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Instruction Manual
245364-V
May 2002
6-2 Service and Maintenance Rosemount Analytical Inc. A Division of Emerson Process Management
Model 755A
6-2 DETECTOR COMPONENT CHECKS
a. Detector
Before concluding that the detector is
defective and must be replaced, verify
that all components and circuits that could
cause erratic readings are operating
properly.
To isolate the detector as a source of a
problem, the source lamp, photocells and
suspension should be checked for proper
operation.
b. Source Lamp
To verify that the source lamp in operating
properly:
1. Verify that lamp is lit.
2. Voltage at U7 pin 2 should be 2.2±0.2
VDC.
If lamp is not operating properly, replace
per instructions in Section 6-4 on page 6-
5.
c. Photocell
To verify that photocell is operating
properly, perform the following steps:
1. Keeping power source ON,
disconnect the leads of the photocell
from connector J12. See Figure 6-1
on page 6-4.
2. Note the current measurement
between the gray and orange wires
(between 300 to 450 mA).
3. Measure between the orange and red
wires. The reading should be
approximately the same as step 2.
If photocell readings not correct, replace
photocell per Section 1-1a on page 6-7.
d. Suspension
If the suspension has been damaged, the
cause may by improper operating
conditions.
Maximum permissible operating pressure
for the detector is 10 psig (69 kPa gauge
pressure). To ensure against over-
pressurization, a pressure relief valve
may be inserted into the sample inlet line.
In addition, a check valve should be
inserted into the vent line, if connected to
a manifold associated with a flare or other
outlet that is not at atmospheric pressure.
If the detector is over-pressurized, the
suspension could break.
To verify correct operation:
1. Turn electrical power to analyzer
OFF.
2. Remove optical bench assembly (see
Section 6-4 on page 6-5, steps 1
through 4.)
3. With 100% nitrogen flowing through
the analyzer, note the position of the
suspension.
4. Admit air and note response of
suspension. It should rotate
clockwise (as viewed from the top)
and to the right (as viewed through
the window).
Failure to rotate indicates that the
suspension has been damaged and that
the detector assembly must be replaced.
See Section 6-3a on page 6-3.