132 Maintenance and Service 083730300A DCN8101
5.7.14.11 PRESSURE/FLOW SENSOR ASSEMBLY
The pressure/flow sensor PCA, located on the top of the absorption bench, can be
checked with a voltmeter using the following procedure which, assumes that the wiring is
intact, and that the motherboard and the power supplies are operating properly:
FOR PRESSURE RELATED PROBLEMS:
1. Measure the voltage across C1:
• - it should be 5 ± 0.25 VDC
• If not, then the board is bad.
2. Measure the voltage across TP4 and TP1.
• With the sample pump disabled it should be 4500 mV ±250 mV.
• With the pump energized it should be approximately 200 mV less.
• If the voltage is incorrect, then S1, the pressure transducer is bad, the board is bad, or
there is a pneumatic failure preventing the pressure transducer from sensing the
absorption cell pressure properly.
FOR FLOW RELATED PROBLEMS:
1. Measure the voltage across TP2 and TP1 - it should be 10 ±0.25 VDC.
• If not, then the board is bad.
2. Measure the voltage across TP3 and TP1:
• With proper flow (800 sccm at the sample inlet) this should be approximately 4.5V (this
voltage will vary with altitude).
• With flow stopped (sample inlet blocked) the voltage should be approximately 1V.
• If the voltage is incorrect, the flow sensor is bad, the board is bad, or there is a leak
upstream of the sensor.
5.7.15 MOTHERBOARD
5.7.15.1 A/D FUNCTIONS
The simplest method to check the operation of the A-to-D converter on the motherboard
is to use the Signal I/O function under the DIAG menu to check the two A/D reference
voltages and input signals that can be easily measured with a voltmeter.
1. Use the Signal I/O function (see Section 5.7.4 and Appendix A) to view the value of
REF_4096_MV and REF_GND.
• If both are within 3 mV of nominal (4096 and 0), and are stable, ±0.5 mV then the
basic A/D is functioning properly. If not then the motherboard is bad.
2. Choose a parameter in the Signal I/O function such as SAMPLE_PRESSURE,
SAMPLE_FLOW, CO_MEASURE or CO_REFERENCE.
• Compare these voltages at their origin (see interconnect drawing, P/N 04215 and
interconnect list, P/N 04216) with the voltage displayed through the signal I/O
function.
• If the wiring is intact but there is a large difference between the measured and
displayed voltage (±10 mV) then the motherboard is bad.
See also Section 5.7.4.
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