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There are many different sizes of Oxygen cylinders available and you may need to plan how long they will last so that you can ensure you have enough Oxygen for your needs.

There is a simple formula to calculate how long a cylinder will last but to start with you will need to know a few figures.

The first unit you will need is the cylinder pressure. This should be on the cylinder or you can ask your supplier. In the example we will use the pressure is 2000 psi. We will need to take off this figure a residual pressure of 200 psi. This will allow 200 psi for errors in calculating or initial cylinder fill errors and give a short extra supply.

The next figure you need is the cylinder constant. This is a figure you need in order to calculate the time it will last. These are set figures you will need and these are:

A "D" cylinder the constant figure is 0.16, an "E" size is 0.28, an "M" size is 1.56, a "G" size is 2.41, and an "H" and "K" size is 3.14. Do not worry about remembering the constant figure, you can refer to the fact sheet we have out in the download area of this course. Use the figure that is relative to the size cylinder you have. In the example we will give, we are calculating the time a "D" size will last so we will use the constant figure of 0.16.

The final figure we need to know is the flow rate in litres per minute. The usual maximum flow rate regulators give is 15 litres a minute. This may be higher in specialist applications like in scuba diving where the rate could be 25 litres a minute. In the example, we will give we will use the usual maximum of 15 litres per minute. You may want to calculate how long a cylinder will last for a nasal cannula and you could change the flow rate to 6 litres a minute.

Now let's calculate how long a "D" cylinder will last 15 litres a minute.

The formula is:
Tank pressure minus residual pressure multiplied by the cylinder constant divided by the flow rate. This will give you the time in minutes that the cylinder will last.

So to use our example, 2000 psi minus 200 psi equals 1800 psi.

Multiply 1800 constant 0.16 and this equals 288.

Finally, divide this figure of 288 by 15 litres a minute to equal 19.2 minutes which we would usually round down. So a "D" sized cylinder will last 19 minutes at 15 litres per minute.