An automatic sprinkler head is a detector and an actuator in one object: a glass bulb filled with a liquid that expands and bursts the bulb at a rated temperature, releasing the plug and starting flow from that head alone. Because the bulb has thermal mass, it lags the hot gas passing it, and the response time index quantifies that lag. It is defined as \( RTI = \tau \sqrt{u} \), where \( \tau \) is the sensing element's time constant and \( u \) the gas velocity, and it has the memorable units \( \mathrm{m}^{1/2}\mathrm{s}^{1/2} \). A low value, below about 50, marks a fast-response head with a thin bulb, used where early actuation protects people; a value above about 80 marks a standard-response head. Two consequences follow. Actuation depends on gas velocity as well as temperature, so the same head in a faster ceiling jet opens sooner. And the bulb colour codes its rated temperature, so a head can be read off a ceiling. After this Concept you can predict which of two heads opens first in the same fire and justify choosing a fast-response bulb.
An automatic fire sprinkler system is a decentralized hydraulic network of independent thermal sensors, in which each sprinkler head acts as its own autonomous heat detector and control valve, activa…