From Maintenance of Process Instrumentation in Nuclear Power Plants

6.3 LCSR

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.

6.3.1 Test Equipment

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...

Products & Services
Resistive Temperature Devices (RTD) Elements
Resistive temperature device (RTD) elements are wire windings or other thin-film serpentines that exhibit changes in resistance with changes in temperature. They are usually made of metallic elements or alloys such as copper, nickel, or nickel-iron.
Temperature Sensor Chips
Temperature sensor chips are semiconductor dies or packages that have built-in temperature sensors.
Temperature Sensors
Temperature sensors are measurement devices that infer temperature by sensing some physical characteristic (i.e. resistance, emf or thermal radiation).
Digital Thermometers
Digital thermometers are portable temperature sensing devices that have permanent probes and a digital display.
Chart Recorders
Chart recorders and strip charts are data acquisition tools used to generate a plot, graph or other visualization of data versus time.

Topics of Interest

6.4 Self-Heating Test 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...

6.5 Noise Analysis Technique The output of all process sensors in nuclear power plants normally contains fluctuations due to random flux, random heat transfer, turbulence, vibration, and other...

6.6 NRC Regulations The NRC Regulatory Guide 1.118, NUREG-0800, and NUREG-0809 all relate directly or indirectly to sensor response-time testing. For example, Regulatory Guide 1.118 provides...

6.8 Summary The response time of a temperature sensor depends on installation and process conditions. In particular, the process temperature and flow have a significant influence on response time.

AMS Analysis and Measurement Services Corporation ANO Arkansas Nuclear One ANSI American National Standards Institute APD amplitude probability density AR autoregressive (refers to...