x Table of Contents 083730300B DCN8145
5.7.12.5 IR Photo-Detector TEC Temperature ..............................................................................132
5.7.13 Excessive Noise .............................................................................................................................133
5.7.14 Subsystem Checkout .....................................................................................................................133
5.7.14.1 AC Mains Configuration...................................................................................................133
5.7.14.2 DC Power Supply ............................................................................................................134
5.7.14.3 I
2
C Bus .............................................................................................................................135
5.7.14.4 Touchscreen Interface .....................................................................................................135
5.7.14.5 LCD Display Module ........................................................................................................135
5.7.14.6 Relay Board .....................................................................................................................135
5.7.14.7 Sync/Demodulator Assembly ..........................................................................................136
5.7.14.8 Opto Pickup Assembly ....................................................................................................136
5.7.14.9 GFC Wheel Drive ............................................................................................................136
5.7.14.10 IR Source ....................................................................................................................137
5.7.14.11 Pressure/Flow Sensor Assembly ................................................................................137
5.7.15 Motherboard ...................................................................................................................................138
5.7.15.1 A/D Functions ..................................................................................................................138
5.7.15.2 A/D Functions in the T300U ............................................................................................138
5.7.15.3 Status Outputs .................................................................................................................139
5.7.15.4 Control Inputs – Remote Zero, Span ...............................................................................139
5.7.16 CPU ................................................................................................................................................139
5.7.17 RS-232 Communications ...............................................................................................................140
5.7.17.1 General RS-232 Troubleshooting ....................................................................................140
5.7.18 The Optional CO
2
Sensor ..............................................................................................................140
5.8 Repair Procedures .....................................................................................................................................141
5.8.1 Repairing Sample Flow Control Assembly .....................................................................................141
5.8.2 Removing/Replacing the GFC Wheel ............................................................................................142
5.8.3 Checking and Adjusting the Sync/Demodulator, Circuit Gain (CO MEAS) ..................................144
5.8.3.1 Checking the Sync/Demodulator Circuit Gain .................................................................144
5.8.3.2 Adjusting the Sync/Demodulator, Circuit Gain ................................................................145
5.8.4 Disk-On-Module Replacement .......................................................................................................146
5.9 Frequently Asked Questions .....................................................................................................................147
5.10 Technical Assistance .....................................................................................................................148
6. Theory of Operation ................................................................................................................ 149
6.1 Measurement Method ................................................................................................................................149
6.1.1 Beer’s Law......................................................................................................................................150
6.2 Measurement Fundamentals .....................................................................................................................150
6.1.2 Gas Filter Correlation .....................................................................................................................151
6.1.2.1 The GFC Wheel ...............................................................................................................151
6.1.2.2 The Measure Reference Ratio ........................................................................................152
6.1.2.3 Interference and Signal to Noise Rejection .....................................................................153
6.1.2.4 Summary Interference Rejection .....................................................................................154
6.1.2.5 Auto-Reference (A-Ref) Ratio (T300U) ...........................................................................154
6.3 Pneumatic Operation .................................................................................................................................156
6.3.1 Flow Rate Control ..........................................................................................................................157
6.3.1.1 Critical Flow Orifice ..........................................................................................................157
6.3.2 Particulate Filter .............................................................................................................................158
6.3.3 Pneumatic Sensors ........................................................................................................................158
6.3.3.1 Sample Pressure Sensor.................................................................................................158
6.3.3.2 Sample Flow Sensor .......................................................................................................159
6.3.4 The T300U A-REF Carbon Monoxide Scrubber ............................................................................159
6.3.5 The T300U A-REF Valve Assembly ...............................................................................................159
6.3.6 The T300U Nafion
®
Dryer ..............................................................................................................159
6.4 Electronic Operation ..................................................................................................................................161