Principle of Operation
14
SafEye
TM
Open Path Detection System User Guide
•
LNG-LPG systems
•
Offshore Floating Production Storage and Shipping vessels (FPSO) and
fixed oil rigs
2.3
Principle of Operation
The SafEye system detects gases through a dual spectral range
monitoring, analyzing the absorption of radiation caused by gases in the
atmosphere, and comparing it (ratio) to background atmospheric
absorption.
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
“fingerprint.” There are
3 IR
sensors: 2 signals and 1 reference. The detection process involves 2
separate filters: 1 transmitting radiation that is absorbed by a particular
gas and 1 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.
