How To Refill a Scuba Cylinder

Scuba cylinders are tanks made of either steel or aluminum that are designed to hold air under pressure while scuba diving. The cylinders can be filled with either air, nitrox (a mixture of nitrogen and oxygen), or tri-mix (a mixture of helium, nitrogen, and oxygen).  Scuba cylinders are typically designed to be filled to a pressure of 3,000 pounds per square inch, although smaller cylinders do exist for redundant air source purposes.  Be sure you know what the cylinder's rating is before attempting to fill it.  You should always follow certain safety precautions whenever filling a scuba cylinder.

You should always conduct a visual inspection of each tank prior to filling.  It is important to look for any rust or cracks that would compromise the structural integrity of the cylinder.  It is also important to check the condition of the valve to ensure that it too is clear of any rust or debris. Additionally, every cylinder should be visually inspected internally at least annually, to ensure that there is no corrosion on the interior of the tank, which could result in a cylinder failure.

Because of the pressures involved with scuba cylinders it is not uncommon for them to flex and contract with each fill.  Department of Transportation regulations specify that each cylinder must be hydrostatically tested every five years starting from the year of their construction.  After a cylinder has been certified as hydrostatically sound, the date is stamped into the top of the tank.  Be sure to verify that the tank has a current valid hydrostatic test date stamp before attempting to fill it.

  1. After completing these safety inspections place the cylinder into the compressor. 
  2. Connect the compressor input hose to the valve and open the cylinder valve. 
  3. Set the fill pressure on the automatic shutdown valve so that the compressor does not overfill the cylinder. 
  4. Ensure that all of the connections are tight and that there are no leaks before you turn on the compressor.
  5. While the compressor is running, stand by and monitor its gauges.
  6. When the compressor shuts off, close the valve on the cylinder and then loosen the relief valve on the input hose. 
  7. Once the air is released you will be able to disengage the input hose from the cylinder valve.

Cylinders not being used regularly should be stored with at least 100 to 300 pounds of air to prevent moisture from getting inside.


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