This type of detectors shall operate on the principle of absorption of infra red radiation by gas, anywhere along the open path, detected by photoelectric means that produce electrical signals to provide alarms and indications of the concentration of gas along the path.
Open Path Type Gas Detector
Selection Criteria for Open Path Type Combustible Gas Detector:
Following are some minimum requirement applicable for combustible gas detector selection, however requirements vary from industry to industry.
• The system shall consist of a microprocessor controlled transmitter and receiver unit with all electronic circuitry, mounting brackets and all other accessories for proper functioning of the detectors.
• The transmitter ahd receiver units shall be suitable for mounting on standard instrument posts.
• The detector system shall be provided with means to ensure reliable operation without false alarm in all adverse ambient conditions.
• Continuous optical check for beam blockage shall be provided. A short time blockage in the detector path shall not prevent the instrument from functioning. Blockages of greater than 10 seconds shall result in a beam -blocked warning. After 60 seconds, the instrument shall indicate a fault operatlon.
• The output from the unit shall be a standard 4-20 mA signal, where 4 mA shall indicate normal operation. Increasing concentration of gas shall cause the signal to increase linearly to a maximum of 20 mA. Output signal less than 4 mA should indicate "Beam Blocked " or “Fault” at defined levels.