Emerson Rosemount 248 Reference Manual page 46

Section 3: Configuration
March 2014
Intermittent threshold
Fast Key sequence
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
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 3-2. Open Sensor Response
Open sensor holdoff
Fast Key sequence
The Open Sensor Holdoff option, at the normal setting, enables the Rosemount 248 to tolerate
heavy EMI disturbances without producing brief periods of alarm. This is accomplished through
the software by having the transmitter perform additional verification of the open sensor status
prior to activating the transmitter alarm. If the additional verification shows that the open
sensor condition is not valid, the transmitter will not go into alarm.
For users of the Rosemount 248 that desire a more immediate open sensor detection, the Open
Sensor Holdoff option can be changed to a fast setting. On this setting, the transmitter reports
an open sensor condition without additional verification of the open condition.
38
1, 3, 5, 4
25
20
15
10
5
0
0 0.2 0.4
1, 3, 5, 3
High alarm
Normal open sensor responses
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.
0.8 1.0
0.6
Time (seconds)
Reference Manual
00809-0100-4825, Rev CB
Figure 3-3 on page 39) up to
1.4
1.2
Configuration