Emerson Rosemount 644h, Reference Manual

Page: 45 / 78

Reference Manual
00809-0100-4728, Rev HA
December 2002
4-5
Models 644H and 644R
Output Trim or Scaled 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 (see “Loop Test” on page 3-9).
Figure 4-1. Dynamics of Smart
Temperature Measurement
Output Trim
The D/A Trim command allows the transmitter’s conversion of the input
signal to a 4–20 mA output to be altered (see Figure 4-1 on page 4-5).
Adjust the analog output signal at regular intervals to maintain
measurement precision. To perform a digital-to-analog trim, perform the
following procedure:
1. From the
HOME
screen, select 1 Device setup , 2 Diag/Service ,
2 Calibration , 2 D/A trim . Set the control loop to manual and select
OK
.
2. Connect an accurate reference meter to the transmitter at the
CONNECT REFERENCE METER
prompt. To do so, shunt the power
to the transmitter through the reference meter at some point in the
loop. Select
OK
after connecting the reference meter.
3. Select
OK
at the
SETTING FLD DEV OUTPUT TO 4 MA
prompt. The
transmitter outputs 4.00 mA.
4. Record the actual value from the reference meter, and enter it at the
ENTER METER VALUE
prompt. The communicator prompts the user
to verify whether or not the output value equals the value on the
reference meter.
5. If the reference meter value equals the transmitter output value, then
select
1 Yes and go to step 6. If the reference meter value does not
equal the transmitter output value, then select 2 No and go to step 4.
6. Select
OK
at the
SETTING FLD DEV OUTPUT TO 20 MA
prompt
and repeat steps 4 and 5 until the reference meter value equals the
transmitter output value.
7. Return the control loop to automatic control and select
OK
.
Fast Key Sequence 1, 2, 2, 2
Microprocessor
Digital-to-Analog
Signal Conversion
Analog-to-Digital
Signal Conversion
Transmitter Electronics Module
Analog
Input
Analog
Output
HART
Communicator
HART
Output
Sensor and Ohm/mV
Trim adjust the signal here
Output and Scaled Output
Trim adjust the signal here