Teledyne GFC-7000E - Trace CO2 Analyzer - Manual (Page 190)

Model GFC7000E Instruction Manual THEORY OF OPERATION
04584 Rev A1 179
DOS Shell
AI FIRMWARE
Analyzer Operations
Calibration Procedures
Configuration Procedures
Autonomic Systems
Diagnostic Routines
Memory Handling
IDAS Records
Calibration Data
System Status Data
Interface Handling
Sensor input Data
Display Messages
Keypad
Analog Output Data
RS232 & RS485
External Digital I/O
Measurement
Algorithm
ANALYZER
HARDWARE
PC/104 BUS
PC/104 BUS
Linearization Table
Figure 10-19: Basic Software Operation
10.5.1. Adaptive Filter
The MGFC7000E software processes the CO2 MEAS and CO2 REF signals, after they are
digitized by the motherboard, through an adaptive filter built into the software. Unlike other
analyzers that average the output signal over a fixed time period, the MGFC7000E averages over
a set number of samples, where each sample is 0.2 seconds. This is technique is known as
boxcar averaging. During operation, the software automatically switches between two different
length filters based on the conditions at hand. Once triggered, the short filter remains engaged
for a fixed time period to prevent chattering.
During conditions of constant or nearly constant concentration the software, by default, computes
an average of the last 750 samples, or approximately 150 seconds. This provides the calculation
portion of the software with smooth stable readings. If a rapid change in concentration is detected
the filter includes, by default, the last 48 samples, approximately 10 seconds of data, to allow the
analyzer to more quickly respond. If necessary, these boxcar lengths can be changed between 1
and 1000 samples but with corresponding tradeoffs in rise time and signal-to-noise ratio (contact
customer service for more information).
Two conditions must be simultaneously met to switch to the short filter. First the instantaneous
concentration must exceed the average in the long filter by a fixed amount. Second the
instantaneous concentration must exceed the average in the long filter by a portion, or
percentage, of the average in the long filter.
10.5.2. Calibration - Slope and Offset
Calibration of the analyzer is performed exclusively in software.