Reference Manual
00809-0100-4728, Rev HA
December 2002
Models 644H and 644R
3-12
Intermittent Threshold
The threshold value can be changed from the default value of 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 3-13) 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 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 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 HART 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 Model 644 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 2% of the
sensor limits.
Fast Key Sequence
1, 3, 5, 4
0
0
5
10
15
20
25
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Tr
ans
m
itt
er Ou
tp
ut
(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
644-
644_
03
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