Typically you will not need to do this but it can be helpful if troubleshooting your system. The best way and least expensive way to do this with an oxygen sensor. All of our air separators include a low-oxygen sensor that automatically tests the oxygen concentration made by the air separator. An alarm indicates if the oxygen level falls below a standard level usually about 75%.
The additional sensors, mentioned below, simply plug onto the reservoir or breathing hose to give you a percentage of oxygen in the air. It might be necessary to press on the reservoir to create air flow/mixing to get an accurate reading.