Cylinders – Page 3 – Learn to Dive Today Blog

Oxygen tank?

A cylinder, scuba tank, or dive cylinder as they are often called is a pressure vessel containing air under extreme pressure and in most recreational diving cylinders that’s all there is in it.

The air we breathe is made up of 79% nitrogen and 21% oxygen. An enriched air (nitrox) cylinder will contain just that: air, enriched with more oxygen, normally somewhere between 32% and 40% depending on the dive plan. This extra oxygen decreases the nitrogen content of the gas in the cylinder.

A recreation dive cylinder is not an “oxygen tank”. Pure oxygen, if breathed below 10 metres, can result in oxygen toxicity: this can kill you.

The most commonly used dive cylinder is a 12 litre and it has a working pressure of 232 bar (you have around two bar in your car tyres). Pumped to 200 bar, a 12 litre cylinder will have 2400 litres of air inside it (12 litres multipled by 200). In this part of the world (South Africa) a scuba cylinder is required by law to be painted yellow with a grey neck. An enriched air or Nitrox cylinder is required to be marked with an additional green band around the top, preferably so labeled.

Nitrox cylinders showing the green banded marking

Reference to an “oxygen tank” in recreational diving is just that, a cylinder filled with pure oxygen, painted black and white These are primarily for medicinal use in oxygen first aid, in treating decompression sickness or other emergencies. Their other use is or for very shallow decompression stops (6 metres or less) usually done by technical or deep divers. Oxygen tanks are also used (as in the picture below) for blending of enriched air mixes for Nitrox diving.

Correctly marked oxygen tanks at False Bay Underwater Club

If you want to talk about the cylinder of air that a scuba diver wears on his back while diving, the correct terminology (or the term that would be understood by most divers) is dive tank, cylinder or tin of sky (OK, that last one is not entirely serious).

A new addition to the family

Filling cylinders is the bane of the regular scuba diver’s life. It’s expensive (R40 for a tin of AIR?), time consuming, and generally involves either leaving one’s cylinders at a filling station and returning to collect them later (sometimes a risky proposition – you could come back to find your pillar valve has been swapped for an older or dodgier one…), or waiting at the dive centre to have them filled (boring, waste of time, inducement to eat junk food, and so on).

We get free air fills as members of False Bay Underwater Club, but when I dive daily I go through a lot of cylinders. Also, we can’t save up all the empties for a Wednesday evening – the other club members would give us the boot!

This is why we are so proud and pleased to announce the latest addition to the Lindeque family.

We recently acquired a small 70 litre/minute 3 stage compressor. Manufactured in Germany, the unit is well put together and appears rugged. The filter tower is a simple yet effective unit and can be serviced in less than an hour. Driven by a Honda petrol engine it is a bit noisy for running in your living room, but in a parking lot such as Long beach or any other dive site for that matter its perfect. It is manufactured by Mohnsam and has a single filling whip with a pressure relief valve and gauge on the end of the filling whip. I have run the compressor for close to 8 hours now and have had no misgivings.

The unit is light enough for carrying with one hand and slots into a very small space as it is compactly built. On an average day we will often have in excess of 120 bar left after a 60 minute dive at Long Beach, and this will cost as much to top up as an empty cylinder at most fill stations so it is very useful to have the freedom to fill your own cylinders.

With a petrol engine driving the compressor it is versatile and can literally be used anywhere. The drawback with a petrol engine is to ensure the exhaust gas does not come close to the compressor intake so as a precautionary measure I have extended the air intake by means of a section of pool pipe and can thus ensure it is well above and upwind of the exhaust fumes.

Compressor resting on its carry case, filling a cylinder

This baby will be a regular at Long Beach from now on, where I can fill cylinders in the parking area after diving with students, while waiting for the gear to drip dry before I pack it in the divemobile. She’s even small enough to come with us to Knysna, where we can dive as much as we want when we go houseboating.

Strobes

Tony and I love doing night dives. It’s wonderful to watch the sunset, and then be underwater as the sky darkens and the nocturnal marine creatures come out to play. Last year, mostly by accident, we gave each other strobes for Christmas.

A strobe is usually a small, battery powered device that emits a very bright, flashing light at regular intervals. The ones we bought for each other (from Andre) are both rated to 50 metres’ depth and visible for several kilometres on the surface, so useful for emergency signalling both above and below the sea. They burn for hours on only one or two small batteries.

Tony’s is from Seemann Sub (rebranding as Subgear at the moment), and has a small ridged wheel at the end opposite the lit portion that you turn to switch it on and off.

Mine is a Princeton Tec Aqua Strobe that runs on a single AA battery and will burn for eight hours. You turn the cover over the lit portion to switch it on and off.

Both strobes float head up in the water, so when we attached them to our cylinder (just put the lanyard over the pillar valve before connecting the first stage) and submerged ourselves, they floated just behind our heads. You can’t see the light of your own strobe in this position (which is good, because it would get really annoying!) but it’s very visible to your dive buddy. You could also attach it to the shoulder of your BCD – some vests have velcro there for that purpose.

We really enjoyed having these underwater the first time we used them. Lukas was with us on his first night dive, and we had another instructor and his student with us (they also both had strobes), and it was incredibly easy to keep track of where everyone was without having to swim right on top of each other.

These would also be useful on a deep dive, for diver identification, and if you’re diving anywhere that there’s a chance (however slim) that you might get lost on the surface.

Top up my cylinder… and win a repeat customer

Your day’s diving is only over once you are home, equipment rinsed, dried and stored and your cylinders are full. If you own your own gear and live on a coastline like we have in Cape Town then diving is a very cheap sport or hobby. You only expense, other than the trip there and back, would be to fill your cylinders.

Popular cylinders are 10 litre and 12 litre cylinders and if you dived at one of the multitude of easy shore entries around the Cape Peninsula where depths vary from 3 m to 12 metres you can quite easily end up with 100 bar in your cylinder at the end of a 50 minute dive.

If you dived at home in your pool to test your gear or work on buoyancy you would likely have 150 bar after more than an hour in the water. The same cylinder on a 30 metre deep dive for a total dive time of 30-40 minutes including safety stops would be at around 50 bar if you followed your dive plan.

The whole thing is that if you arrived at a dive centre, with a tiny 7 litre cylinder that needs 50 bar (350 litres of air) or a huge 18 litre at 50 bar – requiring about 3000 litres of air to fill to 220 bar – most, but not all dive centres will charge the same flat fee for filling. I understand that everyone has their own business plan and set of procedures but in reality this does not bode well for customer retention

A dive centre that is quick at filling, or that charges less if you only have a “top up” will foster good relations with customers, and if you are happy in their space you will probably buy from that shop

The dive centre that is slow, or charges you full price in fact forces you away as you now go to a different centre to fill, and ultimately make other purchases there too. I drive past three dive centres on most diving days to end up at centre number 4 to fill my cylinders, why? Dive centre 4 is friendly (so are the others), but if I need a top up that’s what I pay for. The staff will always talk diving and show you the latest gadgets, and this fosters good relationships and this is where I shop.

As a diver with your own gear you will always attend to any faults with haste otherwise they spoil the diving experience. As an independent instructor I have eight sets of gear. The gear works harder than the average diver’s gear so a fair amount of maintenance is required. Almost weekly something needs to be fixed: reels wear out, torches get dropped, gloves become holed, fin straps break and the list goes on. Students need everything to work properly if they are to have a good experience so this constant expenditure is necessary.

Where do I buy all of these consumables? Often at the same centre that fills my cylinders. It’s the same place you will go when you need an expensive item such as a new BCD, a download cable for a dive computer, and it is most likely the place you will drop off your regulators for service when the time comes. It is also the place you will buy a bargain from as being a regular customer means the centre knows how you think, the type of stuff you most often buy, and what you do, so when a bargain arrives that they know is just what you need they will do their best to put you and the item in the same area, and hey presto they have a sale…

Like this one!

But more on the new compressor in another post…

Chamber dive revisited

We recently did a chamber dive to 50 metres. A hyperbaric chamber is a sealable chamber, or pressure vessel, somewhat like your dive cylinder (just larger), and has hatches large enough for you to climb in. It is connected to an air compressor or a bank of compressed air, and once you’re in and it is sealed the pressure is increased just as the pressure around you increases as you descend. You need to equalise as you do under water, the only real difference being is that you are dry. A fast descent means constant equalizing and ensuring deep breaths are taken. You will experience nitrogen narcosis, the extent will vary from person to person and you voice will change.

The video below shows us counting backwards from five, showing the correct number of fingers and turning our hands round after each number. It’s hard when you’re narced!

[youtube=http://www.youtube.com/watch?v=zWX-X5RngUs&w=540]

As a recreational diver you will find that your PADI eRDP will not allow you to enter a depth greater than 40 metres when you plan a dive, and the recreational dive planner (RDP or dive tables) will not allow planning deeper than 42 metres. The chamber operator or anyone that does deep technical diving will have a program to enable proper dive planning and ensuring the correct dive profile is maintained, by means of decompression stops.

Checking on the first group of chamber divers

On our dive we descended to 9 metres, and paused whilst two way communication was tested and the operator checked everyone was okay. Thereafter we dropped like a stone down to 50 metres in two minutes. Our bottom time was nine minutes. Nine minutes at this depth gives you quite a decompression commitment and we ascended slowly doing several deco stops on the way up with a total dive time of 39 minutes.

My Mares Nemo Wide showing the dive stats

Our dive computers all joined us for this dive and were placed in a bucket of water (some dive computers will not go into dive mode unless the water contacts are activated). I had a Suunto Mosquito, a Mares Nemo Wide, an Uwatec Aladin Prime and a Citizen dive computer as well as a wrist mount depth gauge.

My trusty Suunto Mosquito showing the dive stats

The computers were all similar in readings and were all between 50.1 metres and 50.4 metres whilst the depth gauge showed 59 metres! It’s safer for your instruments to err on the side of conservatism (i.e. tell you you’re deeper than you are, rather than the other way around). This depth gauge probably didn’t know what hit it!

Analogue depth gauge with red needle showing maximum depth

A very important deep skill on a PADI Advanced course is to compare your depth gauge with your buddy and your instructor. There can easily be huge variations in depth gauges.

Bookshelf: The Coast of Coral

The Coast of Coral – Arthur C. Clarke

I was a huge Arthur C. Clarke fan for many years (still am, I suppose), beginning at the age of about ten and reaching my peak during my university years. He’s a science fiction writer, the kind whose work – when read years later – actually foreshadows developments that are currently just beyond the reach of our technological capabilites, but quite feasible.

He also had a great love for the ocean, and this book is an account of a few months he and his buddy Mike Wilson spent diving the Great Barrier Reef in Australia during the early 1950s. The Aqualung had just been invented and their diving locations were remote, so they did a lot of skin and free diving to conserve their air supply.

The book is partly a travelogue, and partly a description of memorable encounters he had with sea life. It’s clear that the diving adventures Clarke describes heavily inspired Dolphin Island. Wilson took photographs, some of which are reproduced in grainy black and white in the book. There’s a hilarious one of a diver with his cylinder mounted upside down (by current standards) and the hose from his second stage snaking down the side of his body to his bottom.

In his foreword, Clarke mentions that he now reads the sections of the book in which he describes (and advocates) walking out on the exposed reef at low tide with some embarrassment. Coral is sensitive, and shouldn’t be touched. The book is also laced with accounts of spear fishing, a sport I think is ridiculous and distasteful. Disturbingly, they capture a turtle, harrass her extensively (sit on her back for photos) and then try to drag her (alive) out to their boat so they can eat turtle steak for dinner. Fortunately the difficulties they encounter in trying to get her offshore cause them to think better of their plan and release her. But a lot of the thinking is very dated and somewhat repugnant to modern sensibilities.

I was surprised by how funny Clarke’s writing style is – his sense of humour is not something that comes through in his science fiction writings. He speaks of having to “de-louse” one’s vocabulary after having spent any length of time in Australia, and dubs the word “bloody” as The Great Australian Adjective. His argument for turning down an exotic meal cooked by the inhabitants of one of the islands they visit is that he would be very distressed and disappointed were he to develop a taste for the rare meats on offer, and then be unable to obtain them when he returns to London or New York. I informed Tony of this excuse, and expect to hear it next time I cook broccoli.

In all, the book paints a fascinating picture of the Great Barrier Reef prior to it becoming the tourist attraction it is today. The reef’s vast extent is very apparent, and Clarke’s enjoyment of the underwater world is palpable and inspiring. I look forward to diving there one day.

You can order the book here.

Movie: Thunderball

There is much to love in a quality James Bond movie, particularly one which starts with Bond having an extravagant fight with a man in a black dress, totally trashing a large regency-style drawing room in the process. This is one of Sean Connery’s early Bond films – released in 1965.

Two nuclear warheads have been stolen, and must be recovered. Bond travels to Nassau in the Bahamas where he does a lot of diving – some to find the warheads, some to flirt with the ladies, and some to fight with criminals underwater. One free diving episode features a lady diver holding onto the back of a clearly distressed turtle. As soon as she releases the turtle, it ascends for air. Poor dude!

Thunderball heavily inspired the Austin Powers movies, which adds an inadvertent element of humour when viewing them in retrospect. The villain, one Emile Largo, has an eye patch and a white fluffy pet cat, and throws failures and enemies into his pool of reef sharks, who obligingly eat them alive.

The aqualung had only been around for about 15 years when this movie was made, but it has some awesome underwater fight sequences (knives cutting air hoses, masks ripped off…), and features a huge orange sled/DPV capable of transporting up to six divers at once. The divers use harnesses rather than BCDs, the exhausts on their regulators are behind their heads, and they have no octos, but other than that look as good – or perhaps better, because their gear is a uniform basic black (including their cylinders) without bits hanging off – than divers today. Of course, they are a team of crooks, so they have to dress in matching togs.

When Bond eventually locates the sunken plane – incidentally containing his lady love’s completely undecomposed brother – he instructs his pilot to shoot one of the sharks milling around the site “to keep the others busy”. Nice.

The film concludes with an EPIC underwater fight scene – goodies in orange, baddies in black – involving perhaps 30 divers. There is hand to hand combat, lots of spear guns, knife fighting, and a lot of frantic finning. Nearly a quarter of this two hour movie was filmed underwater.

There is so much goodness here… The standard Bond misogyny – women swooning over him and being used and discarded in short order, very short shorts on unashamedly hairy men, a young Sean Connery… and a boat called the Disco Volante. The underwater scenes are very well done, and plentiful. What’s not to love?

The DVD is available here if you’re in South Africa, otherwise click here.

The reason I’ve been wearing flip flops to the office

It’s been a while since I put a picture of my feet on the internet, and this is my first diving injury to speak of in almost 110 dives. So this is to demonstrate that fifteen litre cylinders do not mix well with feet, even feet wearing 5 millimetre booties.

Excuse the brutally chopped toenails. I put the cylinder down on my foot while unpacking the boat after our dive on the Cape Matapan. Next time I’ll just take my own kit (with my 10 or 12 litre cylinder) off and let Tami carry her own fifteen!

A near miss at 26 metres

Diving accidents are rare, yet in almost every case stupidity features highly and Saturday’s dive was no exception. We were a group of seven. Three were former students with various qualifications (minimum Advanced), all having done between 20 and 100 dives. All regular divers with me, they were just tagging along for a fun dive. I had three students doing a deep dive for their Advanced diver qualification. All three had completed most of the dives for this course and deep was one of the last dives. I had assessed all three during previous dives and did not anticipate any problems. Cecil, very capable, excellent dive skills and safety concious; Mark, very capable, good dive skills and Diver X, also capable and on two previous deep dives had displayed good watermanship.

So what went wrong?

We descended in a strong current, staying reasonably close together and doing a nice safe slow descent. (I am not a fan of dropping like a brick.) I paused at 20 metres to make sure there were no signs of stress from anyone. Tami was a little slow in getting down but her buddy was watching and of all the group Tami rates high up on the best of the rest list so I was not concerned. The visibility was good, 10 – 12 metres and I could see from everyone’s bubbles that they were all breathing in a relaxed manner.

We dropped to the bottom, and I handed slates to the three Advanced students, slates with a few questions, a bit of maths, and a simple puzzle. This task is a good indication of nitrogen narcosis and a diver’s state. Some of the questions on these slates are ‘”How much air do you have?” and “What is your depth and who is your buddy?”

I time this exercise, so I check my dive computer during the process. This tells me if the depth answer is right, and at the end of the exercise I ask each diver to signal their air supply. Diver X got most of the answers wrong, and more to the point his air pressure answer was 10 bar. I asked him to look at his gauge as everyone else had close to 200 bar. He indicated he did not understand his gauge so I looked at his gauge and it was ZERO.

He then turned and swam away from me towards Cecil, pointing at Cecil’s body. Having someone point at your torso tends to make a person look down to see what he is pointing at. At this point I had caught him up and started to turn him around. He then spat out his regulator and at this Cecil realised there was some problem and perfectly executed the raised arms so his octo was in clear view.

I shoved my regulator in Diver X’s mouth and looked at his eyes – he had no idea of what was going on. I then gathered the group and we started to ascend with Diver X on my octo. At one point I had to bang him on the chest to get him to understand he should hold onto my BCD as he refused to do so and twice drifted off and lost the regulator. We did a short safety stop and ascended. He did not orally inflate his BCD on the surface so I did it for him.

I am extremely grateful to Grant for racing the boat over and getting us out of the water quickly, as we surfaced far from the buoy line (owing to the howling current) and the unexpectedly rapid ascent (and the fact that my hands were occupied holding onto Diver X) meant that we hadn’t deployed our SMBs. The dive site we were at, the wreck of the SS Cape Matapan, is very close to the shipping lane into Table Bay harbour and very exposed. The southeaster was strong and the sea was choppy with fairly large waves making divers on the surface without SMBs very hard to spot.

What do we learn from an incident like this?

Check, check, check your gear. I doubt Diver X checked his equipment before the dive. Second, he did not do a proper buddy check.
Keep your skills sharp. Diver X has forgotten many of the skills he was taught when he did his Open Water course. Refreshers exist for a reason.
Be fit to dive. Get enough sleep and don’t party the night before a dive – SPECIALLY a deep one, where there is no room for error. DON’T come diving if you’re hung over or stoned.
Be alert before and during the dive. Check your pressure gauge before you stow your gear on the boat, when you kit up before rolling into the water, again when you get to the bottom, and frequently during the dive.

And, if you require a dive buddy with exceptional skills, then Cecil is your man.

I know you will all blame nitrogen narcosis for this incident, but on the way up I stopped at 15 metres, again at 10 metres, and again at 5 metres, and there was no change in Diver X’s behaviour. I had to descend from 2 metres back down to the group doing their safety stop and get them all together so we could surface as a group as we were diving on the edge of a shipping lane (I was concerned that we had possibly drifted into the shipping lane in the current) and I had not surfaced with a SMB as I could not release my grip on this diver to deploy the SMB.

What most people don’t realise is that when you don’t take dive safety seriously you almost always put others at risk. I had five other people with me, their safety being my responsibility. We risked surfacing in a shipping lane, without an SMB in less than perfect surface conditions (to put it mildly). All in all other people were put at risk due to the casual disregard for safety by one diver. Don’t dive stoned, hung over or when not serious: not with me and not with anyone else.

I’m left with one cylinder half filled with sea water, one salt-filled pillar valve, and one first stage and two second stages requiring complete rebuilds or servicing. And hopefully some thoughtful divers who all learned something today.

FAQ: What is a double tank dive?

Carel of Dive Inn goes the extra mile for one of his clients... And demonstrates what a double tank dive is NOT!

From time to time dive charters will offer double tank dives. This means you load your gear on the boat, along with a second cylinder and off you go.

It is common practice to choose two sites, the first being the deeper dive. It’s old school opinion that a deep dive must be done first (debatable, but it does improve your overall dive profile). You then head off and do a dive and instead of returning to the harbour or launch site to change cylinders, you change cylinders on the boat and then move off to a new dive site.

What are the benefits?

Well, your day of diving is much shorter as you will probably have a 30 minute surface interval and then do the second dive whereas driving the boat back to shore, off loading, re loading and so on takes well over an hour in most instances.

It’s important the boat is not overloaded. A double tank dive means each diver takes up two slots on a dive rack on the boat so numbers need to be smaller, you all need to agree on where you are going and you all need to want to do both dives. This is easily arranged if you are a member of a dive club and dive often with the same people. Odds are you all enjoy similar dive sites and this makes planning easier.

This isn't a double tank dive either... Mauro is doing some technical training!