What are the advantages of using a temperature transmitter over direct wiring a sensor to a PLC or DCS?
Cut Wiring Costs
Direct wiring sensors to a control system requires the use of sensor extension wires, which can be three times more than the common shielded copper wire used for a temperature transmitter's 4-20mA signal.
Protect Signals from Plant Noise
In a direct wiring scheme, the low-level signals generated by an RTD (ohm) or thermocouple (mV) are particularly susceptible to the signal degrading effects of RFI/EMI. A temperature transmitter converts a sensor's low-level signal to a high-level analog signal (typically 4-20mA).
Stop Ground Loops
Make sure to choose an isolated transmitter. Input/output/power signal isolation protects against signal inaccuracies caused by ground loops. This is important even when using un-grounded thermocouples because their insulation will eventually break down.
Reduce Hardware Costs
With direct wiring, it is necessary to match the sensor type to input-specific DCS and PLC input cards. Sensor input-specific cards usually cost a lot more per point than a 4-20mA input card.
A microprocessor based temperature transmitter can be scaled over a wide or narrow temperature range. The accuracy of Thermocouple or RTD input cards is a percentage of the entire temperature span. You won't get very good accuracy if you want to measure a span of 100 deg F on an input card with a range of -200 to 1,500 deg F.
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