Teledyne gfc 7000ta operation manual Sensor Inputs, Thermistor Interface

Theory of Operation Model GFC7000TA Carbon Dioxide Analyzer

The A/D can be configured for several different input modes and ranges but in the GFC 7000TA is used in uni-polar mode with a +5 V full scale. The converter includes a 1% over and under-range. This allows signals from –0.05 V to +5.05 V to be fully converted.

For calibration purposes, two reference voltages are supplied to the A/D converter: Reference Ground and +4.096 VDC. During calibration, the device measures these two voltages, outputs their digital equivalent to the CPU. The CPU uses these values to compute the converter’s offset and slope and uses these factors for subsequent conversions.

See Section 6.13.4 for instructions on performing this calibration.

9.3.6.2. Sensor Inputs

The key analog sensor signals are coupled to the A/D through the master multiplexer from two connectors on the motherboard. 100K terminating resistors on each of the inputs prevent cross talk from appearing on the sensor signals.

CO2 Measure and Reference

These are the primary signals that are used in the computation of the CO2 concentration. They are the demodulated IR-sensor signals from the sync demodulator board.

Sample Pressure and Flow

These are analog signals from two sensors that measure the pressure and flow rate of the gas stream at the outlet of the sample chamber. This information is used in two ways. First, the sample pressure is used by the CPU to calculate CO2 Concentration. Second, the pressure and flow rate are monitored as a test function to assist the user in predicting and troubleshooting failures.

9.3.6.3. Thermistor Interface

This circuit provides excitation, termination and signal selection for several negative-coefficient, thermistor temperature sensors located inside the analyzer. They are:

Sample Temperature Sensor

The source of this signal is a thermistor located inside the sample chamber of the Optical Bench. It measures the temperature of the sample gas in the chamber. This data is used to during the calculation of the CO2 concentration value.

Bench Temperature Sensor

This thermistor, attached to the sample chamber housing, reports the current temperature of the chamber housing to the CPU as part of the bench heater control loop.

Wheel Temperature Sensor

This thermistor (attached to the heat-sync on the GFC wheel motor assembly) reports the current temperature of the wheel/motor assembly to the CPU as part of the Wheel Heater control loop.

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