Response time index (RTI) <water mist nozzles>
a measure of nozzle sensitivity expressed as RTI = tu0.5, where t is the time constant of the heat-responsive element in units of seconds, and u is the gas velocity expressed in metres per second. RTI can be used in combination with the conductivity factor (C) to predict the response of a nozzle in fire environments defined in terms of gas temperature and velocity versus time. RTI has units of (m/s)0.5.
Source: Resolution A.800(19), Revised Guidelines for Approval of Sprinkler Systems Equivalent to that Referred to in SOLAS Regulation 11-2/12, 23 November 1995, International Maritime Organization. Regulatory Guidance