressure, and the higher the resulting partial pressures. At 10 meters depth, the pressure is 2 atm. By Dalton’s law, the partial pressure of nitrogen is 1.58 atm and for oxygen is .42 atm (notice how they both add up to 2 atm).

Mainly what this means is that the deeper you dive, the more nitrogen you absorb with each breath.

Tissue compartments

Haldane’s model is built around how the body’s tissues absorb and release nitrogen. There’s just one problem: the body is incredibly complex, and accurately modeling all its tissues is not a tractable problem, not even now, much less over 100 years ago.

What Haldane could determine was that different parts of the body absorb and release dissolved nitrogen at different rates. Instead of attempting a much larger problem, he simply represented the entire body by a number of theoretical tissue compartments. These are called “theoretical” because they don’t correspond to any particular tissue in the body, but rather attempt to capture the idea that the body absorbs nitrogen at different rates. Together, these theoretical tissues are meant to represent the body as a whole and the time scales at which it deals with nitrogen.

Haldane originally used 5 compartments. Later, the US Navy dive tables used 6. Some modern tables use as many as 14. There is no limit to the number of theoretical compartments, but any advantage they may provide rapidly falls off as you add more.

Halftimes

These theoretical tissue compartments (from now on, just tissues, or compartments) represent the different rates different parts of the body absorb and release dissolved nitrogen. The model must then deliver these rates. It does so, in the form of tissue halftimes A tissue halftime is the length of time it takes for a given compartment to halve the pressure gradient.

For example, if a compartment contains absolutely no dissolved nitrogen, and is exposed to air at 1 atm, its halftime is the amount of time until the partial pressure of nitrogen in the tissue is .395 atm (half of the partial pressure of the nitrogen in the air, .79 atm / 2 = .395 atm).

Saturation is reached when the pressure gradient is 0, or the partial pressure of nitrogen in the air is the same as the partial pressure of nitrogen in the tissue. This means after one halftime a compartment is 50% saturated. It is not 100% saturated after two halftimes, since each time the pressure gradient is halved, so after two halftimes a compartment is 75% saturated. After three, 87.5%. Four, 93.8. For simplicity, we say a compartment is 100% saturated after 6 halftimes (it’s actually 98.4%, but that’s close enough).

The US Navy model uses 6 compartments with halftimes of 5, 10, 20, 40, 80, and 120 minutes.

Examples

Halftimes can be confusing, so let’s look at examples. For further simplicity, we refer to a compartment’s saturation level in terms of depth. We definitely wouldn’t say 50% saturated, since that gives no indicated of the partial pressure. Similarly, we don’t say the tissue has 1.185 atm nitrogen, although you could. Instead, we give the depth corresponding to that partial pressure of nitrogen. In this case, the partial pressure of nitrogen in air is 1.185 at 5 meters. So we say this compartment has a nitrogen loading of 5 meters (this is also written as meters /feet sea water, or msw / fsw).

Imagine a dive to 20 meters for 40 minutes. What do the 6 compartments look like? For the 5-minute compartment, 40 minutes is 8 halftimes. Recall that we consider 6 halftimes as reaching saturation, so the 5-minute compartment is completely saturated, and has a nitrogen loading of 20 meters.

The 10-minute compartment has gone through 4 halftimes. After the first halftime, its loading is 10 meters. After the second, 15 meters. Third, 17.5 meters. Fourth, 18.75 meters.

The 20-minute compartment has completed 2 halftimes, so it is at 75% saturation, or 15 meters. The 40-minute tissue has completed one halftime, so 10 meters. The 80-minute has completed half of a halftime, so 5 meters. The 120-minute compartment is at 3.33 meters.

M-values

Notice something interesting about what we’ve covered so far. Nowhere has there been any indication on how this model guides your dives. That’s because it doesn’t! To decrease our risk of DCS when ascending, we have to keep a tissue’s pressure gradient below an acceptable threshold. The model so far has not given these thresholds.

These thresholds can only be obtained experimentally. That’s what we’ve done over the years. After thousands of controlled dives and observing symptoms at the surface, scientists obtain values for acceptable pressure gradients for each theoretical tissue. These values are called M-values.

There are M-values for each compartment for each decompression stop. In no-decompression diving, however, we only have to be concerned with the values for the pressure at the surface, which are sometimes written as “M₀-values.”

Dive table designers can experiment with different M-values, but they should be consistent with the data. If experiments show that exceeding a certain value for a given compartment usually results in DCS, then the final table should limit dive profiles based on that value.

A complete model

We now have all the tools for a complete model. A set of compartments with their halftimes, as well as an M-value for each compartment. Let’s do an example.

Let’s use the same compartment halftimes with M₀-values of 30 meters, 20 meters, 15 meters, 10 meters, 7.5 meters, and 5 meters. This means that the 5-minute compartment should not exceed a nitrogen loading of 30 meters, the 10-minute compartment should not exceed 20 meters, and so on.

With our dive to 20 meters for 40 minutes, our tissue loadings were 20 meters, 18.75 meters, 15 meters, 10 meters, 5 meters, and 3.33 meters. Uh-oh! Our 20-minute and 40-minute compartment have reached their M-values (15 meters and 10 meters). That means it’s time to end the dive or ascend to a shallower depth.

If we ascend shallow enough, the 20-minute and 40-minute compartments, even when saturated, can never exceed their M-values. So as long as we ascend shallower than 10 meters, the 40-minute compartment can never exceed it’s M-value. From this we notice that shallower depths are controlled by slow compartments (high halftimes), while the fast compartments (short halftimes) control deeper dives.

For instance, the 5-minute compartment will reach its M-value very quickly at deep depths. At 40 meters (the recreational limit), one halftime (5 minutes) will load the compartment to 20 meters. Another 5 minutes will have it at 30 meters. Staying at 40 meters any longer will require decompression stops.

Keep in mind that following this model with these M-values does not provide any guarantee that DCS won’t occur. It can still happen, although many years of diving with established tables has shown that the chances are minimal. Still, it doesn’t hurt to dive conservatively.

Conclusion

Phew! That was a lot of material. Give it a little time to sink in. We’ll pick up where we left off in future articles. For example, what about repetitive dives? How do compartments release nitrogen when we are out of the water? At the same rate that they absorb?

We’ll also look at how this information can be used when purchasing a dive computer. Stay tuned!

Feel free to ask any questions or add to the discussion in the comments.