Archive for ‘Other’

Considerations for ‘How Much Gas’

By , 21 September, 2015, No Comment

“1/3 in, 1/3 out and a 3rd in reserve”, “surface with a third of all your gas”, “save a third of your gas for emergencies” are typical statements you may hear when talking about rule of thirds.
The rule of thirds is variously attributed to either the pioneers of cave diving in the UK and the founders of what was to become the Cave Diving Group (CDG) or to their slightly more modern counterparts exploring the springs of North Central Florida.

Regardless of origin, the rule was used by cave divers, allowing one third of the gas supply to be used on the inward journey, one third for the dive out and a third held in reserve in case things did not go according to plan.

In simple terms it seems to work.  Looked at more closely there are flaws, albeit only minor, which need to be addressed as part of your dive plan in order to stay safe if things go wrong.

First let us look at the common belief that we should dive ‘to’ the rule of thirds.  For this example we will assume identical breathing rates and identical cylinder sizes between two dive buddies.  Should one diver have a catastrophic loss of gas at the furthest point of penetration the other diver has a third of his gas ready to donate to the out of air diver.  However if the process of the first diver trying to deal with the gas lost in the first instance takes any time or the act of sharing air and sorting themselves out for the exit takes more time, the diver donating gas will have been using some of his or her exit gas already.  Those few minutes to get organised for the swim out could leave gas supplies perilously close to the limit or even being exhausted prior to the team reaching safety.

What if the air sharing swim out of the cave takes much longer just because the long hose has been deployed?  What if the two divers did not have identical breathing rates?  If the out of air diver has a measurably larger breathing rate then the donor diver’s reserve third might simply not be enough gas!

How can we plan to avoid these potential pitfalls?

Firstly we can look at how we might apply the rule of thirds before we discuss other planning considerations.  When I said that many divers will dive to the rule of thirds the implication is that they will turn the dive when the hit the point of having used 1/3 of their gas.  But turning around takes time.  Making sure everyone on the team has seen the signal and they too have turned takes time.  All this is eating in to the gas for the swim out and as so in to your reserves as well.

Perhaps now would be a good time to have a look at how we go about calculating our turn pressure based on the rule of thirds.

Let’s say we have a starting pressure of 210 bar.  That one is nice and easy, 210 divided by 3 is 70 bar, so each third is 70 bar.  70 bar from our start pressure would mean the turn pressure is 140 bar.

What if the stat pressure is not a nice easy number to divide by 3?  I know a few divers who can seemingly instantly divide almost any starting pressure by 3 and calculate their turn pressure.  I have also seen diver then use this very precise pressure during the dive when they have digital readings or pressure.  However I have also noticed how this seems to entice those divers to go right to the limit of their gas.

Simpler and safer would be to err on the side of caution and make the maths easier.  Just for fun I will look at a couple of examples.

A starting pressure of 220 bar is not easy to divide by three in your head whilst bouncing around on choppy seas discussing the dive plan and turn pressures.  Far easier is to round the number down to the next lowest number that is easily divisible by 3.  In this case that would be 210 bar.  The usual turn pressure for 210 bar would be 140 bar (210 – 70) but in this case we are starting with 10 bar more, so very simply, calculate how big 1/3 is from our rounded down pressure (70 bar here) and subtract that from your actual starting pressure. 220 bar – 70 bar would give a turn pressure of 150 bar.  70 bar in and 70 bar out would leave the ‘largest 1/3’ as the reserve, 80 bar.

What about 200 bar start pressure.  Well 195 bar is easily divisible by 3.  Each 1/3 would be 65 bar.  65 bar subtracted from your actual start pressure would give a turn pressure of 135 bar.

For some that calculation might not be so easy but there is no reason why you can’t write a list of start pressures and turn pressures in your wet-notes so you don’t have to worry that your maths isn’t so good or that you might get it wrong.

When you have calculated or looked up your turn pressure don’t just tell the team what your turn pressure is, tell them how much gas you have and then what your turn pressure will be.  That way you get a few other brains to check that you have got it right.  After all, it is their safety too that could be jeopardised if someone gets it all wrong.

Remember too that for wreck penetration dives, while still using the same approach with the rule of thirds, it is applied a little differently.  We have to take in to account the fact that having exited the wreck, in most cases, we still have an ascent to make and possibly decompression too.


At Davy Jones Tech all this information and more is covered during training, so you fully understand and you can put it into practice.


Contact us now if you want to further develop and expand your diving.

by Matt



Twin set

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A Technical Divers Skills Pt 2

By , 30 August, 2015, No Comment

At Davy Jones Locker, we have a busy technical diving department that caters for people that are new to the discipline and experienced technical divers. Your foundational scuba diving skills need to be second nature so when an emergency arises you can focus on solving the problem and aborting the dive. I feel these six essential skills should be practiced on every dive, whether you’re a experienced technical diver with 1000’s of dives under your belt or someone just starting out.

  1. Predive Check, Descent/Bubble Check, and S-Drill– While there are three skill sets listed here, we group them together because the overall objective is the same for all three: start the dive properly equipped and with fully functioning equipment.
  • Predive Check – Once fully dressed for the dive, as a team, each diver runs through their own equipment to verify primary cylinders are full and valves open with turn pressures verified, stage/deco cylinders are full with regulators pressurized but valves turned off, BCD inflates AND holds gas, dive computers/gauges are turned on and functioning properly, mask/fins/weights/etc. are donned and in good condition to dive.
  • Descent/Bubble Check – Depending on conditions and site, either on the surface or on the initial descent, the team inspects each other’s equipment looking for leaks and trapped or entangled equipment.
  • S-Drill – Each team member takes turns conducting the proper gas sharing procedure with another teammate.
  • The dive does not start until all of these checks have been conducted, any complication must be resolved before continuing the descent.
  1. Trim/Buoyancy/Finning– It’s not just for looks. The importance of being able to hold your position in the water column and prevent silting-out an environment cannot be overstated; and everyone can use a little practice. Every dive, try to spend some time focusing on different finning techniques and trim/buoyancy control. Grab the GoPro and let your buddies film you so you can get some valuable feedback on what you actually look like in the water as well.
  2. Valve Drills– On every single dive, you should practice shutting down and re-opening each valve. Make sure do to this with a teammate so they can verify each valve gets re-opened. Depending on your exposure protection and recent diving activity, you may find it more difficult to reach your valves than you remember. It is important to work on this flexibility and muscle memory on a regular basis, because when you really need it is not the time to realize that you cannot reach a valve.
  3. Remove and Replace Stage/Deco Cylinders and Bottle Swapping– It is important to occasionally practice removing and replacing stage/deco cylinders in order to maintain this muscle memory. Even if the dive does not require you to stage a cylinder, practicing this skill often will speed up and smooth out the process on the dives where it is required. Going over your bottom time because you were fumbling with a stage cylinder is both embarrassing and dangerous. You should also practice swapping bottles with teammates. This can be done while decompressing by swapping stages or lean deco gasses that you are finished with between your teammates. This increases team awareness, communication, and equipment familiarity. It is extremely important to check that no hoses or equipment have been trapped by the stage/deco bottle any time you replace one.
  4. SMB Deployment and Reel Skills– Deploying an SMB and running a reel are skills that deteriorate quickly when not practiced regularly, and sloppy work in these skills can be extremely dangerous. Practice these skills as often as you can, if you don’t use it, you’ll lose it.
  5. Post Dive Briefing– It is extremely important to debrief every single technical dive. Discuss the highs and lows of the dive, where communication was good, where it was bad, and what areas can be improved upon for the next dive. You cannot see yourself in the water, so it is important everyone in the team provides some constructive criticism. This is often done with friendly banter, but it is important to remember that this feedback will help you improve your diving and safety.


While this is not intended to be an all-inclusive list of skills to be practiced for technical diving, these six skills are applicable to most technical diving scenarios, and can be easily practiced on just about every dive.

by Matt



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Jenkins Whip Ray

By , 22 July, 2015, No Comment

I have seen a few now over my time here on Koh Tao and every time they are always impressive. Last night however was the first time I have ever seen one at night and it was awesome. They seem far more relaxed at night. Where as during the day they seem to be either sleeping or scare easily at night it was far more focused on hunting and therefore we were able to spend far more time with it moving about the bottom. I am talking about the Jenkins whipray, a species of stingray. Not as common here as the Bluespotted Ribbon tail rail it grows to an impressive 1.5m across and has a broad, diamond shaped pectoral fin disc and a whip-like tail without fin folds. What is even more impressive about it is a row of large spear-like thorns along the midline. It is less colourful than the blue spotted being a more gray and brown colour on the top and white on its underbelly. I’ve always loved the way rays move gracefully through the water and the Jenkins is no exception. It made it one of my favourite night dives to date and an extremely good reason to go on more of them! Unfortunately the International Union for Conservation of Nature (IUCN) have assessed the species as vulnerable in Southeast Asia, due to intense fishing. To lose such as magnificent creature would be a tragedy and I hope something can be done about this as I hope more can experience diving with these animals.

by Alex


jenkins whip ray

Jenkins Whip Ray






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Cave Diving

By , 19 July, 2015, No Comment

With a couple of my colleagues off on a cave diving trip at the minute, I am left here on Koh Tao (which they take great pleasure in reminding me every day they are away) to introduce and inspire other people  about the challenging, exciting and dark world of cave diving.

The term “cave diving” itself draws wide eyes. Most rural farmers in Thailand firmly believe dragons and ghosts live in those bottomless pits deep in the jungle, and jumping into one of them resembles suicide rather than stimulation to the explorer’s mind.

It’s no wonder Thailand isn’t synonymous with cave diving like Mexico and Florida seem to be. Few explorers over the last two decades have ventured into the jungle at their own expense to lay lines and teach cave diving. Unfortunately, very little information was shared and when the next generation of cave divers came along, and caves had to be rediscovered all over again. Luckily some GPS data was passed on, and cave instructors have driven days through the jungle to locate one of the hundreds of caves.

Thailand has enormous limestone formations, generating its unique topography. Limestone is the perfect breeding ground for dry caves to be formed. National projects have flooded entire valleys to supply hydro-power and preserve the rain-forests. A lot of the previously dry caves in these national parks are now flooded and dive-able. Besides stunning panoramas resembling Lord of the Rings scenery, the caves now offer the prefect training grounds for shallow cave training with plenty of the decorative stalactites the novice diver expects to see.

Most cave courses or trips will start in the more shallow caverns, before heading to one of the many deep, dark sinkholes and thermal vents located further south.

The limestone ridge, stretching throughout southern Thailand is very old and has acidic water from the rain-forest above which has carved enormous, mostly unexplored tunnels. Because of the limestone, Thailand has over 2000 caves, both wet and dry. Today less than 10% of these are known.

Thermal vents are deep springs, bringing up a variety of minerals and gases. This usually provides a turquoise, slightly milky water. And these vents are deep, very deep. One has even been explored up to 239 meters.

Other sinkholes are enormous with entrances well over 300 feet/100 meters in diameter. Divers can easily enter scootering side by side, as the visibility can be well in excess of 100 feet/30 meters.

Most of the caves being dived now are easy access and have no strong flows in or out. This means the caves are dive-able for everyone year-round, as we are not limited by weather conditions.

The variety in cave diving experiences found in Thailand is unequaled in comparison to any other part of the world.

by Matt (Feeling left out)


diving-big-cave (2)

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Christmas tree worms

By , 4 July, 2015, No Comment

Davy Jones’ Locker is located in the Gulf of Thailand on a paradise island named Koh Tao were here we are fortunate enough to be able to dive all year around in some of the most beautiful waters in the world.

Koh Tao offers scuba divers (beginners to advanced levels) the opportunity to experience diving with a huge variety of marine life as the Christmas tree worms.

Christmas tree worms (Spirobranchus giganteus ) are Christmas tree shaped tube-dwelling worms with twin spirals of plumes used for feeding and respiration. They have a tubular, segmented body lined with chaeta, small appendages that aid the worm’s mobility. Because it does not move outside its tube, this worm does not have any specialized appendages for movement or swimming. These worms are sedentary, meaning that once they find a place they like, they don’t move much.

They come in many colours including red, orange, yellow, blue and white and though they are small with an average 3.8 cm in span, they are easily spotted due to their shape, beauty and colour always makes an eye-catching display.

Christmas tree worms are polychaete ciliary feeders that feed using their radioles, the hair-like appendages or “feathers” that circle outward from the central spine, to catch phytoplankton floating by in the water. The food is then passed down a food groove by ciliary tracts — lines of tiny hair-like extensions on the surface of cells that generate water currents to move food or mucus. The food particles are sorted and larger particles are discarded. Sand grains are directed to storage sacs to be used later for tube building.

There are male and female Christmas tree worms. They reproduce by sending eggs and sperm into the water. The eggs are fertilized in the water then develop into larvae and become part of the zooplankton to be carried by the currents to then settle on coral heads and then burrow into the coral to form their burrows.

As you can see this is just one of the fascinating creatures that we encounter every day here at Davy Jones’ Locker. Feel free to pop in for a visit and we will be happy to share these unforgettable experiences underwater with you.




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