Emerson Rosemount, Reference Manual

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121
Reference Manual
00809-0100-4021, Rev GC
Section 3: HART commissioning
April 2014
HART® commissioning
3.14 Calibration
Calibrating the transmitter increases the precision of the measurement system. The user may
use one or more of a number of trim functions when calibrating. To understand the trim
functions, it is necessary to realize that HART transmitters operate differently from analog
transmitters. An important difference is that smart transmitters are factory-characterized; 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, dependent on the sensor
input. The trim functions allow the user to make adjustments to the factory-stored characteriza-
tion curve by digitally altering the transmitter’s interpretation of the sensor input.
Calibration of the 3144P may include:

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
3.14.1 Calibration frequency
Calibration frequency can vary greatly depending on the application, performance
requirements, and process conditions. Use the following procedure to determine calibration
frequency that meets the needs of your application.
1. Determine the Required Performance
2. Calculate Total Probable Error
a. Digital Accuracy = °C
b. D/A Accuracy = (% of transmitter span) x (ambient temperature change) °C
c. Digital Temp Effects = (°C per 1.0 °C change in ambient temperature) x (ambient
temperature change)
d. D/A Effects = (% of span per 1.0 °C) x (ambient temperature change) x (Process
temperature range)
e. Sensor Accuracy = °C
f.
3. Calculate Stability per Month

(% per months) x (Process temperature range)
4. Calculate Calibration Frequency

TPE DigitalAccuracy
 
2
D A
  
2
DigitalTempEffects
 
2
D A

Effects
 
2
SensorAccuracy
 
2
+ + + + =
CalFreq
RequiredPerformance TPE –
 
StabilityPerMonth
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=