The difference between the response time of temperature sensors in the lab and in actual operation can be significant. The technique for testing the response time of temperature sensors in the lab is known as the plunge test. The technique for testing temperature sensor response time in-situ is referred to as the Loop Current Step Response (LCSR) test. This paper presents the theoretical foundation of LCSR (the test theory, the LCSR equation, and the test procedures). It then details the author's experimental research to validate its use in multiple applications (primarily in the nuclear power industry). The validity of the LCSR method depends on how well the design of the temperature sensor satisfies the assumptions of the LCSR test, the quality of the LCSR data in terms of signal-to-noise ratio, the application of proper sampling parameters for the data acquisition phase of LCSR, and the suitability of the algorithms and fitting processes used in the data analysis stage of LCSR. In multiple validation tests the author demonstrated that there is excellent (within ±10 percent) agreement between the median values of the response time produced by both the plunge and LCSR methods.