Temperature Transmitters 
Why Use Transmitters?

Why Transmitters over Direct Wiring? 

Advantages of Using Temperature Transmitters

	Transmitter Installation
	Direct Wiring Costs

Reduced Wiring Costs

Wiring sensors direct back to a control system necessitates the use of expensive sensor leadwire extensions which can cost substantially more than standard copper wires used for transmitter communications. Wiring cost savings are dependent on wiring distances, specific plant architecture, material and labor costs. Click here  to download Rosemount’s Cost Savings Calculator. This interactive tool allows you to compare various wiring architectures based on actual inputs from your plant installation.

Less Susceptibility to EMI Interference for Better Reliability

Today’s industrial environments are continuously subjected to numerous forms of EMI (Electromagnetic Interference) and RFI (Radio Frequency Interference). Common sources include transformers, various AC and DC motors, welding equipment and radio communications equipment. EMI and RFI emissions may be negatively affecting your plant, particularly if direct wiring of instrumentation is used. RTD and thermocouple sensor signals are relatively small in magnitude and hence more susceptible to being affected by various EMI/RFI interference. Furthermore, sensor leadwires can contribute to the problem be acting as an antenna and actually drawing additional amounts of spurious EMI/RFI into the sensing circuit. Any extraneous EMI/RFI "picked up" by the measurement circuit will cause an erroneous Temperature measurement, and at high enough levels, a plant shutdown. On the other hand, an installation using temperature transmitters is able to withstand greater levels of EMI/RFI . A transmitter accomplishes this by converting the low-level (ohm or mV) sensor signal into a relatively robust and high magnitude signal, which is typically a 4-20mA analog signal or a digital fieldbus signal. Lastly, it is important to select transmitters that are themselves resistant to the negative effects of EMI/RFI. Not doing so will introduce another potential 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 Temperature sensor (RTD, thermocouple, etc..) and not to the signal communications equipment (i.e. transmitter, PLC, or DCS input card).

Today’s advanced Temperature transmitters are able to eliminate the majority of this Temperature sensor error by utilizing Transmitter-Sensor Matching. First, an RTD’s precise resistance versus temperature relationship 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 internal memory. As a result, the transmitter is able to process the sensor’s resistance information and calculate the exact Temperature of the process, thereby eliminating the bulk of the sensor’s error.

Improved Reliability with Sensor Diagnostics

All sensors drift. Use of the appropriate Temperature Transmitter enabled with Sensor Drift Alert a the concurrent 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 user when the difference between the two sensor readings is greater than the drift limit. This enables pro-active user investigation of the process and instrumentation long before a severe condition is encountered, improving measurement reliability and helping to ensure a smooth running process.

Reduced Downtime with Cost-Effective Sensor Redundancy

Most Temperature measurement points fail due to a sensor malfunction. A temperature transmitter enabled with 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 secondary sensor, 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 reduce costly process downtime due to sensor failures.

Emerson Process Management, Rosemount Division, 8200 Market Blvd. Chanhassen, MN USA 55317 North America: 1-800-999-9307    International: 952-906-8888  Fax:  952-949-7001

Last Updated 11/19/08 

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