23
Model 764 Differential Pressure Transmitter
Section 4
4.
All O-rings exposed during the preceding steps must be discarded. New
O-rings installed during reassembly must be identical to those removed.
Consult parts list (Table 5.1,
page 33
) for correct part numbers. O-
rings should be given a light application of silicone grease prior to instal
-
lation. Special care should be given to proper assembly. Take care that
O-rings are not pinched between two surfaces and are properly seated in
the
O-ring groove.
5.
Reinstall the housings and 3/8-in. head bolts. Tighten bolts with 5/16-in.
Allen wrench to a torque value of 40 ft-lb lubed. The head bolts should
be lubricated with Molykote G paste or other similar lubricant.
Periodic Calibration
Transmitter calibration is recommended at periodic intervals, determined
primarily by the usage of the transmitter, historical performance, the desired
accuracy of the output signal, or indications that the instrument may be out of
calibration.
A calibration should consist of at least nine checkpoints from minimum to full
scale calibrated pressure and back to minimum. For optimum results, the test
equipment should meet the requirements noted in Table 4.1. See Table 4.2—
Calibration Checkpoints,
page 24,
for recommended transmitter output
values and the associated tolerances, in current and voltage, for 4-20 mA and
10-50 mA outputs.
Table 4.1—Calibration Equipment
Equipment
Requirements
Digital Multimeter
±0.05% precision (minimum)
Power Supply
15-55 Vdc, 60 mA minimum, regulation 3%, ripple 1%
Precision Load
Resistor (if output is
measured in voltage)
200 Ohm, ±0.05%, 1W (10-50 mA Transmitter)
500 Ohm, ±0.05%, 1W (4-20 mA Transmitter)
Pressure Source
To provide zero to full scale pressure (Accuracy: ±0.05%)
Milliamp Meter
To provide 0-50 mA minimum (Accuracy: ±0.05%)
To check for calibration accuracy, perform the following steps.
1.
Shut down the transmitter as follows.
a.
Remove electrical power from the transmitter loop.
b.
Close the transmitter shut-off valves.
c.
Close the main block valves at the process connections.
d.
Open the transmitter drain valves and remove all pressure from the
unit.
2. Remove the calibration access plugs from the transmitter cover using a
