45
Reference Manual
00809-0100-4825, Rev CB
Section 4: Operation and Maintenance
March 2014
Operation and Maintenance
To perform a sensor trim with a Rosemount 248:
1.
Connect the calibration device or sensor to the transmitter. Refer to
or inside of the transmitter terminal side cover for sensor wiring diagrams. If
using an active calibrator, please see
“Active calibrator” on page 35
2.
Connect the communicator to the transmitter loop.
3.
From the Home screen, select
1 Device Setup
,
2 Diag/Service
,
2 Calibration
,
1 Sensor 1
Trim
,
1 Sensor 1 Input Trim
to prepare to trim the sensor.
4.
Set the control loop to manual and select OK
.
5.
Answer the Active Calibration question.
6.
Select
1 Lower Only
or
2 Lower and Upper
at the SELECT SENSOR TRIM POINTS prompt.
7.
Adjust the calibration device to the desired trim value (must be within the selected
sensor limits). If a combined sensor and transmitter system are being trimmed, expose
the sensor to a known temperature and allow the temperature reading to stabilize. Use
a bath, furnace or isothermal block, measured with a site-standard thermometer, as the
known temperature source.
8.
Select
OK
when the temperature stabilizes. The communicator displays the output
value the transmitter associates with the input value provided by the calibration device.
9.
Enter the lower or upper trim point, depending on the selection in Step 6.
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 as shown
on
Figure 4-1. Dynamics of Smart Temperature Measurement
Microprocessor
Digital-to-Analog
Signal Conversion
Analog-to-Digital
Signal Conversion
Transmitter Electronics Module
Analog
Input
Analog
Output
Field
Communicator
HART
Output
Sensor and Ohm/mV
Trim adjust the signal here
Output and Scaled Output
Trim adjust the signal here