what is the heat rise detector ？

About the Rate-of-rise heat detectors (be short for heat rise detector)

Rate-of-Rise (ROR) heat detectors（heat rise detector） operate on a rapid rise in element temperature of  8°C (46.4°F) increase per minute, irrespective of the starting temperature. This type of heat detectors can operate at a lower temperature fire condition than would be possible if the threshold were fixed. It has two heat-sensitive thermocouples or thermistors. One thermocouple monitors heat transferred by convection or radiation while the other responds to ambient temperature. The heat rise detector responds quickly when the first sensing element's temperature increases relative to the other.

heat rise detectors may not respond to low energy release rates of slowly developing fires. In order to detect slowly developing fires combination detectors add a fixed temperature element that will ultimately respond when the fixed temp element reaches the design threshold.