Emerson Bristol 2808 Series, Instruction Manual

Page: 55 / 140

2808B Calibration / 3-1
Section 3
CALIBRATION
3.1 CALIBRATION SETUP
Equipment Required
:
Transmitter calibration requires a laboratory bench setup with the
following equipment:
1.
Test source capable of generating fixed pressure values equivalent to 0%, and 100%
values of transmitter’s range (URL).
2. Pressure monitor device to read test source ( ± .025% accuracy)
3. Electrical supply source capable of producing 24V-DC power to the Transmitter.
4. Digital Multimeter (DMM) with a 5-1/2 digit scale ( ± .005% accuracy)
5. Current Sampling Resistor (250 Ω , ±.01%, 1/4W)
Lab Vs. On-Site Setup : Although it is more convenient and recommended to perform this
procedure using a laboratory setup, calibration can also be performed on site providing that
the connecting line or flange is equipped with a calibration tap and appropriate shutoff and
bypass valves. This added equipment allows you to feed in an external test pressure source
or use the process pressure as a reference signal. In the latter setup, the valves are closed
to seal a fixed pressure in the connecting line. Only fine calibration using the external
adjustments should be attempted in wet, dusty, or hazardous environments.
Before attempting on-site calibration, carefully check the application. If the Transmitter is
operated in a closed control loop configuration, either the Transmitter must be isolated
from the process, or the process must be turned off. If this is not done, a critical process
could accidentally be driven into a dangerous region causing damage to equipment and
property, and injury to persons.
Note : Before starting any test procedures, make sure that the Transmitter is firmly
anchored in its intended operating position. A different mounting position can affect zero
calibration for some ranges and necessitate re-calibration.
Electrical Connections : The electrical connections for a voltage output calibration setup are
made to the Transmitter as shown in Figure 3-1a. However, refer to Figure 3-1b if the
Transmitter is configured for current mode operation. The Current Sampling Resistor will
convert the 4-20mA-output signal to a 1-5V signal, which generally allows for a more
accurate reading. A series milliammeter may also be used as discussed in Section 4.2.
3.2 ACCESS TO ADJUSTMENTS
The external adjustments are for fine offset and fine gain settings. The offset adjustment
screw will not affect the gain. That is, adjusting the offset (A1) screw will move both
calibration points equally. The gain adjustment will also affect the offset of the instrument.
That is, adjusting the gain (A2) screw moves the end point unequally. To minimize the
interaction of the gain, it is best to adjust the offset screw for full scale output while full
scale pressure is applied and adjust the gain screw for a zero output while minimum
pressure is applied.