Emerson Rosemount 644h, Reference Manual

Page: 42 / 78

Reference Manual
00809-0100-4728, Rev HA
December 2002
Models 644H and 644R
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 Model 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.
Trim the Transmitter
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
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 reference
command trims, in engineering (F, °C, °R, K) or raw (
Ω , mV) units, 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.
Use the following procedure to perform a sensor trim with a Model 644H.
1. Connect the calibration device or sensor to the transmitter. Refer to
Figure 2-8 on page 2-10 or inside of the transmitter terminal side
cover for sensor wiring diagrams. (If using an active calibrator, please
see “Active Calibrator” on page 3-10)
2. Connect the communicator to the transmitter loop.
3. From the Home screen, select 1 Device Setup , 2 Diag/Service ,
2 Calibration , 1 Sensor 1 Trim , 1 Sensor 1 Input Trim to prepare to
trim the sensor.
Fast Key Sequence 1, 2, 2, 1, 1