Emerson Rosemount 644 Reference Manual page 54

Section 2: Configuration
April 2015
Using the Tri-Loop to detect sensor drift alert
The dual-sensor 644 transmitter sets a failure flag (through HART) whenever a sensor failure
occurs. If an analog warning is required, the HART Tri-Loop can be configured to produce an
analog signal that can be interpreted by the control system as a sensor failure.
Use these steps to set up the HART Tri-Loop to transmit sensor failure alerts.
1.
2.
3.
4.
5.
Figure 2-24. Tracking Sensor Drift and Sensor Failure with Differential Temperature
44
Configure the dual-sensor 644 variable map as shown.
Variable Mapping
PV Sensor 1 or Sensor average
SV Sensor 2
TV Differential temperature
QV As desired
Configure Channel 1 of the HART Tri-Loop as TV (differential temperature). If either
sensor should fail, the differential temperature output will be +9999 or –9999 (high or
low saturation), depending on the position of the Failure Mode Switch (see
alarm switch" on page
Select temperature units for Channel 1 that match the differential temperature units of
the transmitter.
Specify a range for the TV such as –100 to 100 °C. If the range is large, then a sensor
drift of a few degrees will represent only a small percent of range. If Sensor 1 or Sensor 2
fails, the TV will be +9999 (high saturation) or –9999 (low saturation). In this example,
zero is the midpoint of the TV range. If a ΔT of zero is set as the lower range limit (4 mA),
then the output could saturate low if the reading from Sensor 2 exceeds the reading
from Sensor 1. By placing a zero in the middle of the range, the output will normally
stay near 12 mA, and the problem will be avoided.
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
Sensor Failure
(Failure Mode Switch HIGH)
100 °C
Sensor Drift
3 °C
0 °C
–3 °C
Sensor Drift
–100 °C
Sensor Failure
(Failure Mode Switch LOW)
48).
Reference Manual
00809-0200-4728, Rev RA
"Set the
Figure 2-24.
Configuration