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Cylinders

Know your cylinders - a fascinating insight into a key component of diving.


Types of cylinders

Steel cylinders (right of picture) are made by drawing and spinning and are 4-5mm thick. They are usually galvanised prior to painting.

Aluminium alloy (left) is made by extrusion and forming. They are not as strong as steel and therefore require an 11mm wall thickness. They are anodised prior to painting and do not corrode as much as steel. Aluminium cylinders are easily identified by their flat bottom.

Regulations  - New Standards BS EN 1968 and 1802

In March 2002, European standards for cylinder testing were changed. The familiar old BS 5430 part 1 (steel cylinders) & part 3 (aluminium) were withdrawn and replaced by new standards - BS EN 1968:2002 for Steel Gas Containers, and BS EN 1802:2002 for Aluminium Gas Containers.

IDEST, the Inspectorate for Diving Equipment Servicing and Testing, advised approved test centres to implement the new standards by 31st August 2002. These came into force on 1st September 2002.

Labeling and Marking

The Carriage of Dangerous Goods Regulations (1996) requires that cylinders are either correctly labeled with their contents OR that they are painted in the approved scheme (i.e. for compressed air and nitrox this means the black and white quarters on the shoulder).

EN 1089

A new standard was also introduced - EN 1089 Gas Cylinder Identification. This sets new standards for stamp marking, precautionary labels and colour coding.

Stamps Marking
The test or inspection date stamp punched into the cylinder will from now on show the year in full, followed by the month (as opposed to month then year as now). This could be confusing. 

In addition to stamping the cylinder with the test date, it will also be mandatory to mark the cylinder with punch-out stickers to make it easy to see when the cylinder is next due for test.

Contents Labels
According to the new standard EN 1089 part 2, cylinders must now also have stickers showing their intended contents, e.g., Breathing Air, Nitrox or Trimix.

Colour Coding
All new diving cylinders will have the black and white quadrants painted on the cylinder's shoulder. Many recreational divers will of course have (and will have for many years) the older colouring and may run into problems getting them filled.

Officially, professional divers 'at work' must use the new standard colouring, however the definition of 'at work' is not clear. The Health and Safety Executive have said that it is the prerogative of a dive shop or commercial air station to insist on the correct labeling of cylinders because the person filling the cylinder is 'at work'.

In the event of a diving accident the HSE can come back to the commercial operator and investigate the circumstances surrounding the filling of the cylinder. if anything at all were to go wrong as a result of the cylinder having not in fact been safe or the contents mis-identified, then they may well be legally liable for failing in their "duty of care" to ensure that only safe cylinders are filled.

Air Fills
The new standards make recommendations. This is important because it is not in itself illegal for a dive shop to fill a cylinder which does not have, say, a contents label conforming to EN 1089 Pt 2.

Air cylinders that are not used 'at work' and are currently in test according to BS5430 do not have to adopt labeling or painting. This will be left to the discretion of the owner but you are recommended to follow the requirements of the Carriage of Dangerous Goods Regulations.

It is ultimately up to the filler to decide whether a cylinder is safe to fill or not. If they are not completely satisfied this is the case they have every right to refuse to fill the cylinder.

Hydraulic Testing
The hydraulic test period has now been changed from 4 to 5 years. This has been done mainly to bring it into line with standards for testing other industrial compressed gas cylinders. Similarly visual inspection periods have changed from 2 years to 2½ years after a hydraulic test.

The new standard doesn't extend the test expiry for cylinders with a current test-due date. In other words, cylinders tested before the new standards came into force, and with either no test-due sticker (see below) or a 2-year sticker, will still need their next test at the expiry date of the old 2-year interval test/inspection.

Don't forget that as before, the hydraulic test period is not affected by the date of the visual inspection. For example, say you don't use your cylinder for a year or so then have it visually inspected 3½ years after it's hydraulic test. The cylinder will only then be in test for a further 18 months. This is because you will need another hydraulic test five years after the last, regardless of the visual inspection date.

In this test the cylinder is filled with water and then placed inside a water-filled, high-pressure chamber. The water pressure inside the cylinder is then increased to five thirds of it's maximum working pressure. This is sufficient to cause the cylinder to expand slightly. This expansion causes the water outside the tank to be displaced and this is channeled into marked collection tubes that allow it to be measured. If the tank's expansion is within acceptable limits (< 5%), it successfully passes the test. If not, the tank may not be refilled.

After pressure testing, tanks that pass are cleaned and dried, then stamped with the current month and year and the tester's initials.

Visual Inspection (every 2.5 years)
Essentially this test is a detailed visual inspection, both internal and external with the following failure criteria:

  • Bulge - all fail
  • Dent - fail if any greater than 2 mm
  • Wear - reduction of wall thickness greater than 25%
  • General Corrosion - reduction of wall thickness greater than 20% OR where original surface is not visible
  • Area Corrosion - (i.e., where corrosion is limited to less than 20% of surface) a reduction of wall thickness greater than 25%
  • Isolated pitting - Reduction of wall thickness greater than 40%
  • Thread damage - all physical damage or imperfections (recutting of threads can be achieved up to a maximum tolerance )
  • Weight Check - Weight of cylinder ( minus valve ) less than 95% of the tare weight stamped on the cylinder

Any cylinder that has been subject to impact damage should be pressure tested before use. If in the opinion of the inspector, any of the above criteria are borderline, then hydraulic testing may also be required.

Test procedures
All testing is undertaken by qualified and experienced technicians. The procedure is as follows:

  • Removal of the tank boot, bands and valve.
  • The exterior of the tank is then inspected for impact damage and corrosion.
  • The interior of the cylinder is examined using a special endoscope.
  • Dental mirrors may enable the inspector to examine the area around the inside of the tank neck.
  • The valve is checked for smooth operation and its threads lubricated to help prevent galvanic action between the dissimilar metals of the tank and valve.
  • Valves may also require periodic overhauls, just like scuba regulators.
  • If no corrosion or damage is detected, the cylinder is reassembled and filled.
  • A sticker is then placed on the tank which identifies the facility providing the inspection and the month and year in which the inspection was done.
  • If damage or corrosion is detected, the technician will decide upon an appropriate course of action.
  • Minor scale or the presence of contaminants on the inside of an aluminium cylinder can usually be removed by rinsing with distilled water and drying with warm air.
  • Minor oxidation on the inside of a steel cylinder may be best left untreated due to the fact that the process of removing it might actually weaken the cylinder more than the oxidation itself.
  • Extensive oxidation or deep pitting is treated by degrees of sand blasting.

Test Certificate
Centres and shops should now ensure that the paper test certificates issued with a tested cylinder bear the name of the cylinder's owner, not just whoever dropped it off for testing.

More Stringent Testing
The new standards are more stringent and require that all but the very lightest of corrosion needs to be cleaned. This means that your maintenance costs will go up although they will be slightly less frequent. In addition, the criteria for failing cylinders are now much more strict. The new standard accepts less damage, set (the degree to which the cylinder permanently stretches during a hydraulic test) and corrosion on a cylinder before it's scrapped.

Enriched Oxygen Cylinders
You will find that shops and filling stations probably still demand that inspections and cleaning for Nitrox cylinders remain at 12month intervals.

Mini Cylinders
Cylinders of 0.5 litres or less are NOT covered by the new standards. These will still be tested to the old standards (BS 5430 part 6) with the test intervals of 2 and 4 years.

Cylinder valve

Most are Balanced type which means that high pressure air is acting on both sides of the valve assembly to prevent any stiffness while turning the valve

Some have pressure relief valve fitted called a burst disc.


The cylinder valve is fitted into the neck of the cylinder and is provided with an 'o' ring seal so that the diving regulator can be connected to it. The valve is screwed into the cylinder neck with either a taper thread seal or a parallel thread seal and o ring. It is not recommended that divers attempt to service their own cylinder valves, unless they possess specialist skills. Provided that the cylinder valve is washed in fresh water after use and is kept clean, there should be no need for it to be serviced between the intervals of cylinder test.

The Heath and Safety Executive (HSE) have produced a document entitled The safe use of gas cylinders.

 

 

 

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Page last updated on October 16, 2007
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