The Fyreye MKII Rate of Rise heat detector (MKII-AHR) uses a thermistor arrangement to sense a quick rise in temperature and also a final threshold temperature of 57°C. The Fyreye fixed heat detector (MKII-AHF) has a single thermistor arrangement that gives an alarm at a temperature of 57°C, where the MKII-AHF/ CS90 gives an alarm above 86°C.