Bottle testing – Hydrotech

The metal thickness of the cylinder walls being checked with an ultrasound scanner

The metal thickness of the cylinder walls being checked with an ultrasound scanner

The regulations surrounding the use of high-pressure vessels make testing for safety mandatory at periods determined by the use the pressure cylinder is put to. However, there’s enough ambiguity in the rules to confuse end users, charging stations and even test centres! Under the guidance of Hydrotech, I’ve penned this article to help answer some oft-asked questions amongst the precharged pneumatic (PCP) airgun shooting community.

The first thing to realise is that pressurised dive bottles contain a massive amount of energy, which is why there is so much emphasis given to the safety aspects of filling and using them. When full, a 12-litre cylinder rated to 300BAR (4,351psi) has 3,600 litres of air squeezed into it. For this reason, both the vessel and its fittings need to be extremely strong and well maintained. Knock the valve off and the cylinder will make like a rocket!

The regulations surrounding high-pressure vessels (HPVs) have been drawn to protect those who have to work with such equipment – people like doctors, nurses, firemen and, of course, divers. Airgun shooters are very much on the fringe; we’re really using HPV equipment which, though it suits our purpose, was not necessarily designed for our sport. Hand-operated stirrup pumps don’t come under the regulations, by the way.

Hydrotech-2Rules and regulations

The regulations are there for the protection of those who may come into contact with these devices in their workplace. So when you hand over your air tank to be filled by a commercial facility, they then take on responsibility for the safety of the employee who actually does the filling.

Cylinder testing therefore really only applies to high-pressure equipment that needs to be filled commercially. If you filled your own cylinder at home from your own compressor, there’d be no requirement to have it tested; you’d assume the risk for your own safety.

However, once you entrust your HPV to someone else, that’s when the regulations kick in. They actually cover all highly pressurised containers, regardless of volume, but the way that the regulations are interpreted is where the confusion arises.

There are theoretical situations – and more realistic, practical ones. Let’s start with the means for filling your airgun from a high-pressure air tank. It’ll be one of portable dimensions, with a capacity somewhere between three and 15 litres.

Any air tank with a valve that uses a converter fitted to a diving valve must be visually and hydrostatically tested every 30 months – alternating between a visual and hydrostatic check – and stamped accordingly to indicate the last test type.

Air tanks fitted with a ‘surface use only’ valve, where an integral gauge is fitted as part of the valve assembly, need only be tested every five years – the test being a full, hydrostatic one.

Some shooters and fillers incorrectly think that ‘airgun cylinders’ only need testing every 10 years on the grounds that they are simply storage tanks. This isn’t true – because they are still portable. The 10-year test period applies only to static storage cylinders (like industrial tanks, or a dive centre’s compressor tank).

In a nutshell, the rule to remember is: if it’s portable, your tank will come under either the 30-month or five-year test cycle.

PCP cylindersHydrotech-5

So if portable high-pressure air cylinders have regulations requiring their testing, what about the cylinder (or buddy bottle) attached to your PCP airgun?

The use of air tanks by airgunners and the inclusion of HPVs as an integral part of a PCP’s design are fairly new – and until recently, the airgun industry has rather flown under the radar of the regulating authorities. However, it’s now accepted that airgun reservoirs should be visually and hydrostatically inspected at least every 10 years, as per the standards for all pressurised vessels.

What we refer to as ‘buddy bottles‘ – those cylinders of 500cc or less – and your PCP’s air reservoir will come under the scrutiny of these regulations once they go into a service centre or filling station. It’s because of this that many PCP manufacturers are now date-stamping their guns’ air reservoirs, and have designed them to be easily replaced if necessary at the end of the 10-year lifecycle.

The test

So, what exactly do these mandatory cylinder  tests involve? Well, one of the UK’s top testing centres, Hydrotech, allowed me into its facility in Leicestershire to document each stage of the test procedure.

An endoscope is used to inspect the interior surfaces of the cylinder for signs of corrosion or damage

An endoscope is used to inspect the interior surfaces of the cylinder for signs of corrosion or damage

Each component of the bottle has to be checked. The first task is to unscrew the valve and gauge from the neck, using a special tool while the bottle is held in a cradle with a strap wrench. Once those are out, a special gauge is used to check the threads – both the female thread in the neck of the cylinder and the male thread of the valve. It’s a comprehensive check – the threads are examined for wear, damage and compatibility.

Next, the internal and external visual inspection is carried out. An endoscope is used to inspect the interior surfaces of the cylinder, to look for any signs of corrosion or damage. Some corrosion may be removed by shot-blasting – and in the case of external corrosion, the cylinder may also need to be repainted. Where corrosion has been corrected, an ultrasound scanner is then used to ensure the thickness of the metal in the cylinder’s wall is still acceptable. This is the limit of the 30-month ‘visual’ test.

Bottles that pass the tests are date-stamped and bear the mark of the testing centre

Bottles that pass the tests are date-stamped and bear the mark of the testing centre

In the case of a five-year ‘hydrostatic’ test, the bottle is filled with water and immersed into a safety pressure tank, where it’s then submitted to about 1.5x its working pressure – around 460BAR for a 300BAR cylinder.

The purpose of this test is to measure the expansion of the cylinder; once the pressure is released, the expansion tank measures the degree to which the cylinder has expanded. In order to pass the hydrostatic test, the bottle must not expand more than five per cent of its original size.

Hydrotech applies a ‘compressed air‘ label and a reminder of when the next test is due

Hydrotech applies a ‘compressed air‘ label and a reminder of when the next test is due

Providing the cylinder goes through the test procedures happily, it’s then stamped with the year and month of test as well as the unique mark of the testing centre. Hydrotech then adds a label clearly indicating that the bottle is for compressed air (as opposed to any other gas). A sticker is also applied to give an obvious reminder of the next retest date.

However, should your air tank fail its test – tough! The test centre won’t return it because, on safety grounds, it has to be scrapped, cut into several pieces… and then certified as having been destroyed.

Expect your air tank to be scrapped if it fails its test!

Expect your air tank to be scrapped if it fails its test!

My thanks to Hydrotech for allowing me into its facility and the invaluable advice given to help produce this article. If you have any questions relating to air tanks, I can highly recommend you contacting the Hydrotech Servicing team on 01455 273089 or visit  – Simon Everett


Hydrotech-6You may have wondered what all those hieroglyphic-like marks around your bottle are – by and large, they provide information on the type, size of your bottle and what regulations (or standards) apply to it.

The relevant periodic inspection and test procedure for a seamless aluminium cylinder is indicated by the BS EN 1802 mark – it’s BS EN 1968 for a steel pressure cylinder. These replace the old BS 5430 Part 6 standard, which would have required pressure reservoirs on the actual gun to be tested every five years (or every 10 years if the gas within has a dew point of less than -46 degrees Celsius, which very dry ‘breathing’ air has).

The BS EN 1968 standard states that the normal test period for a gas cylinder containing air is 10 years and it applies to pressurised vessels of under 500cc where they are handled in a professional capacity. In other words, you take it to a gun shop or filling centre to be serviced or filled. Bigger dive cylinders come under the ruling for self-contained surface breathing air – the 30-month/five-year test cycle.

There is also a BS EN 12021 standard for ‘breathing air’, the gas used in PCPs. The standard requires that over 200BAR, water vapour should not exceed 35mg/M3 to prevent corrosion or freezing in use, and from 40 to 200BAR, it shouldn’t exceed 50mg/M3. Ironically, there are no specific set standards for airgun use – but it would clearly make sense for the airgun industry to adopt the existing standards and testing cycles for breathing air when making PCP air rifles.

After all, shooters are already working to those regulations with their air bottles, so by volunteering this protocol, gunmakers wouldn’t have to have a new set of rules drawn, scrutinised and approved by the standards authority.

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4 comments on “Bottle testing – Hydrotech
  1. Tim Fieldsend says:

    V. Interesting article.
    A couple of suplementary questions:
    What strictly is the correct procedure for re-charging PCP air rifles? I think I know but….
    Can or should air bottles be stored & used in horizontal position?
    What is difference between white & yellow “Breathing Air” bottles.

    Maybe you could add this information to the article or point me in the right direction if it has been covered elsewhere.

    A personal reply would be much appreciated but I realize this is probably beyond the call of duty!

    Tim F

  2. Bill Webster says:

    Hi Tim,

    I am not an expert but here are my answers for what they are worth….

    Charging via pump:
    > Pump to desired/max pressure… if you can!

    Charging via scuba cylinder:
    > Briefly open valve on main cylinder to blow out any contaminants.
    > Connect ‘whip’ to main cylinder and valve on gun or buddy bottle.
    > Slowly open valve on main cylinder and allow pressures to equalise.
    > Control valve on main cylinder to allow cylinder to fill to desired pressure as SLOWLY as possible.
    > Shut off valve on main cylinder.
    > Release pressure in charging system via manual relief valve. Do this smartly or you will lose some pressure in gun.
    > Disconnect gun/buddy bottle and re-fit any valve protection.

    Cylinder Orientation
    > Theoretically best to store in vertical position as walls are thicker at bottom (in case of moisture/corrosion) but it makes no practical difference with cylinders used for airgun filling and tested regularly. (But probably best not to store upside-down!)

    Cylinder Colour
    > Breathing air bottles in the UK should be grey with black and white quadrants at the top (valve end).
    > This has not been enforced for years.
    > So there is no difference between ‘breathing air’ bottles of different colours.

    The important thing with cylinders is to be conscious of the energy they contain. Don’t skimp on testing and seek reassurance regarding the cylinders on PCP rifles when they are being serviced.

    Accidents/incidents seem to be amazingly rare, but I know one man who is lucky to have retained the use of his left hand when a PCP reservoir let go…


  3. Frank Guin n says:

    I’ve just been told that my aluminium bottle is now illegal and has to be scrapped.

    Is this true.


  4. alan burgess says:

    What are the current rules in may 2017.
    I believe the rules have changed. What are the current rules in 2017.
    These rules could also change in August 2017.

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