TABLE OF CONTENTS The self-heating test is performed to detect gross changes in response time. It is useful only for RTDs; it does not apply to thermocouples.
The self-heating test does not measure the response time of an RTD; it provides a means for detecting RTD response-time degradation. Typically, both LCSR and self-heating tests are performed on RTDs because the two tests complement each other to provide a complete picture of the RTD's dynamics. The self-heating tests are typically performed together with LCSR tests both when response time is measured at hot standby or at operating conditions and during testing at cold shutdown to verify RTD installation.
Like the LCSR method, the self-heating test is based on heating the RTD with a small DC current (I). It is performed using the same Wheatstone bridge as in the LCSR test. For the self-heating test, the steady-state increase in the RTD resistance ( ?R) as a function of input electric power (p = I 2R) is measured. The result is referred to as the Self-Heating Index (SHI) of the RID and is expressed in the units of ohms per watt ( ?/ ?).
The following derivation shows the correspondence between an RID's SHI and its response time ( ?). For simplicity, the description is given for a first-order system, though RTDs are not typically represented by first-order dynamics. The steady-state relation between temperature and I 2 R heating generated in an RTD is...
