2-19
Installation
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 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 2-16.
Figure 2-16. Tracking Sensor Drift and
Sensor Failure with Differential
Temperature.
3 °C
0 °C
–3 °C
100 °C
Sensor Drift
Sensor Drift
Sensor Failure
(Failure Mode Jumper HI)
DI
FF
ERENTI
AL
T
E
MPERATURE
Sensor Failure
(Failure Mode Jumper LO)
–100 °C
