BS 8478 – Breathing Gases for Diving and Hyperbaric Applications
This standards is virtually impossible to achieve using air produced by standard breathing air compressors and the low levels of contaminants required cannot be effectively measured by conventional methods; it makes a laboratory test the only reliable method.
The HSE is well aware of the practical problems that this standards poses for recreational diving operations. A European version of this Standard is being developed. No doubt, when the EN standard is published, further HSE guidance will follow. In the meantime, ASSET Members are advised to ensure that they are producing air, particularly for the production of Nitrox or Tri-mix, to the highest practicable standard.
Meeting the low oil level required will be the greatest challenge. Adding a high volume, low velocity secondary filter (as recommended in the ASSET Nitrox Manual) that provides maximum "Dwell" of the air being filtered will go some way to achieving this.We also recommend that filters are replaced in good time and that any air purity test is carried out at the end of the filter's life not when it is new. This will ensure a "worst case" measurement.
Additionally, we recommend that, in addition to any quarterly "in house" testing that may be carried out, a laboratory test is carried out at least 6 monthly intervals. A relatively economical and practicable laboratory test is available from Aquatron in Glasgow. http://www.airpurity.com/how_apa.htm
Please also read the following HSE guidance:
BS 8478 – Breathing Gases for Diving and Hyperbaric Applications
British Standard 8478 specifies requirements for the composition and purity of breathing gases, other than compressed air, for diving and hyperbaric applications. It specifies reduced levels of contaminants compared with those required to meet the compressed air standard, BS EN 12021. HSE supports British Standards as a means to enhance safety and provide guidance to industry. In particular HSE endorses the need for increased gas purity, over and above the levels required in BS EN 12021, where equipment is exposed to oxygen levels greater than 21% and for gas mixes used at increased depths. Compliance with a British Standard is, however not a legal requirement but their observance provides a means of demonstrating that appropriate risk controls have been applied. It is open to gas manufacturers/providers to produce gas to their own standard provided these also apply appropriate risk controls. Ultimately it will be up to the courts to decide if such other standards are sufficient.
HSE is aware that areas of the recreational diving industry have concerns with some aspects of BS 8478. It is believed these concerns are based on whether the demanding purity criteria are justified for all gas mixes used in recreational diving, as well as the potential impact the standard could have on the continued use of Nitrox in the recreational sector.
BS 8478 is currently being considered as a basis for a new European standard. When this European standard is finalised it will supersede BS 8478 which will then be withdrawn. The process for developing the new standard is now underway within Europe and the resulting standard will almost certainly be different in some respects from the current BS 8478.
In the interim period before the European standard is finalised and published HSE expects the recreational diving industry to take all reasonably practicable steps to minimise the contaminants in the gases other than air which they provide, and where practicable to meet the contaminant criteria detailed in BS 8478. These steps should include robust systems of work to ensure that any impurities are maintained at the lowest levels practicable as well as periodic tests to ensure the contaminants are maintained at these low levels. BS 8478 currently requires testing at least every 6 months.
In the event of diving incidents where gases other than air have been used, the breathing gas composition and purity criteria in BS 8478 will continue to be referred to as part of the investigation process. Should these investigations lead to legal proceedings, and the gas provider did not follow the requirements in BS 8478, it will be up to a court to decide if the steps taken to ensure the purity of the breathing gas were sufficient.
Recreational diving gas providers may wish to take account of the views of their insurers when deciding their course of action.
The process for developing the new European standard is now underway. Stakeholders within recreational diving are urged to actively participate in this process to ensure the new European standard both enhances safety and is workable for all sectors of the diving industry.
THREAD CHECKING M25, 3/4 - 14 NPSM
We get regular enquiries about how many turns can be allowed on a Not Go Gauge when inspecting cylinder neck and valve stem threads. The following is a reply to this question to one of our members:
The "by the book" answer is that there should be no turns allowed on a Not Go gauge (if you think about it, that's why it's called a Not Go gauge!). However, in many industries, particularly where maintenance activities are concerned, it is common to allow something for reasonable wear and tear.
In the past, the general advice given (and widely accepted) in the UK concerned only the G3/4" thread and that advice was that up to 4 turns on the Not Go gauge could be allowed on the valve, providing that the cylinder thread was good.
However, over the past 20 years or so, M25, G5/8" and 3/4" NPSM threads have become widely used additionally ASSET's membership has become Worldwide, therefore, it has been necessary to revise and rationalise this advice.
To keep matters simple, we believe a reasonable rule of thumb to be as follows: Providing the technician is satisfied that all threads are physically sound and the cylinder neck threads have been gauged and the Not Go gauge does not go then 1 (one) turn on the Not Go gauge might be allowed on the on the valve stem thread for every four good turns engaged. In effect this means approx 1T on a 200 bar Din outlet, 2T an M25 and 3T on a G3/4" and so on.
For 3/4 NPSM threads, there is further guidance in CGA 6.1 2006 as follows:
5.9 Threads an Valving.
Cylinder threads should be examined whenever the valve is removed from the cylinder. Cylinders have a specified number of full threads of proper form as required in the applicable thread standards. Cylinders shall be rejected if the required number of effective threads are materially reduced so that a gas tight seal cannot be obtained by reasonable valving methods. Thread defects include threads that are galled, worn, corroded, broken, cracked, nicked and double threaded (by forcing an incorrect valve into the threads).
This guidance should be applied with caution and is at the discretion of the individual technician who must take into account the circumstances as they present themselves.
CYLINDER STAMPING
We have received some inquiries about the correct order of stamp marking on cylinders.
Due to a misunderstanding, the advice given has been to split the year and month with the inspectors mark.
Having checked in the latest version of the applicable standard - BS EN ISO 13769:2006 the order is given as # YY/MM - Where # is the inspector's mark and the year and month is separated by a forward slash.
There is no need to retrieve cylinders and remark them, however inspectors should change to the format above as set out in the Standard.
Although not required by the standard, in order to prevent confusion over the reversal of the of month and year after 2002, we continue to recommend that until 2013 that the year is stamped in full, i.e. # YYYY/MM
