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Enriched Air Nitrox Formulas February 28, 2011

Posted by Chris Sullivan in Technical Diving, Training.
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Formulas for Nitrox are used for several purposes:

  1. Calculating the Equivalent Air Depth (EAD) to allow the use of standard Air tables to calculated no decompression limits, repetitive groups, or decompression schedules.
  2. Calculating the best or optimum mix for a given depth
  3. Calculating the maximum or contingency depth for a given mix to minimize risk of CNS Oxygen Toxicity
  4. Calculating the oxygen exposure for managing Pulmonary Oxygen Toxicity
  5. There are also formulas for partial pressure filling of tanks, which determine how much Oxygen needs to be put in before topping up with air. A more difficult problem is how much Nitrox to drain from a tank before topping off with air in order to reduce the fraction of O2 in the mix.

In basic Nitrox training we’re given the formulas for 1-3, and a table to manage 4 in recreational diving situations. The PADI table uses the NOAA or “CNS” clock only, with no surface interval credit. Surface interval credit makes sense in technical diving when we’re managing pulmonary toxicity using the Repex method, but when using one table to manage both I’m not convinced that it would be wise to apply it.

Even in basic training, I usually like to point out why the formulas work, instead of using rote learning.  I first starting doing this when I took the course myself in the Turks and Caicos in 2005, and found that the understand helps me both remember the formulas (which I’ve never been good at) and to catch errors in my calculations.

Except in case 5, all of the formulas relate to the pressure of Nitrogen or Oxygen in Absolute Atmospheres. Everything that in Imperial divides, multiplies, adds or subtracts 33 (or 34 in fresh water) is converting between gauge and absolute atmospheres, and the formulas themselves are much easier to remember in ATA than in ATG.


Hard to imagine February 28, 2011

Posted by Chris Sullivan in Training.
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This story of a the death of a 28 year-old woman is troubling. As with most stories like this, there aren’t enough facts to really form an opinion. The woman was with an instructor in Fiji, but the instructor surfaced without her. She was later recovered and her boyfriend tried to revive her. It doesn’t mention what she was doing with the instructor or how many other divers were with them. Based on the article, one might wonder:

  1. How could an instructor lose track of a student in the clear waters of Fiji?
  2. Why wasn’t the EFR trained instructor involved in the resuscitation?

PADI is investigating and well they should, at least based on this story.

After writing this I noticed another story this time describing the instructor as a “dive master”, and that he surfaced to change cylinders. If she was with a buddy, and was sufficiently experienced, that just might have been reasonable if conditions were good. But the article goes on to describe strong currents. This one also reports that the boyfriend gave CPR rather than the “dive master”, which is odd.

Goes to show that precision reporting on dive accidents doesn’t occur in the mainstream press.

Cozumel 2011 Day 3: Cedral Wall February 27, 2011

Posted by Chris Sullivan in Dive Log.
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My third diving day in Cozumel began with the familiar site of Cedral Wall, which I’ve dived on previous trips. This dive was to be the deepest of the trip at 125 feet, although it got shallower fairly quickly allowing a bottom time of 41 minutes plus a 5 minute safety stop. We saw lots of turtles and the current was quite strong in the deeper part of the dive.

Working with the currents is an important skill in Cozumel drift diving. In general, the closer you are to the reef the less current you experience, so if you’re getting ahead of the other divers you can let them catch up by hunkering down. Good buoyancy control is important to keep you and your equipment from damaging the reef, especially when the currents are pushing you up or down. Most reefs have plenty of features that also allow you to hide from the current, often taking the form of ledges which are nice to look under. It can be quite useful to have a dive light handy for that purpose. A third method is to present a small profile to the current with a horizontal body position, but that’s effective only for a short while as the current will eventually accelerate you to equal its own speed.

The one thing that you definitely don’t want to do for any length of time is fight it, especially at that depth as you’ll find yourself low on air in no time. If you fall behind, usually ascending 10 feet or so off the reef and a couple of kicks will catch you up pretty fast.

The dive featured a big Black Grouper and a Parrot Fish that was almost as large. I also photographed a Juvenile Spotted Drum, which was a feature of just about every dive, although I only saw one adult the entire trip. The Juvenile has no spots. These develop later.The lobster population seemed to be doing well with 3 hiding under one sponge, and near the end of the dive we had an encounter with a young Green Turtle.

Juvenile Spotted Drum

The lobster population seemed to be doing well with 3 hiding under one sponge, and near the end of the dive we had an encounter with a young Green Turtle.


Young Turtle Heading Towards Surface


Cozumel Day 2 – Camera Trouble February 25, 2011

Posted by Chris Sullivan in Dive Log.
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Got behind on my logbook for a while so I can’t tell you much about this dive. It was at Paso Del Cedral, which was a nice drift dive. I must have not put the camera in its cradle correctly the night before as the battery died shortly into this, the second dive of the day. From then on I started taking the battery out and charging separately and had no further problems, at least when I remembered to bring the camera back from the boat. So only 1 picture for the dive, a nice close-up of a crab.

A lot of the dives in Cozumel are arranged (at least by my shop, Blue XT Sea Diving) to get shallower so we can have a nice long multi-level dive. Often then end up over sandy areas so we always keep a look out for Rays. This dive didn’t really have a safety stop as it just got shallower as time went by. From a maximum of 57 feet the dive lasted 69 minutes for me, a bit more for Grunt & Jackie. We were rewarded with the sighting of a Spotted Eagle Ray, although I have no photo for reasons cited above.

Cozumel Day 2 – Chun Chacaab February 18, 2011

Posted by Chris Sullivan in Dive Log.
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To my surprise on day 2 we headed once again to the south of the island to dive Chun Chacaab, which more or less starts where Maracaibo shallows leaves off. The dive was more arduous than we normally have in Cozumel, because the current was present but going in all directions, along with a fair amount of surge. That meant swimming instead of merely steering with the current. It was the same bunch of divers as the day before, namely Joe, Braxton (also known as “Grunt”, or “Ronco” in Spanish), and Jackie. Mago was still our Captain but Blue XT Sea’s regular guide Pedro had taken over the underwater duties.

There were plenty of Turtles and both an adult and a juvenile spotted drum, as well as the usual assortment of Lobsters and Crabs.

Adult Spotted Drum

Juvenile Spotted Drum

Cozumel Day 1 – Lion Fish Safari February 14, 2011

Posted by Chris Sullivan in Dive Log, Ecology.
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The boat from Blue XT Sea Diving was a little late picking me up but I didn’t mind as we were heading for Maracaibo shallows on the southern tip of the island near the lighthouse, one of my favourite spots. The big difference between this and last time I was hear was that 2 years ago there was mention of Lionfish being spotted on the reef but I didn’t see any personally. This time they were everywhere. Our guide speared more than a dozen and eventually had to give up because there were too  many.

We came across a spot where several of them were hanging out, with a Nurse Shark sleeping under a ledge nearby. The shark flinched when the Lion Fish was hit with the spear, but didn’t swim away, nor did seem to smell blood in the water – or perhaps it just wasn’t hungry. We also say porpoises on the surface before the dive, and then under water, although they didn’t come too close.

At the end of the surface interval, Captain Mago saw a large Eagle Ray. At first they thought it was a Manta but we got close and could see that it wasn’t. Unfortunately it was so bright I could hardly see the screen and that made the pictures hard to frame. This is probably the best one that doesn’t have part of the Ray outside the frame.

The highlight of dive 2 in Palancar Caves was running across a large Stingray. I really like the way the sand is rolling off its wing in this picture.

And that was just day 1. Lots more to go!

Cozumel Tomorrow February 11, 2011

Posted by Chris Sullivan in Dive Log.
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14 days in Cozumel, at least 7 of them diving. Can’t wait!!