TABLE OF CONTENTS The LCSR method was developed to measure remotely the response time of RTDs and thermocouples while the sensor is installed in an operating process. The test involves injecting the sensor with an electrical current applied at the end of the sensor's extension leads. The current causes Joule heating in the sensor and results in a temperature transient inside the sensor. The time plot, of either the heating while the current is applied, or the cooling after the current is discontinued, is recorded during the LCSR test. From this plot, the sensor response time is obtained by means of the LCSR transformation.
The LCSR test accounts for all the effects of installation and process conditions on response time and thereby provides a sensor's actual "in-service" response time.
The LCSR test equipment for RTDs and thermocouples are quite different. We will begin by describing LCSR test equipment for RTDs.
For RTDs, a Wheatstone bridge is used to perform the LCSR test (Fig. 6.1). First, the bridge is balanced with 1 to 2 mA of DC current running through the RTD. Then, the current is switched "high" to about 30 to 50 mA. This causes the RTD sensing element to heat up gradually and settle at a few degrees above the ambient temperature. The amount by which the temperature rises in the RTD depends on the magnitude of the heating current used and on the rate of heat transfer between the RTD and its surrounding medium. Typically, the...
