Emerson Rosemount 248 Reference Manual page 45

Reference Manual
00809-0100-4825, Rev CB
PV range values
Fast Key sequence
The PV URV and PV LRV commands, found on the PV Range Values menu screen, allow the user to
set the transmitter's lower and upper range values using limits of expected readings. The range
of expected readings is defined by the Lower Range Value (LRV) and Upper Range Value (URV).
The transmitter range values can be reset as often as necessary to reflect changing process
conditions. From the PV Range Values screen select 1 PV LRV to change the lower range value and
2 PV URV to change the upper range value.
Note:
The rerange functions should not be confused with the trim functions. Although the rerange
command matches a sensor input to a 4–20 mA output, as in conventional calibration, it does
not affect the transmitter's interpretation of the input.
Intermittent sensor detect (advanced feature)
The Intermittent Sensor Detect feature guards 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 to 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 are tracked normally by the transmitter's output. A T greater than
the threshold value activates the intermittent sensor algorithm. True open sensor conditions
cause the transmitter to go into alarm.
The threshold value of the Rosemount 248 should be set at a level allowing for 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.
Transmitter behavior with intermittent sensor detect OFF
When the Intermittent Sensor Detect feature is switched OFF, the transmitter tracks all process
temperature changes, even if they are the result of an intermittent sensor. (The transmitter
behaves as though the threshold value had been set at 100%.) The output delay because of the
intermittent sensor algorithm will be eliminated.
Configuration
PV URV = 6
PV LRV = 7
Section 3: Configuration
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