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Research was conducted to develop plunge test procedures to evaluate the thermal sensitivity of recessed, flush and concealed (RF&C) sprinklers. Such sprinklers cannot be assumed to be covered directly by the RTI concept, primarily developed for sprinklers fully exposed to the hot gas flow generated by a fire. The first phase of the study was to determine the performance of RF&C sprinklers in full scale fire tests. Fifteen full-scale fire tests were conducted. Using a thermal response model it was possible to determine which of the RF&C sprinklers had thermal sensitivities consistent with IS0 sensitivity requirements for fully exposed sprinklers. Plunge tests were then conducted in the Factory Mutual Research Corporation plunge test tunnel with the sprinkler models used in the fire tests. A s p d plate to mount the recessed, flush and concealed sprinklers in the plunge test tunnel was fabricated with a closed plenum to contain the portion of the sprinklers that would typically be mounted above the ceiling in normal installation. A comparison of fire test results and plunge test results indicated that ceiling pressures generated by the fires needed to be simulated in the plunge test which was accomplished by evacuating the plenum of the mounting plate. Several combinations of plunge test temperatures and velocities, along with selected pressure differential accomplished by evacuating the plenum, were used in the plunge testing to simulate diverse ceiling flows generated by fires. It was observed that thermal sensitivity of RF&C sprinklers could be evaluated, consistent with fire test results, if maximum allowed plunge test response times were less than or equal to those determined from IS0 thermal sensitivity limits.