Emerson Rosemount 3144P Reference Manual page 112

HART Commissioning
December 2019
3.14.4 Advanced diagnostic
Thermocouple degradation
Problem
description:
Our solution:
How it works:
Take away:
112
Figure 3-11: Tracking Sensor Drift and Sensor Failure with Differential
Temperature
Thermocouples can fail unexpectedly, potentially causing lost
production and increased maintenance costs when unplanned service
is performed.
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.
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.
"Thermocouple Diagnostic monitors the health of a thermocouple
loop"
Reference Manual
00809-0100-4021
Rosemount 3144P