Sorry, everyone, but there will be no fish identification article today. If you’re new here, check out the fish id archives.

I’m currently in Bonaire, but I’ll be back on Monday. On the good side of things, I’ve finished my divemaster certification!

Have a good weekend, and I’ll see you on Monday.

When you train to be a Rescue Diver, and especially when you reach a professional level, you will be expected to have a pocket mask.

I looked around at some before ordering, beginning at scuba dive shops. I noticed that they aren’t so cheap. Neither are the official Red Cross masks.

It didn’t take long until I found these pocket masks at Amazon. $8 each is the lowest price I found, and much lower than the price quoted at dive shops. As long as it has an O2 inlet and covers the mouth / nose, you’re good.

Two things I really enjoy are TED talks and underwater photos / videos. Lucky for me I came across this David Gallo TED talk.

Gallo begins his short (5 and a half minutes) presentation with photographs of bioluminescent deep sea creatures before showing videos of cephalopods. Amazing creatures both in the deepest ocean and in the Caribbean shallows.

I hope you enjoy it as much as I did.

Yesterday I overhead a man at a dive shop. He was complaining to his wife that he felt he was carrying too much weight on his dives. In his words, “I’m overweighted for the beginning of my dive, but if I carry less I’ll balloon to the surface near the end of my dive. I start to feel it when I’m down to about 1200 PSI.”

I say he’s not overweighted, but rather weighted just right. If you remember how to do a proper buoyancy check, you’ll recall that you do it with a near empty tank. Your weights should be able to keep you underwater with an empty cylinder.

Weights have two purposes: to get you underwater at the beginning of the dive and to keep you underwater until the end, when you decide it’s time to come up. The correct weighting is the smallest amount that satisfies these requirements—something this guy forgot.

The time has come. I’ve been toying with the idea for a while, but I’ve finally taken the plunge and signed up or scuba diving insurance.

Why insurance?

I dive more than three times a year, and plan to dive frequently in the future. It’s said that the only way to prevent decompression illness (DCI) is to not dive, so it makes sense that the more you dive, the more risk you incur. You can follow every table or computer to the letter and still get sick.

I know I sound like an insurance salesman, but when you purchase insurance, you are purchasing peace of mind. DCI is expensive, especially if they have to helicopter you off a boat. To make matters worse, most insurance policies don’t cover scuba diving accidents, so you can easily accrue tens of thousands of dollars in debt from a single incident!

Check your policy. Find out if you’re covered. You’ll probably find that you’re not as protected as you thought. For me, the small cost per year is well worth the peace of mind.

How?

I’m sure many companies will be happy to underwrite you for additional coverage. For most people, however, the easiest way to get coverage is through the Divers Alert Network (DAN). DAN is a non-profit medical and research organization that supplies resources for recreational divers.

I signed up for insurance through DAN and it was easy. There are only two steps:

1. Become a DAN member

   Only DAN members are eligible for insurance. This comes with some perks like a subscription to their magazine, but nothing I was terribly interested in. Individuals pay $35 a year for membership or you can subscribe your household (people with the same mailing address) for $55 a year (these prices are current as of 2010 and in US dollars).

2. Sign up for DAN insurance

   Once you have membership, you can immediately sign up for insurance. DAN offers three insurance programs: the standard plan, the master plan, and the preferred plan.

   The standard plan is rather skimpy, which is reflected in the $25 a year cost. There is a $45,000 lifetime maximum payout for the plan.

   The master plan is a little better, offering a $125,000 lifetime maximum payout. It includes a few additional items, such as payment for lost diving equipment, dismemberment (yikes!), and disability. For these additional benefits the price goes up to $35 a year.

   The preferred plan offers $250,000 coverage per incident. It includes slightly higher payouts for each item compared to the master plan, but the cost doubles to $70 a year.

I went with the Goldilocks plan, right in the middle. It balanced decent payout with a cheap yearly cost. For individual membership in the master plan, you’re facing $35 + $35 = $70 a year: a perfectly reasonable cost for active divers.

The entire process took less than 15 minutes, including signing up for DAN membership. DAN will want to know who your primary insurance provider is since some parts of a diving incident may be covered by your primary plan, so you may want to have that information at hand.

DAN is a US organization, so this only applies to American divers. I’m curious about divers from other countries, particularly those with public health care: does your healthcare cover diving accidents, along with hyperbaric treatment, or do you take out private insurance?

For the first time in our fish identification series, we’re going to look at a sea creature that’s not actually a fish. I think leopard sharks are awesome, and hope to come across one in a dive someday.

Physical description

I assume most people can tell a shark when they see one, so I’ll focus on the features that make leopard sharks unique. Clearly the first distinguishing feature is the source of their name. Leopard sharks have long, slender bodies covered in dark ink stain-looking spots. Their underbelly is all white.

They have two dorsal fins. The first is about halfway along its back and the second fin, almost the same size, is close to to the caudal (tail) fin. Leopard sharks also have an anal fin on their rear underbelly. Their caudal fins have a long upper lobe which extends out much further than the lower lobe. The forward dorsal fin and pectoral fins give them their tell-tale “sharky” look.

Leopard sharks have mouths on the bottom of their heads, probably for easy consumption of bottom-dwelling food like crabs and clams. They grow to lengths of 1.2 – 1.5 m (3.9 – 4.9 ft).

Geography and habitat

The leopard shark is found along the Pacific coast of North America, as far north as Oregon all the way down to Mexico. They like muddy or flat ocean bottoms in relatively shallow water (5 m / 15 ft).

They are often found in bays or estuaries, although there have been plenty of sightings in kelp beds and rocky reefs. They usually are found relatively dormant on the ocean bottom. They are harmless to humans, but don’t go harassing them.

Overfishing

Leopard shark populations were rapidly declining during the 80s due to overfishing for food and aquariums. However, regulations introduced in the 90s brought them back up to a sustainable amount. Even though I don’t like the aquarium trade, it is reassuring to see regulations in place that protect threatened species through compromise and sustainable practices.

Further reading

Wikipedia
Animal bytes

Overpressure relief valves are pretty much standard on today’s BCDs. You can identify them as the vent-like areas, either on the shoulder and / or bottom rear of the jacket. Often they have a ball-and-string assembly attached called a “dump valve.” Pulling on this will release air from the BCD through the valve. This allows the diver to vent air easily from almost any orientation underwater, without awkwardly holding their inflator hose toward the surface.

As their name implies, the primary purpose of overpressure relief valves is to prevent the accidental over-inflation of BCDs. BCDs contain what are called bladders, or basically, air bags. These air bags have a finite amount of air they can hold, and exceeding this amount, either by over-filling it or by ascending to a depth that increases the air’s volume enough, would ordinarily cause it to rupture.

The trick is to put a hole in the bladder, but a hole that only leaks air when there’s too much of it. This is accomplished by holding a plug in place over the hole with a spring. This plug effectively corks up the hole. The spring has to be perfectly strong enough to hold the plug in place while air enters the BCD, but weak enough so that the plug pushes out, releasing air, when the pressure inside the bladder gets too high.

BCDs have some maximum pressure they can withstand. This is easy enough for the manufacturer to find out. Just fill the BCD until it explodes. The pressure right before it pops is the maximum pressure, although they’ll lower this a bit for a built-in tolerance.

With this maximum pressure p_max, we can figure out the strength, or stiffness the spring should be. We do this using Hooke’s law, which states that the force of the spring is equal to the deformation of the spring times a spring constant (the stiffness of the spring), or,

F = -k x,

where k is the spring constant that we want to find. We know what force is keeping the plug in place, it is the maximum pressure times the area of the part of the plug that feels the BCD’s air pressure (pressure is force per unit area), or p_maxA. The spring is also be deformed slightly to hold the plug firmly in place. How much the spring is deformed gives us x, which tells us the force exerted by the spring on the plug. With that number we can rearrange and compute the spring constant as

k = -p_maxA / x.¹

If the pressure increases, the spring won’t be strong enough to hold the plug in, and the BCD will release air. You can also manually override the valve by pulling the plug yourself, which is what you are doing when you pull the cord.

Like most parts on a BCD, the overpressure valve is an incredibly simple device. The simplicity of this device helps make BCD inspections a fairly easy process, especially when compared to overhauling a regulator.

1. We are ignoring the mass of the spring and the plug.

Here’s a random set of underwater photo / video links across the web.

The beautiful nature blog featured a few underwater photographs.

Amsterdam couple Cor Bosman and Julie Edwards have their photo albums online. Their photography has been featured in articles and magazine covers.

And here’s a video of a clever little penguin.