Turning Up The Heat (1)

It's not often hear about new advances in temperature measurement, but several underlying capabilities become possible with the application of hot technologies like fieldbus and EDDL
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Temperature sensors are in contact with the process, subject to high heat, vibration, corrosion, abrasion, pounding pressure, and stress from flowing fluids, Thermowells, when used, protect against some of these stressors, but not all. As a result, temperature sensors and their wiring are susceptible to degradation.
However, diagnostic technologies now allow plants to leverage thermocouple condition monitoring and failure prediction features that were previously unavailable in transmitters. Deployment of these technologies helps reduce process downtime and decrease energy costs. Specifically, the two open technologies brought together for detection, delivery and display of diagnostics are Foundation Fieldbus and Electonic Device Description Language (EDDL).

Conventional two-wire, loop powered temperature transmitters with a 4-20 mA analog output have a power limitation as they must consume less than 4 mA. This limits processing power of the microprocessor, as well as the level of complexity of the device firmware.
In comparison, two-wire, bus powered fieldbus devices do not have this power limitation; a temperature transmitter can consume 12 mA or more, if need be. This enable the use of more powerful microprocessor, and subsequently more sophisticated device firmware, allowing for new powerful diagnostics.

As a further advantage, and unlike other communication protocols used in temperature transmitters, devices using Foundation fieldbus have a syncronized real-time clock that enables process and diagnostic alarms to be time-stamped in the transmitter.
The time-stamps allow event recordings to be accurate, even if the communication is not instantaneous. Alerts are sent when state changes. This is the most efficient use of communication, eliminating the need to wait for polling from the system.
Device diagnostics like those for thermocouple degradation, temperature tracking, and statistical process monitoring are prioritized and categorized as per NAMUR NE. 107 recommendation. This allows the system to alert the right person without flooding others in alarms.

Making Predictions

A thermocouple is made up by joining two wires of dissimilar materials in one end. A low voltage as a function of the temperature difference between the measuring junction and open end (cold junction) is generated, known as the Seebeck effect.
The transmitter measures the voltage at the open end, and computes the temperature from the voltage based on polynomials for each type of thermocouple. The temperature at the cold junction (transmitter terminals) must also be included, known as cold junction compensation.
Thermocouples are vulnerable to a number of failure risks. Wire thinning increases the resistance of thermocouples as does improper wiring connection at terminals. Overtime, thermocouples may deteriorate, and burn out. If thermocouple conductors corrode, they also become brittle and may break due to vibration, particularly thermocouple types based on iron. The result of a degrading thermocouple or wiring is undesirable downtime.

Deterioration in thermocouples occurs slowly, and may cause inaccurate measurements. Thermocouple Degradation Diagnostic can help detect a failing thermocouple and allow preventive maintenance to be scheduled. Scheduled maintenance avoids the removal of a thermocouple from a process while in operation, which is not always possible.
Because thermocouple degradation is aggravated by long distances and multiple wiring terminations, field-mounted transmitters are preferred as a first step to reduce this problem. Next, thermocouple diagnostics may be used to monitor the health of the thermocouple. As thermocouples deteriorate, the resistance increases away from the baseline value that was recorded at installation. Thermocouple Degradation Diagnostic is able to measure and monitor the resistance of the thermocouple. If the resistance exceeds a threshold or "trigger" level, the operator will receive an alert. This alert will prompt the user to perform several preventive maintenance actions. The alert also allows time for a planned shutdown to be schedules prior to possible loop failure.

For many years, temperature transmitters have had diagnostics that, at time of sensor installation, are able to detect if sensor wiring is open or shorted. Temperature diagnostics have also provided the ability to detect if a sensor breaks and fails completely during operation. However, the ability to predict deterioration is new.

This is a good start, provided the diagnostics are passed to the operators so they can take evasive action. If a sensor breaks, the operators may have minutes or hours to respond before the process is affected. If device diagnostics are displayed on the operator consoles, operators can put loops in manual and take over control of the process. This is not deal, since it may be strenuous on operators to run the process on manual for long periods of time, and control will not be as tight as during automatic control. Therefore predictive diagnostics is important.

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