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Cylinder Capacities

By Graeme Davison
graeme_davison@mac.net.com

Many of us dive abroad and have to suffer the vagaries of the USA system of cylinder capacities. The USA system is based around the capacity of the cylinder (in cubic feet) at a defined operating pressure (in PSI). The real problem comes from the changing operating capacities of the cylinders (i.e. a 80 Cubic foot aluminium cylinder is rated at 3000 PSI, whereas a 104 Cubic foot steel cylinder is rated at 2600 PSI) and the possibility that a cylinder may not be pumped to its exact working pressure (due to under/over filling).

For those of us who keep a note of the Starting and ending cylinder contents, so that we can calculate and record our Surface Breathing rate this is a problem. In addition we may have hired equipment which has a contents gauge in PSI or have taken our own, calibrated in BAR and are mixing the two systems.

To make my life easier I sat down and calculated the following table, which takes the USA standard cylinders and equates them to the water capacity and operating pressure system we use in the Europe.

Capacity in Cubic Feet Working Pressure WC Litre
PSI BAR
50Al
3100
210.8
6.7
71.2 St
2470
168.0
12.0
80 Al
3000
204.1
11.1
80 Gen
3500
238.1
9.5
100 Al
3300
224.5
12.6
95 St
2640
179.6
15.0
100 Gen
3500
238.1
11.9
104 St
2640
179.6
16.5
120 Gen
3500
238.1
14.3


Key: Al = Aluminium; St = Steel; Gen = Sherwood Genesis

Some other useful conversion figures which are

  • PSI to BARS divide by 14.5038 (1 Standard Atmosphere = 1.01325 bar)
  • Litres to Cubic feet multiply by 0.035
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