3-15
On-line Operations
Sensor Current
The
Sensor Current
command enables or disables the Active Calibrator
Mode
(1)
feature. The transmitter ordinarily operates with pulsating
current so that open sensor conditions can be safely detected, but some
calibration equipment requires steady current to function properly.
Active Calibrator Mode is only available on single sensor calibrations.
The transmitter automatically defaults to Active Calibrator Mode
whenever performing a sensor trim. When the sensor trim is complete
the transmitter will automatically disable Active Calibrator Mode
whether or not Active Calibrator Mode was disabled prior to the trim.
The Active Calibrator Mode is volatile and will automatically be
disabled when power is cycled or when a Master Reset or Master Test is
performed via HART.
CALIBRATION
Calibrating the transmitter increases the precision of your
measurement system. You may use one or more of a number of trim
functions when calibrating.
To understand the trim functions, it is necessary to understand that
smart transmitters operate differently from analog transmitters. Smart
transmitters are factory-characterized; they are shipped with a
standard sensor curve stored in the transmitter firmware. The
transmitter uses this information to produce a process variable output,
in engineering units, dependent on the sensor input. The trim functions
allow you to make corrections to the factory-stored characterization
curve by digitally altering the transmitter’s interpretation of the sensor
input.
The trim functions should not be confused with the rerange 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.
Deciding Which Trim
Procedure to Use
Sensor Trim
Perform a sensor trim if the unit calibration of the transmitter does not
match your plant standard calibration equipment. The sensor trim
function calibrates the sensor and transmitter together in engineering
units using a site-standard input source. Unless your site-standard
input source is NIST-traceable, the sensor trim function will not
maintain the NIST-traceability of your system.
Transmitter-Sensor Matching
Perform the sensor matching procedure if you wish to enhance the
temperature measurement accuracy of your system, and you have a
sensor with Callendar-Van Dusen constants. Sensors with
Callendar-Van Dusen constants are NIST-traceable when ordered from
Rosemount Inc.
Model 3144
HART Fast Keys
1,2,3,3,3
Model 3244MV
HART Fast Keys
1,2,3,3,5
(1)
Previous versions of the Model 3144 and 3244MV transmitters use the RTD current or
T/C Pulsing Mode commands instead of Active Calibrator Mode where steady current is
desired.
