Rosemount 248, Reference Manual

Page: 42 / 80

Reference Manual
00809-0100-4825, Rev BB
August 2005
Rosemount 248
4-4
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 4-3).
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 .
Scaled Output Trim
The Scaled D/A Trim command matches the 4 and 20 mA points to a
user-selectable reference scale other than 4 and 20 mA (2–10 volts, for
example). To perform a scaled D/A trim, connect an accurate reference
meter to the transmitter and trim the output signal to scale as outlined in
the ”Output Trim” procedure.
HARDWARE
Maintenance
The Rosemount 248 has no moving parts and requires minimal scheduled
maintenance.
Sensor Checkout
To determine whether the sensor is at fault, replace it with another sensor or
connect a test sensor locally at the transmitter to test remote sensor wiring.
Do not remove the thermowell while in operation
.
Select any standard,
off-the-shelf sensor for use with a Rosemount 248, or consult the factory for a
replacement special sensor and transmitter combination.
Fast Key Sequence 1, 2, 2, 2
Fast Key Sequence 1, 2, 2, 3