3Advanced Diagnostics - Emerson Rosemount Reference Manual

Section 4: F fieldbus Configuration
OUNDATION
April 2014
6.
7.
Note
When changing the sensor type, the transmitter reverts to the factory trim and any other trim
performed on the transmitter is lost.
4.10.3 Advanced diagnostics
Thermocouple degradation diagnostic
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.
Thermocouple diagnostic must be connected, configured, and enabled to read a thermocouple.
Once the diagnostic has been activated, a Baseline resistance value is calculated. Then a
threshold Trigger must be selected, which can be two, three, or four times the Baseline
resistance, or the default of 5000 ohms. If the thermocouple loop resistance reaches the Trigger
Level, a maintenance alert is generated.
Caution
The Thermocouple Degradation Diagnostic monitors the health of the entire thermocouple
loop, including wiring, terminations, junctions, and the sensor itself. Therefore, it is imperative
that the diagnostic baseline resistance be measured with the sensor fully installed and wired in
the process, and not on the bench.
Note
The thermocouple resistance algorithm does not calculate resistance values while the active
calibrator mode is enabled.
164
Set SENSOR _CAL_LOC_X to the calibration location.
Set MODE_BLK.TARGET_X to AUTO.
Reference Manual
00809-0100-4021, Rev GC
Foundation™ fieldbus configuration