Aviation authorities require the use of supplemental oxygen when the cabin altitude is above a certain level. As a general rule you use oxygen when flying more than 30 minutes above 10000 feet by day and 5000 feet by night. These numbers do vary per country so its worth to check the rules before you go up there.
Most general aviation aircraft flying at high altitudes have either a fixed installation or a portable version, should the pilot wishes to use this on more than one aircraft. Keep in mind that even a portable installation will add to the weight and balance of the aircraft, these steel cylinders do weigh a bit.
Having an oxygen system onboard means that it will be part of the preflight and have the operation manual together with the aircraft papers. Make sure that, should the flight require this, you must refill with the correct type of oxygen or have the system always topped off.
Both portable or fixed oxygen systems will be of one of three types: constant flow, altitude adjustable or compensating. The choice is up to the pilot and depends on the dis- and advantages of each of the models and the possibilities of the aircraft.
A simple system, not too expensive and easy to use. The system has one or more cylinders to increase capacity, regulator and a manifold. The oxygen is stored in the cylinder(s) at about 2200 psi and the regulator (with flow indicator) reduces that to 50 / 75 psi and maintains this until the cylinder is almost empty and pressure starts dropping below the preset value. This system uses a mask with a re-breather bag.
With this system the pilot can preset the altitude the aircraft is flying on the regulator and the system will adjust the air flow accordingly. It will reduce the amount air used from the cylinder and as such extend the time the aircraft can remain at these altitudes.
It operates along the same principle as the altitude adjustable model but now the compensating system regulates the air flow automatically depending on cabin altitude. And as you may have guessed, this system is the more expensive one compared to the other two.
The last two sometimes use a diluter demand system where oxygen is delivered when the user inhales through the mask. It has a automatic mix ensures that cabin air is mixed with extra oxygen, altitude dependent. The masks must seal properly.
The oxygen used should be aviation grade pure (100%) and not the industrial or medical variant. These contain too much water and this may freeze in the regulator causing starvation and possible hypoxia problems.
Masks or Cannula
Using a mask can be more restrictive as eating, drinking or talking are more difficult compared to a cannula. A cannula is attached around the head with openings for air flow directly into your nose. Cannulas can be used to 18000 feet according to some manufacturers specifications. Check to see if your masks are compatible with the system in the aircraft. They also have a flow meter for the pilot to check.
You can keep them clean by using mild water and soap, then disinfected them.
All of these systems also can be built into the aircraft. In this case the manifold and tubing to the masks is part of the aircraft. In contrary to a portable system where the regulator and manifold is close to the cylinder. And you will also have or deal with the tubing, going through the cabin.
Oxygen systems are not normally found on your every day aircraft, expect to see them on high performance aircraft as a backup when the aircraft is pressurized or as the main system if the aircraft can not be pressurized.
The time a cylinder can be used depends on the size of the cylinder and its pressurization (180 liter to 1000 liter) and on how many persons in the cabin will use supplemental oxygen. But you have to make sure this information is within easy reach in the aircraft flight manual.
When installed, these systems should be part of the aircraft checklist and if any high altitude flight is planned, a go or no-go item. Instructions on how to use the oxygen system must be in the POH (if oxygen is installed permanently in the aircraft) or in the manufacturers handbook, if portable, and be kept with the system at all times.
Dangers with oxygen
There is a slight drawback when using 100 % pure oxygen. Some materials that are fire proof in a normal atmosphere are susceptible to combustion in a pure oxygen environment. Oils and greases may ignite and can not be used for lubrication and sealing of the system. Smoking also also prohibited.
For those of you flying a lot in the flight level regions: get a pulse oximeter. This device is able to measure the color change of the red blood cells (thus the oxygen saturation level) in a non-intrusive way, just by a clicking small device around your fingertip. This would be a very good investment in safety.