Emerson Rosemount 644 HART Reference Manual page 52

Configuration
July 2016
1. Configure the dual-sensor Rosemount 644 variable map as shown.
Variable
PV
SV
TV
QV
2. 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 (HART)" on page 15).
3. Select temperature units for Channel 1 that match the differential temperature units of the
transmitter.
4. 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
T 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.
5. 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
Figure 2-25. Tracking Sensor Drift and Sensor Failure with Differential Temperature
42
Mapping
Sensor 1 or Sensor Average
Sensor 2
Differential Temperature
As Desired
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)
Figure 2-25.
Reference Manual
00809-0100-4728, Rev MB
Configuration