Calculation Assumptions

Cylinder sizes 10l,12l and 15l with 232 bar working pressure. 50 bar residual pressure, 182 bar pressure for breathing, no account has been made for inflation / deflation of BCD/dry suit and other potential gas losses. Assumes rectangular profile. Breathing rates set at 30/24/18 and 40 Surface Litre per Minute (SLM), surface pressure 1 bar, with 1 bar pressure increase for every 10m of depth. NDL based on PADI Recreational Dive Table (Air) - depth used 10,16,20,25 and 30 metres and assumes first dive with no surface interval obligations.

Cylinder size, gas consumption and diving depth

How big does my cylinder need to be? There is no hard and fast answer - 90% of divers in the UK will dive with a 12l steel cylinder. Your breathing rate and your dive depth will determine what type of cylinder you will require. Using some theoretical assumptions (listed in the side panel), we have created a table showing how long a cylinder would last at depth.

These table should not be used for dive planning. Every diver has their own breathing rate, this table should only be used as guideline to demonstrate a theoretical profile with completely stable breathing rate using theoretical breathing rates commonly used in planing. Additional training is required to ensure proper use of such calculations.


Let us have a look at the 12l cylinder. At the surface and being a heavy breather the cylinder would last 73 minutes, while at 30m it would only last 18 minutes. A very relaxed and experienced diver, can make the same cylinder last 30 minutes at 30 metres. Now compare it to your non-decompression limit (NDL) a dive to 30 metres without decompression would be a maximum of 20 minutes. (As a reference only, in an emergency your breathing rate becomes very high, the same cylinder only last 14 minutes - so the real question is always, in an emergency do you have enough gas to make it to the surface).


As most divers dive between 20m (NDL: 45 minutes) and 30m (NDL: 20 minutes) and want to avoid decompression obligations, the 12l cylinder last anywhere between 18 to 40 minutes at depth for them. Now,  allowing time to descend and ascent, not spending the full time at depth, plus finishing the diving at the shallow end - most divers will dive 45 to 60 minutes with a 12l cylinder. If you are of small stature or a child, you may select a 10l cylinder for comfort and size.


For divers wanting to stay down a bit longer at the deeper bit (careful about your decompression obligation), or want to carry a bit of extra gas to make them more comfortable and also don't mind the extra weight of a 15l cylinder, they could stay at 30 metres for an extra 5 to 8 minutes.

Steel or Aluminium

There are really to types of cylinders - Steel and Aluminium - both a advantages and disadvantages.


Steel Cylinders  - Most UK divers will use steel cylinders. There are negatively buoyant (i.e. sink) and therefore require you to carry less lead weight. The typical working pressure for a cylinder is 232 bar, but there are cylinders available which can take 300 bar (check that your local dive shop is able to fill them). On the other side, steel cylinders are heavier and need better care than aluminium cylinders as they can rust.


Aluminium Cylinders - The key advantages are the material is lighter than steel, meaning the cylinders are lighter. Aluminium doesn't rust, so they are less susceptible to corrosion. However, they are also some disadvantages, the material is softer, so they damage easier. Also, to get the same gas volume and gas pressure into the cylinder, the cylinder walls have to be thicker, which makes the cylinders larger. As they are lighter (especially when they are empty the are extremely buoyant), you must ensure you have adequate weight to compensate for that fact.

Cylinder Valves

DIN ValveCylinder Yoke (K-Valve) There are two types of cylinder valves. The DIN Valve (on the left) and the K-Valve / A-Clamp (on the right). The type of valve your cylinder needs depends on your first stage regulator.


Recent changes in legislation means if you want to dive on Nitrox (Enriched Air), you will need an M26 DIN Valve (If you have a DIN valve). We recommend that you ensure if you need to buy a new cylinder and regulator that they are Nitrox approved (even if you are not a Nitrox diver yet). 


Also, if you wish to dive with 300bar cylinders, you must have DIN valve. K-Valves are not rated for that pressure and are limited to 232 bar working pressure.


On holidays you are likely to be given a DIN valve and a screw insert (as shown on the right) will allow you to adapt it to a K-Valve when needed. The dive operators will have enough inserts available, so you don't have to worry too much about it.

Cylinder Care and Markings
Minimize
Cylinders

Cylinder Boots

First of all cylinders can't stand on their own, a cylinder boot helps it to stand up and protects the bottom from damage. This is a definite must for your dive cylinder. Do take the boot off from time to time and see if there is any damage to the cylinder.

 

Basic Cylinder Care


Handling - First of handle them with care, don't drop and bang them. You should remember, a filled cylinder is a bit like a small bomb (you might have heard a tyre burst (that is only 2 to 3 bar), imagine 232 bar exploding (a full cylinder exploding in your car will rip open your car like a can of sardines - so do handle them carefully).


Cylinder Walls

Moisture - The single biggest damage to a cylinder is moisture. If moisture gets into the cylinder or you suspect water ingress into the cylinder, get it checked out and tested immediately. Moisture under pressure will have an accelerating effect inside the cylinder and it can corrode very quickly. The easiest way to avoid this is by getting your gas fills from a reputable dive centre source (the double filter usually) and always keep the cylinder under pressure (try to keep it above 20bar all the time). 


Storing - Keep the cylinder in a cool and dry area. Also try not to store it full pressure for long periods of time. If you need to lower the cylinder pressure, release the gas slowly. A fast gas release (apart from the noise it makes), will cool the cylinder down quickly and moisture can form on it (and risk getting into the cylinder).


Tests - By law you must get your cylinders inspected and tested in regular intervals. A visual inspection every 2.5 years and a full hydrostatic test (where they strip down the cylinder, check it and then test how much it expands under pressure) every 5 years. If the cylinder is out of test, the filling stations will not fill it. The reason for the test is to check for corrosion (visual), but as you can see from the photo (courtesy of  Massdiving.com), the walls of a cylinder become thinner over time from the constant pressurisation. 


Cylinder Hydrostatic Markings

Cylinder Neck Markings - Therefore it is advisable to understand what the markings on the cylinder actually mean. 

In the UK, the approved Government Agency rating should be EN144/BS (in the US you will see DOT/CTC).
The metal type tells the tester what the "expansion" tolerances are during the test - 3A stands for Carbon Steel.
Working Pressure - is the maximum normal operating pressure for the cylinder, for testing the tester will over pressurise the cylinder to a set fixed percentage.
Serial Number  - a unique number issued by the
Manufacturer (in this example Faber, but it could also read luxfer, PST in the UK - being the most popular manufacturers).
Hydrostatic Test Date is 6/2012 on this example, meaning the next visual inspection is due at the latest in 12/2014 (with a little diamond in between to show it is from an approved test station).
The PLUS symbol means a 10% Overfill is allowed.


General advise - If you buy a second hand cylinder, always ask for the seller to supply a valid testing certificate with the cylinder. Any cylinder more than 12 months from its last test (unless you know where the cylinder has been), we wouldn't touch it.