Keys to achieving high accuracy and reliability in temperature measurement

01/24/2014


Maximizing accuracy and reliability

When selecting transmitters, there are many features and options that can work together to build confidence in your readings:

• Intelligent filtering—In most plant environments, electrical surges from lightning or other static discharge are common as are power surges and dips from your local grid. There can be other hostile conditions caused by vibration, high humidity, extreme ambient temperatures, corrosive atmospheres, and so on, that can adversely affect transmitter performance. Fortunately, high-quality manufacturers have design features and configuration options that address these issues and help provide a reliable temperature measurement.

Figure 3: With a dual sensor system, the transmitter can be programmed to switch the transmitter input automatically from the primary sensor to the secondary sensor should the primary sensor fail. This feature allows the assembly to maintain measurement c• Transmitter-sensor matching—All RTD sensors have inherent inaccuracies or offsets from an ideal theoretical performance curve. Transmitter-sensor matching can create precise compensation for these inaccuracies. Some transmitters offer this as a built-in function using a Callendar-Van Dusen (CVD) equation program. This equation describes the relationship between resistance and temperature of specific RTDs. The matching process allows the user to enter four sensor-specific CVD constants into the transmitter. The transmitter solves the equation to match the transmitter to that specific sensor, thus providing outstanding accuracy. Accuracy improvement for sensor matching is typically 7:1 and system accuracies of ± 0.025 °F (± 0.014 °C) are possible.

• Hot device switchover—With a dual sensor system, the transmitter can be programmed to switch the transmitter input automatically from the primary sensor to the secondary sensor should the primary sensor fail. This feature allows the assembly to maintain measurement continuity (Figure 3).

Figure 4: When two sensors are operating in parallel and connected to the same transmitter, the transmitter can be programmed to compare the two sensor readings and notify the control system if the readings begin to diverge due to degradation of one of th• Sensor drift detection—When two sensors are operating in parallel and connected to the same transmitter, the transmitter can be programmed to compare the two sensor readings and notify the control system if the readings begin to diverge due to degradation of one of the sensors that is causing its measurement signal to drift away from the actual value (Figure 4).

Regardless of the hostile conditions that might exist at the point of measurement, a properly specified temperature measurement system configured with options and features as described above can go a long way to ensure that an accurate and reliable measurement is continually reported to the receiving system.

David Anderson is a senior marketing engineer for Emerson Process Management.

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This is the fourth part of this temperature series. Read the earlier articles attached below. 

To learn more about measuring temperature in process manufacturing applications, go to www.rosemount.com/tempguide and order a free copy of The Engineer’s Guide to Industrial Temperature Measurement.

www.emersonprocess.com

Key concepts:

  • All devices in your temperature measurement chain will affect the overall accuracy and reliability.
  • The accuracy of all devices needs to be similar or you could be wasting money.
  • Devices such as transmitters can offer features to make the task easier and more reliable.

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