Spectrex Xenon 700 SIL Open-Path Gas Detection System - Manual (Page 17)

TM 799200 Rev (C), January 2014
Principle of Operation 5
2.3.2 Spectral Finger Print
Each hazardous material is detected at a specific wavelength selected
according to its specific spectral absorption or "finger print". There are three
IR sensors: two signals and one reference. The detection process involves
two separate filters, one transmitting radiation that is absorbed by a
particular gas and one that is not sensitive to it.
2.3.3 Optical Path
The presence of hazardous airborne vapors, gases, or aerosols in a
monitored area is detected when the defined substance crosses/enters the
optical path between the radiation source unit and the detector.
Hazardous gases /vapors present in the atmosphere cause absorption of the
radiation pulse in the optical path between the radiating source and the
detector unit at some specific wavelengths. This causes a change in the
signal intensity received by the detector, which is translated into an output
related to the detector’s measuring scale.
The system analyzes the defined open path at the spectral bands specific to
the materials being monitored. The Automatic Gain Control (AGC) unit
compensates for environmental disturbances such as fog, rain, etc., through
a constant comparison with its dual spectral beam.
2.3.4 Microprocessor Based
The incoming signals are analyzed by the built in microprocessor.
A sophisticated mathematical algorithm calculates between the various
functions of the detected signal thresholds. Statistics, ratio algorithms, data
communications, diagnostics and other functions are performed.
2.3.5 Gas Sensitivity
The SafEye IR Model uses wavelengths around 3.4µ spectral band to
measure air flammability potential between the source and detector. At this
wavelength, all hydrocarbon materials have a strong absorption peak. This
peak enables the detector to achieve both regular sensitivity of 0-5 LEL.m or
high sensitivity of 0-2 LEL.m according to functions set up. However, since
the desired detection information is air flammability and the actual
measurement is radiation absorption around the 3.4μ spectral band, the
detector has a different sensitivity to different gases or gas combinations.
This difference in sensitivity is irrelevant if in the protected area the gas
composition is known. However, if in a protected area the gas composition
can vary substantially, then the difference in sensitivity should be
considered in determining the Detector calibration.