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Neil Harris decants pure oxygen on board Big Cat Reality.



By Andrew Whitehead

Published in Dive Log Australasia
November 2001


Nitrox is oxygen enriched air, or air with a greater than normal percentage of oxygen.  It is also known as Enriched Air Nitrox (EANx).  Divers who enrol in the Nitrox Diver Specialty course at Absolute Scuba have various motives for completing this course.  These include increased bottom time, reduced chance of DCI, reduced post-dive fatigue and sharper awareness during the dive. The use of Nitrox pushes you towards using better equipment and having a more disciplined approach to diving.  Most of the benefits of using Nitrox can be boiled down to one thing – safety! 


The five students on the September course completed their self-study requirements and arrived for the evening theory session at the shop in Capalaba, Brisbane.  I completed the TDI Nitrox Diver course in June 1997 and have been using Nitrox ever since.  On this occasion, I sat in on the course as Neil Harris delivered TDI’s online presentation, complete with humorous anecdotes and good advice.  Naturally, we covered oxygen toxicity, partial pressures and nitrogen toxicity in some detail. The TDI course is very comprehensive and also includes a few simple formulae, dive planning, blending and analysing gas mixes. 

During the theory session, Neil presented the various bits of equipment that are used in Nitrox diving.  Out came the Nitrox computers, oxygen-cleaned cylinders, DIN valves, high performance regulators and oxygen analysers.  Since most of the students had air computers, they would be able to dive these to their deco limits and still have a safety margin due to the reduced nitrogen in their breathing gas.

Dive Planning

Apart from the practical aspects of analysing your gas mix, filling in the tag on your tank and the log sheets, the basic change to diving preparation is the calculation of your Maximum Operating Depth (MOD) for the particular gas mix in order to minimize the risk of oxygen toxicity.  To avoid errors, we use a simple table which shows the MOD for various oxygen mixes at various partial pressures of oxygen, depending on the circumstances.  As a very careful diver, I always use the recommended maximum partial pressure of 1.4 Bar.

Dive planning using Nitrox can be made very simple.  The most common mixes that people use are 28%, 32%, 36% and 40%.  These allow you to dive to a maximum depth of 40, 34, 29 and 25 metres respectively.  Since I rarely dive below 30 metres, I generally ask for an EANx 36 mix and set my depth alarm accordingly.  Easy!

The Gear

This is a course which involves careful dive planning and the use of quality gear.  During the dives that made up the practical part of the course, the students used 12 litre working capacity (100 cu ft) oxygen-cleaned steel tanks with the distinctive green and yellow Nitrox band.  The tanks all have high pressure DIN valves so they can be filled to their maximum working pressure of 230 Bar.

I went along as a pleasure diver equipped with a 15 litre (120 cu ft) and two 12 litre (100 cu ft) steel tanks, all filled at the shop with EANx 36.  I also took my Poseidon Jetstream and Scubapro G250 Nitrox regulators with analogue gauges attached. In addition, I wore a Nitrox computer on my wrist pre-programmed with this gas mix. A depth alarm at 29 metres limits the partial pressure of oxygen to a maximum of 1.4 Bar when using this mix.  As Bret Gilliam (TDI) says, this is not technical diving, just safe recreational diving.

The Vessel

On Friday night, we met at Newport Waterways, Scarborough, and loaded our gear, and two “G cylinders” of oxygen, on board “Big Cat Reality”. This is a 25 metre, luxury live-aboard vessel which is available for charter to the recreational scuba diving industry as an overnight dive vessel.  Big Cat Reality primarily provides a service on weekends, as a luxury dive charter vessel with facilities to accommodate up to 24 divers and 7 crew including a K180 dive compressor, and rescue vessel.  She has the latest navigation equipment with a professional and experienced crew. Big Cat Reality has a 10 metre beam which provides a large stable dive platform.  This is a good thing because it was going to be windy!

Nitrox Fills

Under Neil’s careful supervision, the students learnt how to calculate the correct amount of pure oxygen to put in their tanks, so that it would end up with the required mix when topped up with air. For example, how much oxygen do you add to a tank to get a 32% mix when the previous dive left it with 70 Bar of EANx 36?  Sounds complicated!  Once again the ‘rocket science’ myth is removed by using a simple table to arrive at the correct answer.

The air from the on-board compressor was passed through a portable filter with DIN fittings. When each tank was full, the diver analysed the mix, filled in the tag and the log sheet with the pressure, gas mix and maximum operating depth.

The Dives

We crossed Moreton Bay on Friday night and anchored in the shelter of Tangalooma Wrecks on the west coast of Moreton Island.  At first light, we moved north to Curtin Artificial Reef for a dive at high tide. 

Curtin Artificial Reef

We descended the anchor chain and located the Transfield Barge near the anchor.  A big Cobia came out of the hold and on to the sand. Inside the barge, a 150mm Nudibranch was making its way across the sand! Over near the north end of the wreck, a very large black stingray rested on the bottom.

Neil’s Group went over to the tug Loevenstein, then explored the gravel barge Estrella Del Mar then back to the Transfield Barge.  Mike and I did not have the URGQ Chart with us and had to rely on memory to navigate from the sand carrier Centipede, to the yacht Solace, to the tug Loevenstein, and into the Etmore at 22 metres.  We then retraced our route back to the anchor.

As it was a neap tide, there was not much water movement in the outgoing tide.  We were able to have a slow drift dive on Curtin Artificial Reef after morning tea.  Neil took us on one of his guided drift tours and we had a lot of fun scooting in and out of the wrecks.  We started at the southern end of the reef at the old tug Melbourne, then to Barge 25, the Transfield barge, the upside down Utah, and then stopped at the Bremer. The students with reels practiced sending their safety sausages to the surface to indicate our position.  After completing our drifting safety stop, Big Cat Reality was right there to pick us up.

Flinders Reef

After a superb lunch, we headed out to Flinders Reef, 5 kilometres off Cape Moreton.  The crew spotted some large green turtles near the boat.  During the afternoon dive, you could hear the whale song underwater!  There were small cod, big Wobbegongs, and the usual abundance of reef fish. The ‘no extraction’ policy is working. The marine critters are really becoming abundant on this totally protected reef.

One of the students was a bit heavy on the air consumption so I lent him my 120 cu ft tank for the night dive and then for the rest of the trip.  Neil spotted a big crayfish and a large male green turtle. He also claims to have seen a Spanish Dancer as long as his forearm, pink in colour with two pink shrimp on its back. He even got it to swim, magnificent!!! The large parrot population is improving and we spotted a huge Wobbegong.

We moved over to the sheltered anchorage at Yellow Patch and had a barbeque dinner on board.  Next morning, we anchored on the Cementco which was scuttled in 1985 near Flinders Reef.  We swam around the outside of this upside down barge and peered into the various holes underneath the hull reaching 26 metres.  Then we went for a little swim through the wreck where there were quite a few baby cod.  Hopefully they will grow up!

The second dive was at the northern end of Flinders Reef where we saw more turtles, egg cowries, Nudibranchs, and a 3 foot long potato cod that was not interested in leaving the cleaning station. We examined one of the new permanent moorings before ascending to Big Cat.

Smith Rock

After lunch skipper James took us over to Smith Rock. We had a great time swimming around the wreckage of the former Liberty ship Marietta Dal which hit Smith Rock and sank in 1950. Those old Liberty ships were welded together and had a bad habit of breaking in half!  The wreckage is strewn all over the place at about 12 metres.  We missed the bulldozers this time but saw a lot of interesting country and a good mix of fish.

The Future

You can tell who was diving on Nitrox on the way home from a trip.  The air divers are all asleep!  The future of diving is in Nitrox and ultimately in high-tech rebreathers.  We need to move away from the common alloy 88s with their unstamped K valves to the new steel tanks which are 100s or 120s.  These can be oxygen cleaned and fitted with DIN valves rated to 232 bar.  You can then take advantage of the increased bottom times that are available when using Nitrox, by using these bigger tanks.

The students all demonstrated a high level of competence in their diving skills and also had a great time doing it.  They were subsequently awarded their Nitrox Diver certificates.

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