Emerson Rosemount 644 Reference Manual page 53

Reference Manual
00809-0200-4728
same as the units of the HART Tri-Loop. Use any combination of RTDs or thermocouples as
long as the units of both match the units of the HART Tri-Loop.
Using the Tri-Loop to detect sensor drift alert
The dual-sensor 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.
Procedure
1. Configure the dual-sensor transmitter variable map as shown:
2. Configure Channel 1 of the HART Tri-Loop as TV (differential temperature). If either
3. Select temperature units for Channel 1 that match the differential temperature
4. Specify a range for the TV such as -148 to 212 °F (-100 to 100 °C). If the range is
5. Configure the DCS so that TV < –148 °F (–100 °C) or TV > 212 °F (100 °C) indicates a
Emerson.com/Rosemount
Variable
PV
SV
TV
QV
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
Setting the alarm switch).
units of the transmitter.
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.
sensor failure and, for example, TV ≤ 26.6 °F (–3 °C) or TV ≥ 37.4 °F (3 °C) indicates a
drift alert. See Figure 2-26.
Mapping
Sensor 1 or sensor average
Sensor 2
Differential temperature
As desired
Configuration
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