50% nitrous oxide / 50% oxygen is a medical gas mixture which is available as a compressed gas, in cylinders.

If the cylinder temperature is allowed to fall below -6 degrees Celsius, there is a risk that the two gases in the mixture will start to separate.  To prevent this from happening, the cylinders should be protected from the cold and stored above 10 degrees Celsius for at least 24 hours before use.

If cylinders are allowed to get too cold, they should not be used to administer gas to the patient until it has been allowed to remix properly. This may be done by bringing the temperature up to 10 degrees Celsius for at least 24 hours.  Alternatively, they may be remixed by agitating the cylinder to obtain a homogenous mixture as described in more detail below.

Before use, to ensure the gas is properly mixed, three options are available:

1. Cylinders should be stored for at least 24 hours at a temperature above 10 degrees Celsius.  Care should be taken not to expose the cylinder to excessive heat.

2. Alternatively cylinders may be remixed by maintaining them at a temperature above 10 degrees Celsius for at least 2 hours and for:

a. small portable cylinders (such as the D and CD sizes), inverting them 3 times immediately before use. Cylinders should not be immersed in water as this can lead to water entering the cylinder.

b. larger cylinders (such as the F and HX sizes), laying them down and lifting them up several times to help them to remix.

3. Cylinders used for manifold supplies (such as size G), should be stored in the manifold room for 24 hours before use.  The manifold room should be kept at a temperature of at least 20 degrees Celsius. 
Note: it is not practicable to agitate this size of cylinder.