Which type of fire detection system measures temperature rise compared to a reference temperature?

The correct choice is the thermocouple, which is specifically designed to measure temperature changes by comparing the temperature at a particular point to a reference temperature. This is accomplished through the thermoelectric effect, where two different metals joined together produce a voltage that varies with temperature.

In fire detection systems, the thermocouple can effectively monitor for rapid increases in temperature that could indicate a fire. Its ability to provide immediate feedback on temperature changes makes it invaluable in environments where detecting fire quickly is critical.

Other devices like photodetectors, ionization detectors, and smoke detectors operate on different principles. Photodetectors are designed to detect light, specifically the wavelengths emitted during combustion or smoke generation, rather than measuring temperature. Ionization detectors rely on the movement of ions or changes in electromagnetic fields caused by the presence of smoke. Smoke detectors identify particles produced by combustion rather than changes in temperature. Each of these alternatives serves important functions, but they do not measure temperature rise relative to a reference point like the thermocouple does.