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Ref. 185040646 MR730/720/700/480 Technical Manual ¯ Revision F ¯ Issued March 2001
Pushing the mute button mutes the audio alarm for ten minutes and restores power to the heaterplate during this time.
The Airway Temp Low LED will continue to flash. This alarm is disabled during warmup. See § 2.6.2.
If the airway temperature drops below 5°C the display will show ‘LO’ and an audible alarm will sound.
3.4 AUDIO ALARM
A choice of the ISO draft standard audio alarm or a gliding tone can be selected by the programming control link (L1).
Refer Appendix A3.
A momentary depression of the mute button will silence the audio alarm for about 3 minutes (except for a low
temperature alarm in no heater wire mode. See § 3.3.3. The relevant LED indicators remain flashing until the alarm
condition clears. Any new alarm condition occurring within a mute delay will cause the audio alarm to be re-enabled.
3.5 SAFETY PROTECTION (ALL MODELS)
3.5.1 TEMPERATURE DEVIATIONS
An over temperature alarm condition causes an immediate shut down of both heater circuits. In the case of a Low
Temperature alarm, pressing Mute causes heating to recommence, the Low Temperature LED however remains on until
the fault has cleared. There are two independent backup over temperature protection circuits.
Firstly, an over temperature protection circuit independent of the microprocessor control continuously monitors the
airway thermistor sensor. This backup protection is preset to disconnect the heating circuits at 41.0 °C.Secondly, a
bimetallic thermostat fixed to the heaterplate operates if the heaterplate temperature exceeds 118 ± 6 °C and cuts out the
mains active supply. On cooling, this protector can only be reset by gaining access to the under surface of the
heaterplate and manually depressing the red reset button.
Heaterplate temperature is limited to 110 °C max during normal operation (80 °C for MR720). Optional heaterplate limits of
50, 80 and 90 °C can be selected. (Refer Appendix A3).
3.5.2 MICROPROCESSOR WATCH DOG
A control loop in the operating software continuously monitors for correct operation. Should a fault be detected a
hardware reset circuit attempts to reinitialise the microprocessor. If this is not successful additional hardware disables
the heating circuit, blanks the display and turns the Humidifier Fault red LED on.
The watch dog circuit is tested at turn on by deliberately halting the processor and forcing initialisation.