A closed circuit breathing apparatus (2) and a method of operating the same, the method comprising: determining whether a sufficient amount of replenishing gas is being supplied to a breathing circuit (4) sensing pressure within the breathing circuit (4) using a pressure sensor (6) generating a pressure signal representative of the sensed pressure determining whether the pressure signal is representative of breathing and generating an alert if it is determined that both an insufficient amount of replenishing gas, such as oxygen, is being supplied to the breathing circuit (4) and that the pressure signal is representative of breathing.
