Reference Manual
00809-0100-4360, Rev BA
August 2008
Rosemount 1151
4-4
Calibrate the Sensor Digital Trim (Sensor Trim and Analog Output Trim)
In order to understand the digital trim function, it is necessary to understand
that smart transmitters operate differently from conventional analog
transmitters. Smart transmitters are characterized, which involves comparing
a pressure input with the output of each transmitter’s sensor module. The
information obtained in the comparison is stored in the sensor module
EEPROM during the characterization process. In operation, the transmitter
uses this information to produce a process variable output, in engineering
units, dependent on the pressure input. The digital trim function allows
corrections to be made to this factory-stored curve.
The digital trim procedure is a two-step process. The first step, called sensor
trim, consists of matching the digital process variable reading of the
transmitter to a precision pressure input. The second step, called
4–20 mA trim, consists of adjusting the output electronics.
This procedure should not be confused with reranging. Although you can still
match a pressure input to a 4 or 20 mA output through a reranging function,
you have not affected the transmitter’s interpretation of that input. A sensor
trim allows you to alter the transmitter’s interpretation of the input signal. A
4-20 mA analog output trim allows you to alter the transmitter’s conversion of
that interpretation into an analog 4–20 mA output.
The transmitter can only be as accurate as the equipment used to perform the
digital trim. Use precise equipment under stable, ambient conditions for best
results. If such equipment is not available, it may be better to return the
transmitter to a local Rosemount service center for verification of the trim
values. To eliminate the possibility of over-trimming the transmitter, the
Rosemount 1151 Smart will accept only trim values that are within 5 percent
of its original characterization.
Sensor Trim
The sensor may be trimmed in two ways: sensor trim and zero trim. They vary
in complexity, and their use is application-dependent.
Table 4-1. Recommended Calibration Tasks.
Transmitter Bench Calibration Tasks Field Calibration Tasks
Standard
Calibration
Tasks
1151DP
1151GP
1151HP
1151LT
1. Set output configuration parameters:
a) Set the Transmitter Range Points.
b) Set the Output Units.
c) Set the Output Type (linear or square root).
d) Set the Damping Value.
2. Optional: Perform a Full Sensor Trim
—Pressure source required.
3. Optional: Perform an Analog Output Trim
—Multimeter required.
1) Reconfigure parameters if necessary.
2) Zero trim the transmitter to compensate
for mounting position effects or static
pressure effects.
1151AP
Standard Bench Calibration,
except step 2:
2. Optional: Perform a Full Sensor Trim if
equipment is available (accurate absolute
pressure source required); otherwise,
perform the Low Trim Value section of
Full Sensor Trim procedure.
1) Reconfigure parameters if necessary.
2) Perform Low Trim Value section of
Full Sensor Trim procedure to correct for
mounting position effects.
NOTE
A HART-based communicator is required for all sensor and output trim procedures.
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