Spectrex Series 300 Open-Path Gas Detection System - Manual (Page 11)

Spectrex Inc. - SafEye TM 300 Gas Detector Manual – TM 794100, Rev. D, May 2004
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2.3.1 Definitions of Terms
The following list defines gas concentrations measurement terms that are used in
this manual:
LEL Lower Explosive Limit - The minimum concentration of a substance
(gas/vapor) in air mixture that can be ignited. This mixture is
different for every gas/vapor, measured in % of LEL.
LEL.m Integral of Concentration in LEL units (1 LEL = 100% LEL) and the
operation distance, or path length, in meters (m).
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.