Emerson Rosemount 644, Reference Manual

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69
Reference Manual
00809-0200-4728, Rev RA
Section 5: Operation and Maintenance
April 2015
Operation and Maintenance
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.
Figure 5-1. Trim
Application : Linear Offset Application : Linear Offset and
Slope Correction
Solution : Single-Point Trim Solution : Two-Point Trim
Method :
1. Connect sensor to transmitter. Place
sensor in bath between range points.
2. Enter known bath temperature using the
Field Communicator.
Method :
1. Connect sensor to transmitter. Place
sensor in bath at low range point.
2. Enter known bath temperature using
the Field Communicator.
3. Repeat at high range point.
One-Point Trim Two-Point Trim
Transmitter System Curve
Site-Standard Curve
R
e
sist
anc
e
(ohms)
Temperature
R
e
sist
anc
e
(ohms)
Temperature