E+C August 2018

TEMPERATURE MEASUREMENT + INSTRUMENTATION

monitoring tend to use thermocouples. With ei- ther RTDs or T/Cs, it's important to ensure that the temperature transmitters have the curves and lin- earisation data built into the memory without the need for custom programming. Transmitters superior to direct wiring Most temperature applications in power plants involve directly wiring a temperature sensor to the control system. Often engineers wire direct because they mistakenly believe this is a cheap- er and easier solution. Despite the large installed base of direct wired sensors, the trend is towards using transmitters in conjunction with temperature sensors. Transmitters save time and money in in- stallation, improve measurement reliability, reduce maintenance and increase uptime. A transmitter converts the mV signal from an RTD or thermocouple to a 4-20 mA signal or to a digital fieldbus output such as HART, Foundation

Fieldbus or Profibus PA in the case of a smart transmitter. Either of these outputs can be trans- mitted over a twisted pair wire for a considerable distance. Smart transmitters incorporate remote calibration, advanced diagnostics and built-in con- trol capabilities – and some are capable of wireless operation. Direct wiring requires sensor extension wires from the sensor to the automation system input modules. For thermocouples, these wires are ex- pensive and sometimes fragile. RTDs can use in- expensive copper wires, but some RTDs have up to four wires. In power plants the automation system can be located far from the temperature sensors, which can lead to great installation expense, depending on the number of sensors and the distances in- volved. Over long wiring distances, Electromagnetic Interference (EMI) and Radio Frequency Inter-

The longer the standby power station is in operation, the more important the choice of fuel choice.

Electricity + Control

AUGUST 2018

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