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5-7
Flue Gas Analysis System Maintenance & Troubleshooting 5
TELEDYNE BROWN ENGINEERING
Analytical Instruments
original detector is probably defective and requires replacement. If
problems still occur in the detector circuit, then the dummy sensor
will give a signal that can be tracked through the system.
• Measure the voltage at the tiepoint of R7, R8 and R10 with
respect to terminal 14 which is the center of the sensor bridge.
With the zero potentiometer adjusted up and down, the voltage
should change by about
1
/4 volt. If this does not occur, then
there is probably something wrong in the Wheatstone Bridge
circuitry, i. e., the resistors that constitute the bridge along with
P1, the zero potentiometer.
• Measure the voltage at the output of A1 (A1-1). (The easiest
place to check the output voltage of A is at the tiepoint of R15,
R30 and C11.) Use the tiepoint of C7 and C8 (oxygen
analyzer) for power supply common. The signal should be
tracked through the system. With a voltmeter attached to A1–1
move the zero potentiometer up and down. Pin 1 of A1 should
reflect this voltage movement by a couple of volts.
• With the zero potentiometer set so there is a voltage at A1-1,
follow the signal through the circuit, i.e. through to A1-7 and
see if the meter deflection is appropriate to the voltage at A1-7.
The meter should indicate full scale deflection with 1 V at
A1–7. If it does not, replacement of A1 is indicated.
• The preceding discussion concerning signal checkout assumes
that the power supplies have been verified and are operating
properly (see Section 5.2.1.). The regulated power supply
should be checked at C8 and C7 for +/- 15 VDC.
• The output of the current supply for the detector can be checked
at the two test points (located on each side of R6). The voltage
should be 0.3 V, which indicates that 300 mA is flowing
through the detector.
5.2.3.2 No Alarm With High Gas Level
If there is trouble with the alarm comparator, A3 or transistor
Q1, there will be no alarm indication even though the gas level goes
above the setpoint. Also, there will be no alarm if the relay coil K2
fails, although relay coil failure is not likely.