Calibration; Trim The Transmitter - Emerson Rosemount 248 Reference Manual

Section 4: Operation and Maintenance
March 2014
4.2

Calibration

Calibrating the transmitter increases 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 realize that smart transmitters operate differently
from analog transmitters. An important difference being that smart transmitters are fac-
tory-characterized, meaning 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 248 may include the following procedures:
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4.2.1

Trim the transmitter

One or more of the trim functions may be used when calibrating. The trim functions are as
follows
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Sensor input trim
Fast Key sequence
Perform a sensor trim if the transmitters 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 your site-standard input source is
NIST-traceable, the trim functions will not maintain the NIST-traceability of the system.
The Sensor Input Trim command allows the transmitter's interpretation of the input signal to be
digitally altered (see
sensor and transmitter system to a site standard using a known temperature source. Sensor
trimming is suitable for validation procedures or for applications that require calibrating the
sensor and transmitter together.
44
Sensor Input Trim: digitally alter the transmitter's interpretation of the input signal
Output Trim: calibrates the transmitter to a 4–20 mA reference scale
Scaled Output Trim: calibrates the transmitter to a user-selectable reference scale.
Sensor Input Trim
Output Trim
Output Scaled Trim
1, 2, 2, 1, 1
<HotXRef>Figure 4-1). The sensor input calibration trims the combined
Reference Manual
00809-0100-4825, Rev CB
Operation and Maintenance