Diving cylinders designed for different gas mixtures
The selection and operation of diving cylinders also has to do with the type of gas they are filled with. Typical diving cylinders are filled with compressed air. Normal compressed air (not oxygen), compressed into cylinders using high-pressure compressors. This air, is only additionally filtered during the compression process to rid it of typical impurities and excess moisture. For specific applications – very deep dives, cylinders are filled with a mixture of oxygen, helium and nitrogen, or trimix; for medium depths, a mixture of oxygen and nitrogen with increased oxygen content, most often between 28 and 36% (oxygen), called nitrox, is used; and for shallow depths – for decompression, mixtures of oxygen and nitrogen with high oxygen content, most often between 40 and 99% (oxygen), and pure 100% oxygen are used.
When cylinders are filled with gases with a higher oxygen content than in air, one must additionally pay attention to their cleanliness and the lubricants and seals (orings) used. This is especially important when the cylinders are filled with pure oxygen or nitroxes above 40%, i.e. mixtures used in technical diving (i.e. deep dives for sport or exploration purposes). It is also important to consider the methods of making the nitrox mixture. Even nitroxes in the 22-40% range may require specially prepared cylinders if they are made by the method of evaporative mixing, i.e. by dropping a certain amount of 100% oxygen into the cylinder and topping it off with air.
In summary, specially clean cylinders, with properly prepared valves, are required whenever the cylinders have to deal with a mixture of more than 40% oxygen at some point in their use.
Marking of diving cylinders
It is also necessary to remember to clearly mark nitrox cylinders by placing inscriptions (in yellow and green) EAN, Enriched Air, or Nitrox / Nitrox on them.
In addition, such a cylinder is marked with the diver’s name and the percentage composition of the mixture, the date of filling, the blender’s designation and the MOD value, which is the maximum depth at which the mixture can be used.
Similarly, special markings have cylinders dedicated to other gases. Cylinders for pure oxygen are labeled Oxygen; 100% Oxygen or Oxygen and cylinders filled with trimix, i.e., a mixture of oxygen, helium and nitrogen, are labeled TMx and the percentage composition of oxygen and helium (the percentage composition of nitrogen is implicitly a complement to 100%), e.g., TMx. TMx 20/20 (20% oxygen, 20% helium, 60% nitrogen); TMx 12/60 (12% oxygen, 60% helium, 28% nitrogen).
Capacity of diving cylinders
A simplified method of counting the capacity of diving cylinders only requires multiplying the water capacity of the cylinder times the pressure in atmospheres of the gas in it. This is the method constantly used by divers, and in practice “above water” especially in the working pressure ranges up to 200 atm no more accurate calculations are used.
That is:
- A 10-liter cylinder tapped to 200 atm contains 10 liters x 200 atm = 2,000 liters of gas – 2 m3
- A 15-liter cylinder tapped to 200 atm contains 15 liters x 200 atm = 3,000 liters of gas – 3 m3
Accurate calculation of the amount of gas compressed in the cylinder requires consideration of the compression characteristics of real gases based on the Van der Waals equation.
Approximate pressure-dependent actual air content contained in a 10- and 15-liter cylinder:
Temperature 20 degrees C (air), 10-liter capacity cylinder:
- 0(1) atm – 10 liters (10 liters for perfect gas)
- 100 atm – 1053 liters (1000 liters for perfect gas)
- 200 atm – 2042 liters (2000 liters for perfect gas)
- 300 atm – 2763 liters (3000 liters for perfect gas)
15-liter capacity cylinder:
- 0(1) atm – 15 liters (15 liters for perfect gas)
- 100 atm – 1580 liters (1500 liters for perfect gas)
- 200 atm – 3063 liters (3000 liters for perfect gas)
- 300 atm – 4145 liters (4500 liters for perfect gas)
That is, as we can see from these calculations, looking simplistically or based on perfect gas calculations, a 10-liter cylinder at 300 atm has the same amount of gas (3,000 liters) as a 15-liter cylinder tapped to 200 atm (3,000 liters).
However, looking precisely:
A 10-liter cylinder tapped to 300 atm contains 2763 liters of air
a
A 15-liter cylinder tapped to 200 atm contains 3063 liters of air.
Cylinder transportation
Transporting cylinders is a major challenge for divers. It is important that the cylinders themselves are not damaged and that they do not damage other equipment, but most importantly, the transported cylinders should not cause great danger in the event of a traffic accident.
On the one hand, it is important to realize that in many countries regulations restrict the carriage of loaded cylinders on the other hand, in practice there are no problems with carrying one or two cylinders in the car. However, it is important to remember that the cylinder in the event of an accident is a very dangerous item. Always attach the cylinders to special mounts in the luggage racks. It is a good idea to separate the cargo area with a strong grille, etc. In addition, you need to be careful that, for example, the fall of the cylinder does not cause damage / break off its valve. This risks an uncontrolled outflow of gas and the cylinder, which is rejected by the force of recoil, as well as the remnants of the valve, can pose a great danger.
It is also important to remember that even a fall from a small height of a heavy cylinder, with a high probability of bending the valve and the inability to screw the automatic – this is the most common result of falling cylinders. Such a valve is entirely replaceable.
Therefore, a good solution is to borrow cylinders at the dive site. Since dive sites are very often where we fly by plane, so transporting cylinders due to weight is not an option, so divers borrow cylinders on site anyway, and in practice the price of dives at warm-water bases includes cylinders and ballast.
However, if we need to transport the cylinders by airplane (as in the case of an argon cylinder, for example) we need to prepare them for this. First of all, it must be empty, but for that we need to check that we can unscrew the valve without any problems, because the airport security control will want to check the inside of the cylinder after unscrewing the valve.
Cylinders with or without feet?
When transporting cylinders and especially when dressing or storing them, it is important whether the cylinder can be placed freely or not. Due to the pressure pushing on the inside of the cylinder, its bottom as well as the top is spherical.
To keep the cylinder from tipping over, but also to keep the cylinder from banging around underneath, manufacturers put rubber covers – “feet”, sometimes called “slippers” – on the cylinders. Some divers use these feet, some pull them off, and the Internet is full of discussions of how it’s better. In extreme cases, one can conclude that the presence or absence of “feet” kills so we will describe it in a little more detail.
The fashion for pulling off rubber feet came from cave diving. In cave diving, from what regulars say, feet are pulled because they weigh on the surface and as you carry your gear through passages on the surface reaching remote siphons, every kilogram counts. They are certainly right.
But the advantages or disadvantages of feet in open-water diving, where we don’t carry our equipment very far, are more debatable.
First, the anti-foot arguments.
Argument one – underfoot the cylinder rusts faster. Of course, if someone first uses the cylinder without feet and then puts his feet on the damaged paint, they will rust more because moisture stays under the feet longer. But if the alloys are from new, the paint under them is in better condition than that in other parts of the cylinder because it is completely shielded from mechanical damage. The undamaged paint on good cylinders is fully waterproof.
Argument two – if you don’t have feet, it’s not tempting for a diver to leave a standing cylinder unattended and the cylinder won’t capsize. Well, of course, there is something in this, but I know many people who have their feet in cylinders and also cylinders do not put because it is known that it is not allowed/should not be. The feet are there to protect the base of the cylinder when dressing and in that one moment when the cylinder/twin standing on the foot makes it easier to dress the equipment.
Argument three – wing stacking. With cylinders of twelve (12l) it does not matter at all with tens the wing lies a little differently but debatably or worse.
Argument four – the v-ballast (ballast attached to the twine in the form of a triangular casting). If you have a twine with alloys then making a v-ka requires an exact fit (it is more difficult), but, of course, it is possible to order such a v-ka and a sizable v-ka, for example, 9 kg.
Argument five – feet affect trim (minimally). But this is a disadvantage for divers in wetsuits where the center of gravity has to be shifted toward the head. People diving in drysuits tend to shift the center of gravity toward the legs so the feet improve (minimally) trim.
In general, underwater in general it is irrelevant whether we have our feet on the cylinders or not. The whole discussion is about surface. Personally, I see the primary advantage of feet when loading cylinders in a dive center where it is easier to handle cylinders/twins as they have feet because it is easier to position them.
Arguments from proponents of feet on cylinders:
- Feet allow you to more easily store cylinders in an upright position.
- The feet prevent the underside of the cylinder from flopping.
- The feet make it easier to dress the equipment.
To have or not to have “Feet”? If you don’t have it, don’t put it on – you’ll never paint the resulting damage again so that when you apply the slippers, it won’t rust.
If you have one, consider whether to pull it off, because the operation is not repeatable in the other direction due to the scratches on the paint created after the first dive.
Fortunately, steel cylinder manufacturers have begun to emulate aluminum cylinder manufacturers and are making flat-bottomed cylinders. That is, on the inside the cylinder is spherically terminated, but on the outside there is an overflow and the cylinder has a flat bottom. In a little while, probably all cylinders will be like this and cavers will have to lug this excess around the caves. All in all, maybe they deserve it for so much fuss.
