Testing Cylinders

There are is so much that goes on behind the scenes in a dive center, but I am going to concentrate on the servicing of your Dive Cylinders.


  1. Types of Cylinders
  2. Visual Check
  3. Hydrostatic Test
  4. Testing of Aluminum Cylinders

1. Types of Cylinders

There are 2 main type of cylinders we use at Divewise, Steel and Aluminium and they come in sizes from 1 to 18 Litres in volume. Cylinders have to have a Visual Check every year and a Hydrostatic check every 5yrs as dictated by Law in Malta

Testing Cylinders

2. Visual Check

So what do the checks comprise of for Steel Cylinders:

This Check starts with the draining of any gas within the cylinder, draining must be done slowly as doing it fast causes condensation to form on the outside and inside of the cylinder which is one of the causes of rust in a cylinder, the other is total letting the gas out of the cylinder and no pressure left to stop water ingress.  

Removal of the boots, splitting of the twin sets down to single cylinders and taking any handles, netting and stickers if the cylinder have them on.  The exteriors are checked visually for any damage, pitting due to rust spots or damage due to collision. The Rust spots if any are cleaned away with a high speed wire brush and then checked to see how deep the pitting is, depending on this they can fail at this stage. The next stage is to remove the Valve this gets a full service (cleaning and all new o rings) before getting put back on later.

The Cylinder is then visually checked using a small strip light that can illuminate the interior,

Behind the scenes

During this they will look for any rusting, or blemishes on the surface, If all is good then the next to be checked is the thread of the cylinder for wear and tear, a tool is used for this, again it can fail if there is too much play or rust on the threads .

If the inside has some sight rust or blemishes then they are given a sand blasting treatment to remove.

Cylinders are turned upside down on this machine and then aggregate fired at the surfaces to clean.

Excessive rust or deep pitting can fail the cylinder. Once complete the cylinder and Valve is put back together with a new “O” Ring and tightened back up. A cylinder test sticker is applied to the outside and a test certificate supplied. Pictures of damaged cylinders that have been cut open

3. Hydrostatic Test

The test starts off as above doing all the visual stuff but after the internal visual check and sand blasting if needed then the Hydrostatic test is carried out, this comprises of filling he cylinder with water and placing in a sealed container also full of water, then increasing the water pressure to the test pressure to either 5/3 or 3/2 of the working pressure.  Below is a standard type machine which carries out these test. The Volume of the cylinders are measured before the test and again after ,if the volume increases above the tolerated level then the cylinder will fail and will be removed from service .The valve again gets serviced and a Hydrostatic sticker and certificate is supplied.

Behind the scenes

Depending on the Paintwork on the outer surface of the cylinder they sometimes has to be sent to a paint shop for a new coating. 

The single cylinders are reassembled and normally have new nets if needed. The twin sets have their Manifolds serviced when the valves are serviced. When we reassemble the Twin sets it is advisable to tape around the cylinder where the bands are attached to stop the paintwork getting scratched. This is due to the bands being moved when divers must readjust them. 

4. Testing of Aluminum Cylinders

Aluminium Cylinders have to be serviced at intervals every 2 years by having an Ultrasonic and neck test. The visual test will look for pitting which happens if two metals like Stainless steel back plates or tank band rigging kits and the cylinder are together with these so it is good practice to wrap tank bands in cloth tubing to keep metals separate. Corrosion can be made worse inside if Salt water has entered the Cylinder. Aluminium does corrode over time just in the air. But this will be picked up during the visual check.  It would be advisable if the build-up of a powder on the outside then periodically clean it down.  

Aluminium Cylinders are cleaned out using a liquid called Biox they are then sealed with a plug and then placed into an ultrasonic bath for a set period of time. They are then emptied and rinsed out with fresh water and dried before 

Re-attaching valve and rigging. The items below are the tubing and kits we use on our cylinders. 

Behind the scenes

So as you can see there is a lot goes in to dealing with cylinders every year to make them ready for use the next year…if you want to know more we have a few courses for you to have a look at – Equipment Specialty and Gas Blending. 

Steve John Dixon

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