Emerson Rosemount 644 HART, Reference Manual

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Reference Manual
00809-0100-4728, Rev MB
Operation and Maintenance
July 2016
Operation and Maintenance
5.3 Calibration overview
Calibrating the transmitter increases the measurement precision by allowing corrections to be made to
the factory-stored characterization curve by digitally altering the transmitter’s interpretation of the
sensor input.
To understand calibration, it is necessary to understand that smart transmitters operate differently from
analog transmitters. An important difference is that smart transmitters are factory-characterized,
meaning that they are shipped with a standard sensor curve stored in the transmitter firmware. In
operation, the transmitter uses this information to produce a process variable output, in engineering
units, dependent on the sensor input.
Calibration of the Rosemount 644 may include the following procedures:

Sensor Input Trim : digitally alter the transmitter’s interpretation of the input signal

Transmitter Sensor Matching : generates a special custom curve to match that specific sensor curve,
as derived from the Callendar-Van Dusen constants

Output Trim : calibrates the transmitter to a 4–20 mA reference scale

Scaled Output Trim : calibrates the transmitter to a user-selectable reference scale.
5.3.1 Trimming
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.
One or more of the trim functions may be used when calibrating. The trim functions are as follows:

Sensor Input Trim

Transmitter-Sensor Matching

Output Trim

Output Scaled Trim
5.4 Sensor input trim
The Sensor Trim command allows for alteration of the transmitter’s interpretation of the input signal. The
sensor trim command trims, in engineering (°F, °C, °R, K) or raw (ohms, mV) units, the combined sensor
and transmitter system to a site standard using a known temperature source. Sensor trim is suitable for
validation procedures or for applications that require profiling the sensor and transmitter together.
Perform a sensor trim if the transmitter’s digital value for the primary variable does not match the plant’s
standard calibration equipment. The sensor trim function calibrates the sensor to the transmitter in
temperature units or raw units. Unless the site-standard input source is NIST-traceable, the trim
functions will not maintain the NIST-traceability of your system.