Emerson Rosemount Reference Manual page 111

Reference Manual
00809-0100-4021, Rev GC
5.
Figure 3-10. Tracking Sensor Drift and Sensor Failure with Differential Temperature
Advanced diagnostic
Thermocouple degradation
Problem description: Thermocouples can fail unexpectedly, potentially causing lost
production and increased maintenance costs when unplanned service is performed.
Our solution: Thermocouple Degradation Diagnostic acts as a gauge of general thermocouple
health and is indicative of any major changes in the status of the thermocouple or the
thermocouple loop. The transmitter monitors for increasing resistance of the thermocouple
loop to detect drift conditions or wiring condition changes. The degrading thermocouple can
be caused by wire thinning, sensor breakdown, moisture intrusion or corrosion and can be an
indication of an eventual sensor failure.
How it works: The thermocouple degradation diagnostic measures the amount of resistance
on a thermocouple sensor path. Ideally a thermocouple would have zero resistance, but in
reality it has some resistance especially for long thermocouple extension wires. As the sensor
loop degrades (including sensor degradation and wire or junctions degradation), the resistance
of the loop increases. First, the transmitter is configured to a baseline by the user. Then, at least
once per second, the degradation diagnostic monitors the resistance in the loop by sending a
pulsed current (in microamps) on the loop, measuring the voltage induced and calculating the
effective resistance. As the resistance increases, the diagnostic can detect when the resistance
exceeds the threshold set by the user at which the diagnostic will provide a digital alert. This
feature is not intended to be a precise measurement of thermocouple status, but is a general
indicator of thermocouple and thermocouple loop health by providing trending over time.
The thermocouple degradation diagnostic does not detect shorted thermocouple conditions.
Take away: "Thermocouple Diagnostic monitors the health of a thermocouple loop"
Target applications: Control loops, safety loops, "problem thermocouples"
HART® commissioning
Configure the DCS so that TV  –100 °C or TV  100 °C indicates a sensor failure and, for
example, TV  –3 °C or TV  3 °C indicates a drift alert. See
(Failure Mode Switch HIGH)
100 °C
3 °C
0 °C
–3 °C
–100 °C
(Failure Mode Switch LOW)
Sensor Failure
Sensor Drift
Sensor Drift
Sensor Failure
Section 3: HART commissioning
Figure 3-10.
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