why transmitters over direct wiring

8/17/2019 Why Transmitters Over Direct Wiring

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Why Transmitters over Direct Wiring?

http://www.rosemount.com/products/temperature/why_transmitters.html

Advantages of Using Temperature Transmitters

1. Reduced Wiring Costs 4. Improved Reliability with SensorDiagnostics

2. Less Susceptibility to EMI Interference 5. Reduced Downtime with Cost-EffectiveSensor Redundancy

3. Better Accuracy and Repeatability

Transmitter Installation

Direct Wiring Costs

Reduced Wiring Costs

Wiring sensors direct back to a control system necessitates the use ofexpensive sensor leadwire extensions which can cost substantially morethan standard copper wires used for transmitter communications.

Wiring cost savings are dependent on wiring distances, specific plantarchitecture, material and labor costs.

Click here to download Rosemount’s Cost Savings Calculator . Thisinteractive tool allows you to compare various wiring architectures basedon actual inputs from your plant installation.

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Less Susceptibility to EMI Interference for Better Reliability

Today’s industrial environments are continuously subjected tonumerous forms of EMI (Electromagnetic Interference) and RFI (RadioFrequency Interference). Common sources include transformers,various AC and DC motors, welding equipment and radiocommunications equipment.

EMI and RFI emissions may be negatively affecting your plant,particularly if direct wiring of instrumentation is used. RTD andthermocouple sensor signals are relatively small in magnitude andhence more susceptible to being affected by various EMI/RFIinterference. Furthermore, sensor leadwires can contribute to theproblem be acting as an antenna and actually drawing additionalamounts of spurious EMI/RFI into the sensing circuit. Any extraneousEMI/RFI "picked up" by the measurement circuit will cause anerroneous Temperature measurement, and at high enough levels, aplant shutdown.

On the other hand, an installation using temperature transmitters is ableto withstand greater levels of EMI/RFI . A transmitter accomplishes thisby converting the low-level (ohm or mV) sensor signal into a relativelyrobust and high magnitude signal, which is typically a 4-20mA analogsignal or a digital fieldbus signal.

Lastly, it is important to select transmitters that are themselves resistantto the negative effects of EMI/RFI. Not doing so will introduce anotherpotential EMI/RFI failure point into the measurement circuit.

Improved Measurement Accuracy for Optimal Yields

In a typical application, the majority of Temperature measurement error is inherent to the Temperaturesensor (RTD, thermocouple, etc..) and not to the signal communications equipment (i.e. transmitter, PLC, orDCS input card).

Today’s advanced Temperature transmitters are able to eliminate the majority of this Temperature sensorerror by utilizing Transmitter-Sensor Matching. First, an RTD’s precise resistance versus temperaturerelationship is determined by sensor calibration. Next, the resultant sensor calibration information is

"Matched" to a device enabled with Transmitter-Sensor Matching by loading it onto the transmitter’s internalmemory. As a result, the transmitter is able to process the sensor’s resistance information and calculate theexact Temperature of the process, thereby eliminating the bulk of the sensor’s error.

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Improved Reliability with Sensor Diagnostics

All sensors drift. Use of the appropriate Temperature Transmitter enabled with Sensor Drift Alert a theconcurrent monitoring of two separate sensors or one dual-element sensor.

By setting a user definable drift limit between the two sensors, the transmitter will automatically alert the userwhen the difference between the two sensor readings is greater than the drift limit. This enables pro-activeuser investigation of the process and instrumentation long before a severe condition is encountered,improving measurement reliability and helping to ensure a smooth running process.

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Reduced Downtime with Cost-Effective Sensor Redundancy

Most Temperature measurement points fail due to a sensor malfunction. A temperature transmitter enabledwith Hot Backup capability can be wired to two separate sensors or one dual-element sensor.

Should the primary sensor fail, the transmitter will automatically and seamlessly switch to the secondarysensor, thereby avoiding a lost reading and allowing the user time to replace the failed sensor.

Hot Backup is an easy and cost-effective way to provide redundancy for critical applications and reducecostly process downtime due to sensor failures.

why transmitters over direct wiring

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