Skip to content

Various sized scuba cylinders

Get your own scuba kit, part 3: SCUBA Cylinder

Getting to know your scuba cylinder

The most basic piece of equipment is unfortunately also the most overlooked – your scuba cylinder. You could go diving without fins, as many disabled divers do, or even go without a BCD as some harness configurations allow, but there is simply no SCUBA dive without a dive cylinder.

The types of high-pressure cylinders

There are three main types of high-pressure cylinders – Aluminum, steel and composite cylinders.
You won't find composite cylinders in dive centres as they have bad buoyancy characteristics and most are not cleared for underwater use at all by the manufacturer. However, firefighters use them a lot. They consist of an aluminum or steel liner which is wrapped over by a material such as carbon fiber, glass fiber or Kevlar bonded with a suitable epoxy.
Aluminum cylinders are the most widespread. They have low maintenance costs and offer a great service-life if treated right. Due to Aluminum not being the strongest alloy these tanks have comparable thick cylinder walls making them quite heavy.
Steel cylinders are very common around Europe. Steel has great strength which allows steel cylinders to have much thinner walls, around a third of a typical aluminum cylinder, or hold greater pressure. Unfortunately, steel rusts fast and therefore steel cylinders are always painted, have a boot and usually a net around them. Taking care of any damage to the coating as soon as possible is essential to prevent the cylinder from rusting.
Contrary to the popular belief, steel cylinders are a lot lighter than aluminum cylinders that hold the same water volume. As the steel cylinder walls are only a third of an aluminum cylinder, the overall material used in a steel cylinder is much less, making it lighter.
The reason we need less weights with a steel cylinder is that a 10L steel tank will displace less water than a 10L aluminum cylinder. The increased water displacement of an aluminum tank gives it greater buoyancy.

Hardness, toughness and strength

These are tricky terms to define. I’m sure in everyday life all of us use these words interchangeably but in metallurgy these have very distinct meanings.
A hard material is a material that doesn’t get easily scratched. Think of a diamond which is one of the hardest substances around. It is pretty difficult to scratch a diamond, but every child could make it shatter with the use of a hammer.
That is where toughness comes into play. Toughness is the material's ability to resist fracturing. You can take the hammer that easily shattered the diamond and whack a solid block of steel with it for weeks and your chances of shattering it are non-existent. But it is rather easy to scratch the block of steel.
The last attribute is strength. You can think of strength as how much force is needed to pull on the material until it deforms.
Let’s take a diamond – It is very hard, has a high strength but its toughness is very low. Cookie dough on the other hand is not hard at all, has a very low strength but it is, as stupid as this may sound, very tough.
The dilemma engineers find themselves in, as they increase the hardness of an alloy, its toughness reduces. They have to strike a balance between the two while keeping the strength at an optimal level. To achieve this they use a process called Hardening, followed by Tempering. As the name suggests, Hardening makes the alloy hard, but unfortunately at the same time its toughness decreases as the material becomes brittle. This is done by heating the alloy to 800 – 910 Celsius and then rapidly cooling it. To make the alloy tough again it gets tempered which is heating the alloy to 200 – 700 Celsius and then gradually cooling it down.

The composition of a high-pressure cylinder

An alloy is the combination of different metals or elements. Pure elements have very little use in engineering as their characteristics make them undesirable for the uses we intent to. By mixing them though, we can create harder, tougher or stronger materials than either one of the elements would have on its own.
Pure iron or pure aluminum are neither very tough nor very hard. By adding carbon to iron it gets both harder as well as stronger. The usual carbon content in a steel high pressure cylinder is usually 0.3% - 0.5%. The same applies to aluminum, but instead of carbon, silicone is added – usually around 1%.
Common elements that are added are:

  • Nickel to steel to create stainless steel (13%)
  • Manganese which increases the alloy's impact strength (0.7% - 1.5%)
  • Chromium to harden the alloy (1%)
  • Molybdenum which can counteract the brittleness caused by Chromium (0.2%)
  • Copper which allows for better hardening (0.3%).

The most common aluminum alloy for SCUBA cylinders in use today is A6061, which consists of the following elements: Aluminum (95.85–98.56%), Silicon (0.4% - 0.8%), Magnesium (0.8% - 1.2%), Chromium (0.04% - 0.3%), Copper (0.15% - 0.4%) and trace amounts of Iron, Manganese, Zinc and Titanium.
A common steel used in SCUBA cylinders is 4130, which is made up of: Iron (97.37% - 98.22%), Chromium (0.8% - 1.1%), Molybdenum (0.15% - 0.25%), Carbon (0.28% - 0.33%), Manganese (0.4% - 0.6%) and Silicon (0.15% - 0.35%).

The forces that act on a SCUBA cylinder

The pressure inside most cylinders in SCUBA is around 200 bar. The force that this pressure exerts on the walls of the cylinder is immense. Imagine a 2 Euro coin and putting a small car like the Toyota Corolla on top of it. That is pretty much the force that every 2-euro coin sized piece of your cylinder has to endure. The exact numbers are 203.94 Kg on every square centimeter of cylinder wall. Transfer this to a 300 bar cylinder and you slowly get to the size of a small pickup truck sitting on each 2 euro coin sized piece of metal of your tank.

A hand grenade from the second world war releases an energy of around 250 kJ. During detonation of a 12 L SCUBA tank the compressed air releases 465 kJ of energy in an adiabatic expansion. That is nearly double the energy of a World War II hand grenade. The total energy stored in a 12 L SCUBA tank is actually much higher at 1270 kJ, which could be released in an isothermal expansion. During the rapid expansion like a detonation of a cylinder, a lot of energy is lost due to the air getting very cold.

Even though the stress on the cylinder material is pretty big, if these forces above were the only ones acting on a cylinder, they would last a lot longer than any diver could possibly want to use them. Unfortunately, there are a lot of external factors that shorten the life of our Scuba tank drastically.

The things our cylinder has to endure

Take for example the very common practice that dive tanks stay on dive boats forever. This is done frequently on larger boats that are equipped with a compressor. You combine two different metals, the aluminum from the cylinder and the brass from the valve in an extremely corrosive environment. The result can be an incredible fast destruction of the cylinder neck due to galvanic corrosion.
Misadjusted or poorly operated compressors can lead to moisture inside the cylinder, corroding it from the inside. Oxygen partial pressure is usually elevated because cylinders get stored under pressure, increasing the oxidization process rapidly. Corrosion inside the cylinder can only be spotted during visual inspection or when it is too late resulting in a catastrophic failure.
Other things our cylinder might encounter is rough handling or a rough ride on the back of a pickup truck. These could lead to dents or gouges which will compromise the strength of the cylinder walls.
It is not unheard of that dive centers or laymen try to modify their cylinders. Painting cylinders or stripping paint with the wrong technique, e.g. with applying heat, can result in catastrophic changes of the molecular structure of the metal. Cases of divers welding handles to their cylinders to make them easier to carry have been presented to testers in the past. Needless to say, that these are absolute no goes and usually result in catastrophic outcomes.

As tough as our cylinders might appear, they deserve much better treatment then we give them. A properly cared for SCUBA cylinder will easily last for a couple of decades or more. A poorly treated cylinder that e.g. has ingested water can be damaged beyond repair in as little as a few weeks.

Cylinder markings

There are a quite a few markings on a scuba cylinder. They give you all the information you need about it – starting from who and when it has been manufactured to its technical details. Unfortunately, a lot of divers are never taught what specific markings mean. Let’s have a look at a typical Aluminum cylinder from Luxfer:

The markings of a scuba cylinder
  • M25x2 – Valve thread size. There are many different thread sizes, but the most common ones are: M25x2 and M18x1.5 in the European area and 3/4’’ NPSM in the American area.
  • UK - Country of manufacturing
  • LUXFER - Manufacturer
  • P3173V/LuxUK 1234A – Cylinder serial number
  • AA6061 T6 – Alloy used for the cylinder
  • 6mm – Minimum wall thickness
  • 8Kg – The weight of the empty cylinder excluding valve
  • 7L – The inner water volume
  • PW232 Bar – The working pressure of the cylinder
  • PT348 Bar – The test pressure that has to be reached during hydrostatic test
  • PS287 Bar at 60ºC – The pressure that can develop inside the cylinder at 60ºC. This is not the working pressure!
  • CE 0038 – The standard that the cylinder complies with. Other common ones are BS EN 5045 (British & European norm), ISO XXX (International standard norm), DOT XXX (Department of transport in the US, TC XXX (Transport Canada).
  • 2002/02 – The year of manufacturing. A hydrostatic tester will stamp close to this marking subsequent hydrostatic tests.

Interested to know more about scuba cylinders? In this blog you can find interesting information and updates about them.


10 Tips for Underwater Photography

With underwater cameras getting better and cheaper every year, you might want to invest in your own before your next holiday. But underwater photography is harder than it looks – trust me, I learned that the hard way. The first time I took pictures under water, my SD card was filled with fish butts, badly focused subjects, and overall everything was blue, blue and (you guessed it) - blue!
But underwater photography can be so much fun! Once you get the hang of it can be an amazing memory of your time spent below the surface.
Here are some of the most essential tips for new underwater photographers to make your pictures pop:

1. Get to know your camera

Before you take your camera on it’s first dive, make sure you are familiar with its functions. Get to know what each button does - with and without the housing. It makes focusing on your subject so much easier and you don’t have to think about what to press when suddenly a group of dolphins swims past you.

2. Check the housing for leaks before every dive

Underwater cameras are expensive, so it’s a good idea to check them properly before every use. Make sure the housing’s O-ring is clean and no hair or sand is stuck to it – a single hair can be enough to cause a flood and ruin your dive day – not to mention the camera. It is a good idea to put the camera in a bucket of water and check for bubbles before you go diving. This can help to detect tiny leaks before it’s too late.

3. Improve your buoyancy & watch where you put your feet

I’m sure you have seen it before: A diver with a huge camera taking pictures – while lying on top of the reef and breaking corals. Not a good role model for other divers.
Before you focus on photography, make sure you get the basics down: Practice your buoyancy (maybe with the PADI Peak Performance Buoyancy Specialty)!
If you are able to control your body properly, your pictures will be better instantly. If you are able to stay very calm, fish might hang around you a little longer, giving you time for the perfect shot. And of course, you cause less damage to the reef: “Take only pictures, leave only bubbles!”

4. Stay shallow

It’s no secret that water absorbs colour pretty quickly and under water pictures tend to look very blue and green. The easiest way to fight this is to stay shallow. Less light gets absorbed, the colours are more vibrant and (bonus point!!) your air supply will last longer.

5. Learn how to use the white balance

Of course, you don’t only want to stay shallow to take nice pictures. Therefore, it is important that you learn how to use the cameras white balance settings. Make sure you set the white balance manually, every time you go up or down a bit. This can help to make your pictures much more life-like and vibrant.

6. Take many pictures of the same subject

Fish are difficult to photograph: they tend to move – a lot! Getting your perfect picture is not always as easy as it seems. Once you get back on land you might find that most of your shots show a fish’s bum. An easy fix is to be patient and take several pictures of the same subject. Even if a lot of your pics will not turn out great right away – it certainly increases your chance for the perfect shot!

7. Get close to your subject and don’t use the zoom

Even the clearest water has a lot of tiny little particles in it. If you zoom in on your motive, you actually zoom in on all those particles, making the picture less clear. Get as close to your subject as possible and avoid shooting through too much water.

8. Avoid using the flash (or get an external flash)

Similar to the zoom, an inbuild flash can make or brake a picture under water. It can create beautiful lightening for close-up macro photography, but it is less than ideal for wide angle pictures. Similar to the zoom, your flash will highlight all the little particles under water and make your picture look like it was taken in a snow storm. We call this “backscatter”. If you are new to underwater photography, it could be best to keep the flash turned off. Later, you might want to invest in an external video light or flash.

9. Rinse your camera housing with fresh water after the dive

Taking proper care of your camera after every dive is essential. During the surface interval, the best place for your camera is in a bucket of water. This avoids the built up of salt crystals on the housing and keeps the temperature down so your camera lens doesn’t fog up on the next dive. Once you are home, make it a habit to soak your camera housing in a bucket of fresh water to remove the last bit of salt before you open and clean it.

10. Use computer software to make the most out of your pictures

Most underwater pictures don’t turn out perfect by themselves – but with the right software you can make the most out of them.
Many photographers use enhancement software to correct the colours, increase the sharpness and to get rid of unwanted objects (talking about you, fish butt!). They can be a great help to turn a good picture into a great one – but don’t forget that a good composition and learning how to use your camera properly is the most important thing for any picture.

If you just bought a new camera and are on your way to Zanzibar, why don’t you take the PADI Digital Underwater Photography Specialty course.
It only takes one day and we will put all these tips into practice.

own mask

Get your own scuba kit, part 2: Own mask

Masks - How we see the underwater world

Whether you’re a brand-new open water diver or an instructor with 2'000 dives: Your own mask can make all the difference in your experience underwater. For beginners, knowing that your mask will have minimal leaking provides you with confidence. It therefore allows you to put more focus towards improving other important skills in the water, such as buoyancy and air consumption. For experienced divers, a reliable mask reduces the inconveniences of having to clear it when trying to capture that perfect picture of a sea turtle or just zone out on a school of fish.

A mask should be the one of the first investment towards equipment when buying your kit. Firstly, it’s the most personal piece of equipment we can own as divers. Most of us actually spit into the mask to make sure it doesn’t fog up. Secondly, it’s a piece of equipment we can carry anywhere. No matter how light you travel you can always find space to pack your mask. And finally, with proper care, a good mask can last you several years, if not your entire diving career.

Step 1: Find your own mask

Now, masks come in all sorts of shapes, sizes, styles, and colours. When choosing a mask to buy it helps to do some research. A proper fit is the most important feature to a mask. Be sure to do a fit check before purchasing: First, you need to put the mask on your face without putting the strap on your head. Then, while the mask is on your face, you should breathe in through your nose to create suction. If the mask stays on your face without air entering into it, then we know it creates a good seal.

Step 2: Look after your mask

After finding your own mask it’s also very important to properly break it in before you go diving. This can be done several ways, the two most common ways are the following: Either rub toothpaste on the inside of the lenses and leave it overnight. The next morning, wash it away while rubbing the paste a bit more. The other option is to burn the inside of the lenses and properly rinse it out afterwards. Either method you choose will help to keep the mask from fogging.
If your own mask is cared for properly, then it can last you your entire life. That means washing it thoroughly with fresh water after each dive and protecting it when you pack it away. Also, to avoid losing your mask while diving you just have to follow one simple rule: Don't wear it on your forehead! On the surface of the water the mask will not get lost if it is worn on the face or around the neck. Following this rule can help you to keep your mask for many years and dives.

own fins

Get your scuba kit, part 1: Own fins

When I first bought my own scuba equipment, I remember being very excited!
As soon as you start to buy your own equipment you will want to dive more often. It is also an easy way to improve your experience underwater: Everything fits you better, you will be more comfortable diving with it, which means you will enjoy diving more.
One of the first part of scuba equipment we buy are own fins. Why? They are an essential piece our diving kit. Without them, we are going nowhere!
Fins are designed to move you and your gear through the water with minimum effort and maximum efficiency. The correct pair of fins can improve buoyancy and air consumption. Even though rental fins, like our ScubaPro Go fins can be great, having your own fins makes a big difference. There are many different types, styles and brands, so let's look at what to think about when choosing your own pair.

Full Foot or Open Heel

Full foot fins fit like shoes. They are great for travelling, warmer water diving, boat diving and snorkelling. They are lighter than open heel and do not require booties.
Open heel fins require booties to be worn and are great for colder water diving and shore diving.

Split or Paddle

Split fins are designed to offer the same or more propulsion as blade fins with less effort. This can help reducing air consumption.
Paddle fins have a single blade design. They tend to have stiff blades that require more leg muscle to get them moving. They are great for divers who need to swim faster or against a current.

Choose your own fins

Over the years, I have tried out numerous types and styles of fins before choosing the right pair and I advise you to do the same. While some divers love split fins, others prefer paddle fins. Once you find the right fins, remember to take good care of them and they will last for years.