95
Reference Manual
00809-0100-4021, Rev JA
HART Commissioning
April 2017
HART Commissioning
3.11.4
Intermittent threshold
The threshold value can be changed from the default value of 0.2 percent. 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
) up to the threshold value (100 percent of sensor
limits if Intermittent Sensor Detect is OFF). Unless a rapid response rate is necessary, the suggested
setting is ON
wi
th 0.2 percent threshold
.
Figure 3-9. Open Sensor Response
A. Normal open sensor responses.
B. 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 percent of sensor limits if Intermittent Sensor Detect
is OFF) is possible when an open sensor condition is detected.
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 percent 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 percent 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 (
Δ
T) within the
threshold value will be tracked normally by the transmitter’s output. A
Δ
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 transmitter 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
percent of the sensor limits.
HART 5 Fast Keys
2, 2, 7, 5
HART 7 Fast Keys
2, 2, 7, 5
A
B
High alarm
25
15
20
10
5
0
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Tr
ansmi
tte
r output (mA)
Time (seconds)
Summary of Contents for Rosemount 3144P
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