Rosemount 248, Reference Manual

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Reference Manual
00809-0100-4825, Rev BB
August 2005
Rosemount 248
4-2
CALIBRATION
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 248 may include the following procedures:
• 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.
Trim the Transmitter
One or more of the trim functions may be used when calibrating. The trim
functions are as follows
• Sensor Input Trim
• Output Trim
• Output Scaled Trim
Sensor Input Trim
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 Figure 4-1). The sensor input
calibration trims the combined 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.
Fast Key Sequence 1, 2, 2, 1, 1