Oxygen is measured in a wide variety of processes, to confirm its presence or absence. The most common application however is measuring oxygen as an 'impurity' in other gases, e.g. nitrogen generators. Oxygen is required in aerobic processes and also in situations that require user intervention, e.g. to ensure that it is safe to enter a chamber for maintenance work. Some processes will only run efficiently in the absence of oxygen, e.g. furnace atmospheres; others just require a low level of oxygen for safety reasons, e.g. inert gas blanketing.