The most important rule of rebreather diving is to always know what gas is in the breathing loop. This process begins with analyzing the contents of your cylinders.
Sensor calibration is performed assuming a certain percentage of oxygen, however the only way to know what percentage of oxygen the cylinder actually contains is to analyze it. If the actual percentage of oxygen in the cylinder is different than what is assumed, it could lead to an incorrect calibration resulting in a different gas mix in the loop than what is displayed on the HUD and controller. This could result in oxygen toxicity or errors in decompression calculations.
It is highly recommended that both oxygen and diluent cylinders, along with any bailout cylinders, be analyzed and the contents labeled on the cylinder prior to assembling the unit.