An apparatus for monitoring a wearer of breathing apparatus, comprising: a pressure detecting device comprising: a pressure transducer arranged to detect a pressure of a gas supply of the breathing apparatus; and a mobile monitoring unit arranged to be attached to the wearer comprising: a motion sensor arranged to monitor the motion of the wearer; and an alarm (44, 46); wherein the mobile monitoring unit has a plurality of operational states including: an ON state in which the alarm is activated when the motion sensor has not detected motion for a predetermined period of time; and an OFF state; and wherein: the pressure detecting device further comprises a wireless transmitter arranged to wirelessly transmit a preset number of ON signals when it is determined that the pressure of the gas supply has exceeded a predetermined on threshold; the mobile monitoring unit further comprises a wireless receiver arranged to receive signals wirelessly transmitted by the wireless transmitter of the pressure detecting device; and wherein the mobile monitoring unit is arranged to be switched from the OFF state to the ON state when the wireless receiver receives at least one ON signal from the wireless transmitter. a pressure detecting device comprising: a pressure transducer arranged to detect a pressure of a gas supply of the breathing apparatus; and a mobile monitoring unit arranged to be attached to the wearer comprising: a motion sensor arranged to monitor the motion of the wearer; and an alarm (44, 46); wherein the mobile monitoring unit has a plurality of operational states including: an ON state in which the alarm is activated when the motion sensor has not detected motion for a predetermined period of time; and an OFF state; and wherein: the pressure detecting device further comprises a wireless transmitter arranged to wirelessly transmit a preset number of ON signals when it is determined that the pressure of the gas supply has exceeded a predetermined on threshold; the mobile monitoring unit further comprises a wireless receiver arranged to receive signals wirelessly transmitted by the wireless transmitter of the pressure detecting device; and wherein the mobile monitoring unit is arranged to be switched from the OFF state to the ON state when the wireless receiver receives at least one ON signal from the wireless transmitter. a motion sensor arranged to monitor the motion of the wearer; and an alarm (44, 46); wherein the mobile monitoring unit has a plurality of operational states including: an ON state in which the alarm is activated when the motion sensor has not detected motion for a predetermined period of time; and an OFF state; and wherein: the pressure detecting device further comprises a wireless transmitter arranged to wirelessly transmit a preset number of ON signals when it is determined that the pressure of the gas supply has exceeded a predetermined on threshold; the mobile monitoring unit further comprises a wireless receiver arranged to receive signals wirelessly transmitted by the wireless transmitter of the pressure detecting device; and wherein the mobile monitoring unit is arranged to be switched from the OFF state to the ON state when the wireless receiver receives at least one ON signal from the wireless transmitter. an ON state in which the alarm is activated when the motion sensor has not detected motion for a predetermined period of time; and an OFF state; and wherein: the pressure detecting device further comprises a wireless transmitter arranged to wirelessly transmit a preset number of ON signals when it is determined that the pressure of the gas supply has exceeded a predetermined on threshold; the mobile monitoring unit further comprises a wireless receiver arranged to receive signals wirelessly transmitted by the wireless transmitter of the pressure detecting device; and wherein the mobile monitoring unit is arranged to be switched from the OFF state to the ON state when the wireless receiver receives at least one ON signal from the wireless transmitter. the pressure detecting device further comprises a wireless transmitter arranged to wirelessly transmit a preset number of ON signals when it is determined that the pressure of the gas supply has exceeded a predetermined on threshold; the mobile monitoring unit further comprises a wireless receiver arranged to receive signals wirelessly transmitted by the wireless transmitter of the pressure detecting device; and wherein the mobile monitoring unit is arranged to be switched from the OFF state to the ON state when the wireless receiver receives at least one ON signal from the wireless transmitter.