The air we breathe

Dive gear maintenance: Cylinders

Cylinders are made of either steel or aluminium (aluminum to the Yanks), and both are prone to corrosion. Nothing rusts quicker than a damp cylinder. Most people will fill the cylinder with air after a dive, and then store it.

The air we breathe
The air we breathe

When you get home rinse the cylinder well, several times. The plastic boot collects salt and sand but it does not come off easily so drilling a few large drain holes in the plastic at the bottom of the boot makes rinsing easy and the boot does not need to come off so often.

The net protects the cylinder from nicks but once in a while loosen the net and clean away any rust that has formed on scratches (with a scouring pad) and paint the spot. Cylinders require an annual viz, this is a visual inspection for dings, dents, corrosion and general condition. The inside is inspected, for rust, pitting and discoloration. Every four years the cylinder is subjected to a hydro test. This involves filling it with water and pumping it up to a specific pressure, way above the normal operating pressure, and the difference in size is measured to ensure it is structurally sound. The stamp from the hydro inspection is prone to rust and difficult to keep rust-free so as soon as it is returned from inspection you should paint it. If you don’t have any paint, use your (or someone else’s!) clear nail varnish.

The colour of your cylinder is also important. By law (in South Africa) it must be yellow with a grey neck. Compressor operators are not obliged to fill the cylinder if it is the wrong colour, and nor will they fill it for you if it is out of date (i.e. more than 12 months have passed since the last inspection). So painting it random colours will be a problem when trying to have it filled.

The air pumped into a dive cylinder goes through a drying process so there should not be moisture in your cylinder. If you have had a cylinder at depth that has been breathed empty then there is a strong likelihood it has water in it. Should you have moisture enter the cylinder it will rust, and it will do so very quickly. The cure? If it is light rust it can be cleaned with a wire brush (quite a special type of wire brush) attached to a drilling machine.  It can also be chemically cleaned or if really bad it can be ”rumbled”. This is a process whereby the servicing dive centre adds a few handfuls of stones and some water to the cylinder and it is placed on a pair of slow moving rollers that rolls it for some time cleaning off the interior rust. It is then washed, dried and inspected again.

It is not a requirement when servicing a cylinder, but having your pillar valve serviced at the same time is a good policy. If you have the training you can do it yourself but it is best left to the professionals. My cylinders have just had their annual service, a repaint and a pillar valve service. A few new tank nets to replace the badly worn ones and they are all set for a year of trouble free diving.

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Tony

Scuba diver, teacher, gadget man, racing driver, boat skipper, photographer, and collector of stray animals

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