Sorts, Markings, and Valves
Understanding the markings on scuba cylinders can be quite a headache in some cases.
First of all, it is important to know that some of the markings are mandatory in every country, but that these obligations can also differ per country.
In this post "Scuba Cylinders Compared" I would like to review some of the best-known brands and types of scuba cylinders in this blog.
Permanent stamp markings
The shoulder of the cylinder carries stamp markings providing the required information about the cylinder.
Stamp markings on an American-manufactured aluminum 40 cu ft 3000 psi cylinder.
Stamp markings on an American-manufactured aluminum 80 cu ft 3000 psi cylinder.
Stamp markings on a British-manufacture aluminium 12.2-litre
Stamp markings on an Italian-manufactured steel 7-litre 300-bar cylinder.
Universally required markings include:
Identification of the manufacturer
Manufacturing standard, which will identify the material specification
Date of manufacture
Mark of the accredited testing agency
Date of each revalidation test
A variety of other markings may be required by national regulations or may be optional.
Cylinder neck and valve
There are several standards for neck threads, these include:
Taper thread (17E), with a 12% taper right-hand thread, standard Whitworth 55° form with a pitch of 14 threads per inch (5.5 threads per cm) and pitch diameter at the top thread of the cylinder of 18.036 millimetres (0.71 in). These connections are sealed using thread tape and torqued to between 120 and 150 newton-metres (89 and 111 lbf⋅ft) on steel cylinders, and between 75 and 140 N⋅m (55 and 103 lbf⋅ft) on aluminium cylinders.
M25x2 ISO parallel thread, which is sealed by an O-ring and torqued to 100 to 130 N⋅m (74 to 96 lbf⋅ft) on steel, and 95 to 130 N⋅m (70 to 96 lbf⋅ft) on aluminium cylinders
M18x1.5 parallel thread, which is sealed by an O-ring, and torqued to 100 to 130 N⋅m (74 to 96 lbf⋅ft) on steel cylinders, and 85 to 100 N⋅m (63 to 74 lbf⋅ft) on aluminium cylinders
3/4"x14 BSP parallel thread, which has a 55° Whitworth thread form, a pitch diameter of 25.279 millimetres (0.9952 in) and a pitch of 14 threads per inch (1.814 mm);
3/4"x14 NGS (NPSM) parallel thread, sealed by an O-ring, torqued to 40 to 50 N⋅m (30 to 37 lbf⋅ft) on aluminium cylinders, which has a 60° thread form, a pitch diameter of 0.9820 to 0.9873 in (24.94 to 25.08 mm), and a pitch of 14 threads per inch (5.5 threads per cm);
3/4"x16 UNF, sealed by an O-ring, torqued to 40 to 50 N⋅m (30 to 37 lbf⋅ft) on aluminium cylinders.
7/8"x14 UNF, sealed by an O-ring.
A VALVE-TO-CYLINDER MISMATCH recently resulted in a deadly outcome in Europe. This issue is preventable with formal, function-specific training and a basic understanding of scuba tank valves and cylinder threads.
Why does this happen?
In the U.S., the most common parallel cylinder neck thread today is the ¾-inch National Pipe Straight Mechanical (NPSM) with 14 threads per inch (TPI). The most common metric cylinder neck thread is the M25x2 – ISO with 12.7 TPI, which is often erroneously called the metric equivalent to a ¾-inch NPSM.
While a valve with an M25x2 thread may initially appear to fit a ¾-inch NPSM cylinder neck, it will necessitate more force to tighten and likely not seal correctly, requiring significant overtightening to achieve a seal. This can have — and has had — disastrous consequences. To further complicate matters, an M25x2 threaded valve will also appear to fit a ¾-inch British Standard Pipe (BSP) cylinder, but only after damaging the threads.
One should never force an O-ring sealed parallel threaded valve into a cylinder. Hand tightening is all that is needed to install the valve, whether U.S. or metric, to where the valve is touching the top face of the cylinder. After hand installation of the valve, one must apply torque to compress the O-ring, ensure a robust seal, and prevent inadvertent loosening.
Torque is a unit of measure defined as pound-feet force (lbf∙ft) or Newton meters (N·m). The torque specification for scuba cylinders is usually 50 lbf∙ft or 67.5 N∙m.
A prudent scuba technician should attend formal training to better understand all aspects of cylinder and valve connections and the tools used to ensure that the threads are correct as well as to ensure an understanding of threads, O-rings, lubricants, and torque specifications. AD
[source DAN org]
Aluminium cylinders are usually manufactured by cold extrusion of aluminium billets in a process which first presses the walls and base, then trims the top edge of the cylinder walls, followed by press forming the shoulder and neck. The final structural process is machining the neck's outer surface, boring and cutting the neck threads and O-ring groove. The cylinder is then heat-treated, tested and stamped with the required permanent markings. Aluminium diving cylinders commonly have flat bases, which allow them to stand upright on horizontal surfaces, and which are relatively thick to allow for rough treatment and considerable wear. This makes them heavier than they need to be for strength, but the extra weight at the base also helps keep the centre of gravity low which gives better balance in the water and reduces excess buoyancy.
Steel cylinders may be manufactured from steel plate discs, which are cold drawn to a cylindrical cup form, in two or three stages, and generally have a domed base if intended for the scuba market, so they cannot stand up by themselves. After forming the base and side walls, the top of the cylinder is trimmed to length, heated and hot spun to form the shoulder and close the neck. This process thickens the material of the shoulder. The cylinder is heat-treated by quenching and tempering to provide the best strength and toughness. The cylinders are machined to provide the neck thread and o-ring seat (if applicable), then chemically cleaned or shot-blasted inside and out to remove mill-scale. After inspection and hydrostatic testing, they are stamped with the required permanent markings, followed by external coating with a corrosion barrier paint or hot dip galvanising and final inspection.
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Our documents are to provide a general understanding of SCUBA diving-related topics, and not to provide specific advice.
Most authorized dealers are able to perform warranty, repair, and service work on your equipment. Availability of sub-assemblies and components, repair parts, specialized tools, maintenance guides, and service manuals does not imply qualification to assemble and/or service scuba equipment. Improper service of dive equipment can lead to serious injury or death.