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Reference Manual
00809-0100-4021, Rev DA
November 2004
Rosemount 3144P
3-20
HART
Intermittent Threshold
The threshold value can be changed from the default value of 0.2%. Turning
the Intermittent Sensor Detect feature OFF or leaving it ON and increasing
the threshold value above the default does not affect the time needed for the
transmitter to output the correct alarm signal after detecting a true open
sensor condition. However, the transmitter may briefly output a false
temperature reading for up to one update in either direction (see Figure 3-4 on
page 3-24) up to the threshold value (100% of sensor limits if Intermittent
Sensor Detect is OFF). Unless rapid response rate is necessary, the
suggested setting of the Intermittent Sensor Detect mechanism is ON with
0.2% threshold.
Figure 3-3. Open
Sensor Response
Intermittent Sensor Detect (Advanced Feature)
The Intermittent Sensor Detect feature is designed to guard against process
temperature readings caused by intermittent open sensor conditions (an
intermittent sensor condition is an open sensor condition that lasts less than
one update). By default, the transmitter is shipped with the Intermittent Sensor
Detect feature switched ON and the threshold value set at 0.2% of sensor
limits. The Intermittent Sensor Detect feature can be switched ON or OFF and
the threshold value can be changed to any value between 0 and 100% of the
sensor limits with a Field Communicator.
Transmitter Behavior with Intermittent Sensor Detect ON
When the Intermittent Sensor Detect feature is switched ON, the transmitter
can eliminate the output pulse caused by intermittent open sensor conditions.
Process temperature changes (∆T) within the threshold value will be tracked
normally by the transmitter’s output. A ∆T greater than the threshold value will
activate the intermittent sensor algorithm. True open sensor conditions will
cause the transmitter to go into alarm.
The threshold value of the 3144P should be set at a level that allows the
normal range of process temperature fluctuations; too high and the algorithm
will not be able to filter out intermittent conditions; too low and the algorithm
will be activated unnecessarily. The default threshold value is 0.2% of the
sensor limits.
HART Fast Keys 1, 3, 6, 3
0
0
5
10
15
20
25
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
Transmitter Output (mA)
Time (seconds)
When Intermittent Sensor Detect is
OFF, a false temperature output is
possible when an open sensor
condition is detected. A false
temperature output in either direction
up to the threshold value (100 % of
sensor limits if Intermittent Sensor
Detect is OFF) is possible when an
open sensor condition is detected.
Normal open sensor responses
High Alarm
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