Emerson Rosemount 3144P Reference Manual page 184

F Fieldbus Configuration
OUNDATION
December 2019
4. Set SENSOR_CAL_WHO to the person responsible for the calibration.
5. Set SENSOR _CAL_LOC to the calibration location.
6. Set TARGET_MODE to AUTO.
4.11.3 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 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.
Important
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.
184
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.
Reference Manual
00809-0100-4021
Rosemount 3144P