Response Time Index (RTI) is a value applicable to

Response Time Index (RTI) is a measure of thermal sensitivity of a sprinkler’s activation. It indicates how quickly the sprinkler responds to the heat from a fire. RTI is calculated by using the operating time, operating temperature, air temperature, air velocity, and conductivity factor of the sprinkler. RTI is independent of the gas velocity but depends on the properties of the sprinkler head such as mass, specific heat capacity and surface area of the thermal sensing element. RTI is used to classify sprinklers into fast response or standard response categories.

References:The Basics of Sprinkler Thermal Characteristics | NFPA;Sprinkler Characteristics According to NFPA 13 - Fire Protection …;Response time index (RTI) - Oil and Gas Drilling Glossary;RESPONSE TIME INDEX OF SPRINKLERS - Department of Building Environment …