Active Calibrator And Emf Compensation; Trim The Analog Output; Analog Output Trim Or Scaled Analog Output Trim; Analog Output Trim - Emerson Rosemount 644 HART Reference Manual

Reference Manual
00809-0100-4728, Rev MB
5.4.2

Active calibrator and EMF compensation

The transmitter operates with a pulsating sensor current to allow EMF compensation and detection of
open sensor conditions. Because some calibration equipment requires a steady sensor current to
function properly, the Active Calibrator Mode feature should be used when an active calibrator is
connected. Enabling this mode temporarily sets the transmitter to provide steady sensor current unless
two sensor inputs are configured.
Disable this mode before putting the transmitter back into the process to set the transmitter back to
pulsating current. Active Calibrator Mode is volatile and will automatically be disabled when a master reset
is performed (through HART) or when the power is cycled.
EMF compensation allows the transmitter to provide sensor measurements that are unaffected by
unwanted voltages, typically due to thermal EMFs in the equipment connected to the transmitter, or by
some types of calibration equipment. If this equipment also requires steady sensor current, the
transmitter must be set to Active Calibrator Mode. However, the steady current does not allow the
transmitter to perform EMF compensation and as a result, a difference in readings between the active
calibrator and actual sensor may exist.
If a reading difference is experienced and is greater than the plant's accuracy specification allows,
perform a sensor trim with Active Calibrator Mode disabled. In this case, an active calibrator capable of
tolerating pulsating sensor current must be used or the actual sensors must be connected to the
transmitter. When the Field Communicator, AMS Device Manager or the LOI asks if an active calibrator is
being used when the sensor trim routine is entered, select No to leave the Active Calibrator Mode
disabled.
5.5

Trim the analog output

5.5.1

Analog output trim or Scaled Analog Output Trim

Perform an Output Trim or a Scaled Output Trim if the digital value for the primary variable matches the
plant's standards but the transmitter's analog output does not match the reading on the output device.
The output trim function calibrates the transmitter to a 4–20 mA reference scale; the scaled output trim
function calibrates to a user-selectable reference scale. To determine the need for an output trim or a
scaled output trim, perform a loop test
Figure 5-4. Measurement Dynamics of a Smart Temperature transmitter
Sensor and Ohm/mV
Trim adjust the signal here
5.5.2

Analog output trim

The Analog Output Trim allows the transmitter's conversion of the input signal to a 4–20 mA output to
be altered
precision. To perform a digital-to-analog trim, perform the following procedure with Traditional Fast Key
sequence:
Operation and Maintenance
Transmitter Electronics Module
Analog-to-Digital Signal
Microprocessor
Conversion
Analog Input
Field
Communicator
(Figure 5-4). Adjust the analog output signal at regular intervals to maintain measurement
("Performing a loop test" on page
Digital-to-Analog
Signal Conversion
Output and Scaled Output Trim
adjust the signal here
HART
Analog
Output
Output
Operation and Maintenance
39).
July 2016
65