2-8
EN
CALIBRATION
Calibrating the transmitter increases the precision of your measurement system.
You may use one or more of a number of trim functions when calibrating.
To understand the trim functions, it is necessary to understand that smart
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, in engineering units,
dependent on the sensor input. The trim functions allow you to make corrections to
the factory-stored characterization curve by digitally altering the transmitter’s
interpretation of the sensor input.
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.
Calibration Overview
Complete calibration of the Model PX2088 Smart Pressure Transmitter involves one or
more of the following tasks:
Configure the Analog Output Parameters
n
Set Process Variable Units (Page 2-3)
n
Rerange (Page 2-5)
n
Set Output Type (Page 2-4)
n
Set Damping (Page 2-6)
Calibrate the Sensor
n
Full Trim (Page 2-10)
n
Zero Trim (Page 2-10)
Calibrate the 4–20 mA Output
n
Digital to Analog Trim (Page 2-11) or
n
Scaled Digital to Analog Trim (Page 2-12)
Figure 2-3 illustrates the Model PX2088 Smart transmitter data flow. This data flow
can be summarized in four major steps:
1. A change in pressure is measured by a change in the sensor output (Sensor
Signal).
2. The sensor signal is converted to a digital format that can be understood by
the microprocessor (Analog-to-Digital Signal Conversion).
3. Corrections are performed in the microprocessor to obtain a digital
representation of the process input (Digital PV).
4. The Digital PV is converted to an analog value
(Digital-to-Analog Signal Conversion).
4690.bk : 4690.s2 Page 8 Tuesday, May 26, 1998 5:28 PM
