CYLINDER

CYLINDER NETS/MESH
These are those plastic mesh things you slide over your precious cylinder in the hope that they will be safe from the ravages of the sea.
Most dive shops are all too keen to sell you these, often with the advice that “they will protect your brand new cylinder from being scratched and chipped” thus extending the life of the cylinder, better still “they grip the tank better”.
Most dive shops are all too keen to sell you these, often with the advice that “they will protect your brand new cylinder from being scratched and chipped” thus extending the life of the cylinder, better still “they grip the tank better”.

Let us look at the facts shall we:

1. When did you last see a cylinder rust from the outside anyway?  The odd chip may expose some of the steel underneath, but as long as the chip or scratch is free to dry, rust doesn’t set in.  The steel is about an inch thick anyway.
   If rust was to start, it would take so long that the cylinder would fail for other reasons long before (e.g. internal corrosion).
   The only time external corrosion may occur is when salt water is left trapped under the cylinder boot for a long period of time (one of the reasons for not using cylinder boots – see later) and not allowed to dry. The mesh wouldn’t prevent corrosion under the boot anyway.  Ask any of the cylinder testing (IDEST) centres when they last failed a cylinder for external corrosion.  Cylinders fail either due to internal corrosion or wear, tear, corrosion and physical stress of the threads of the neck of the tank (pillar valve).  Tank nets help none of these. None of my cylinders are showing any signs of corrosion even after being used without boots or nets for the last 3 years.
2. They don’t actually protect the cylinder anyway.  If you look closely at some of these nets after they have been used a while, you will notice that the mesh is holed or torn and that contact with rocks and wreckage just cuts through the mesh and touches the cylinder anyway.
3. The plastic of the mesh DOES NOT hold onto the cylinder as well as the rubber of the cam band of the jacket/harness.  If you don’t believe this, try taking a piece of soft rubber and whilst pushing down hard, try and slide it across a smooth surface (wet or dry).  Now try it again with the cylinder mesh rather than the rubber.  If your cam bands haven’t got rubber on them use the rubber bit that A.P.Valves use for their jackets.  I have lost count the number of times I’ve seen tanks slip on cylinders with meshes on.  A poorly tightened cam band may slip on a tank without a mesh, but not as easily as with a mesh.
4. The mesh increases drag and may pose a snagging problem on a wreck.  Although the increased drag is small, if lots of bits of equipment all add more unnecessary drag, the combined effect is significant.

Cylinder nets/meshes therefore add no value and cause the problems outlined above.  You have to ask therefore, “Why bother”? You are better off without them and can save yourself the 5 Euro or whatever they cost. If you don’t need it in the water, don’t take it!

CYLINDER MASSES
Different batches of the same size and brand of tank weigh slightly different amounts.  The weights below are typical and are when empty on land for 232 Bar tanks:

• Faber 3L steel                            3.3KG
• Faber 7L steel                            7.5KG to 8.1KG
• Faber 12L steel                          13.3 - 14.5 KG
• Faber 15L steel                          16.6KG

• Luxfer 3L aluminium                  3.7KG
• Luxfer 7L aluminium                  8.8KG
• Luxfer 11,1L aluminium              14.3 KG

CYLINDER BOOTS

In simple terms, boots should not be fitted to cylinders because they are not required.  As mentioned above, boots can cause and hide corrosion and are only usually removed at test.  When did you last check under your cylinder boot(s)?

1. Not having boots will mean that some of the paint on the bottom of the cylinder will wear off, but as stated earlier this does not cause problems in any way, shape or form.  Having no boots means that the bottom of the cylinder gets to dry properly.
2. Cylinder boots may allow the tank to be stood upright.  But, in our first scuba lesson, we are told never to leave a tank upright (for it may topple over and damage gear or injure people).  So go and figure this out.
3. A double cylinder boot is sometimes used to hold the lower end of a twin-set together.  The problem with this is that you still have all the problems of boots (corrosion, drag etc) and the double boot doesn’t hold the cylinders together as well as stainless steel cam bands.  If the double boot was to slide off by accident, the cylinders would only be held together by the manifold.
   Most people who use double boots also use manifolds that rely on “facing” O-rings to form the seal (see later notes on manifolds).  Any twisting or outwards movement of the cylinders would result in serious loss of air/gas.  Using a pair of stainless steel cam bands is more expensive than using a double boot (maybe an extra L40), but if you are going to cut costs, don’t do it with anything involving your air/gas supply (i.e. tanks, manifold & regulators).
4. Cylinder boots also increase drag in the water.  Although the increased drag is small, if lots of bits of equipment all add more unnecessary drag, the combined effect is significant.  This was also the case with the cylinder meshes.  You can now start to appreciate that if the streamlining of every single piece of kit is considered, the overall effect is to reduce drag and snag points significantly.  This not only makes diving safer, it also means we enjoy it more as we can swim with less effort, use less gas and get tangled up less often.

CYLINDERS/TANKS

If you are going to buy or use cylinders that are going to be mounted on your back they should be made of steel.  Aluminium tanks, because of their lower density displace more water and are therefore more buoyant.  If you were to use aluminium tanks on your back and use either a dry-suit or thick wetsuit, you would need a very large amount of weight to keep you neutrally buoyant at the end of the dive when cylinder pressures are low.  Most tanks (Faber, Roth etc) sold in the UK are steel so that’s fine.

On holiday in a tropical location (Caribbean, Red Sea, Far East etc) you will probably find that you are using an “aluminium 80”.  This is an 80 cu ft (1 cu ft = 28.3 litres of gas) cylinder made of aluminium, probably by a company called Luxfer.  This normally doesn’t present problems because you are probably only wearing a thin wetsuit, shortie or no wetsuit. You can therefore get away with the buoyant tank by using a little lead.  I find that by the end of the dive the bottom of the tank starts to lift off of my back.  I find this uncomfortable so I thread 2 two lb weights onto the lower cam band of the single tank adapter.  This seems to solve the problem.

Conversely, any tank that you intend to sidemount as a decompression bottle or stage cylinder should be made of aluminium.  This is because anything negatively buoyant (i.e. steel cylinder) mounted on the side will tend to make you roll over to that side.  Furthermore, if you intend to “drop” (collect & use on way out) a bottle when cave diving, you do not want to become positively buoyant after you have dropped the tank because you were relying on the negative buoyancy of the steel tank at the start of the dive.  For wreck diving you always want to carry your decompression gas with you.

For single tank diving in the Europe, a 15L or 12L tank is probably all right if you don’t stray below about 25 to 30m too often and you are not incurring significant decompression stops.  That said, a twin-set is obviously safer.  Unless you are short, I would advise against a “dumpy” tank because they have a bigger profile, move the centre of buoyancy back too far and don’t enable you to take the weight off your back very easily when sitting and kitting up.  If you are short and find that a correctly positioned normal length tank means that you keep clunking your head on the pillar valve/regulator, then you may have no option but using a “dumpy”.