Open Path Type Gas detector Principle and Logic


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Open Path type gas detector devices measure the sum of the hydrocarbon molecules along a narrow beam, the configuration for displaying the reading from an IR beam gas detector is as follows:

Display in LEL-meters which is an integral of percentage LEL as multiplied by the length of the beam. This is in effect a direct reading of what the device is measuring i.e. the total number of hydrocarbon molecules along the length of the beam.

Open Path type gas detector

Alarm levels should be set at a low level but such that spurious alarms are avoided, typically:

· High level gas – 1 LEL.m,

· High-High level gas – 2.53 LEL.m

As a minimum the detectors will provide the following performance requirements;

  • 2-60Hz maximum Peak to Peak amplitude vibration

  • Diagnostic capabilities, to alert incorrect installation and fault conditions

  • Loss of signal strength of <5 for="">60minutes shall indicate a fault.

  • Detectors are double pass filtered to compensate for fog, rain and mist interference.

  • Misalignment tolerance of 0.25°. Both vertical and horizontal adjustment of 45°, with locking facility shall be provided. Beam misalignment of 1° or less is not adversely affect performance.

  • Error range shall not be exceeded while any one or more interference mechanisms are present.

  • The speed of response shall not exceed 5 seconds for the specified beam path length, T90 less than 3 secs.

  • The detector will be calibrated on methane with a measurement range of 0 to 5 LEL.m for methane with the output being 4mA for 0 LEL.m and 20mA for 5 LEL.m.

These type of detector usually have 7 signals and status that are shown in the control room,

  1. Faulty Status

  2. Calibrate alarm

  3. Beam Block alarm

  4. High gas exec action

  5. High-high alarm

  6. High-high gas exec action.

  7. Inhibit signal

The logic are:

  1. If the detector is in the faulty status (open loop or short). Exec. Action trip command and alarm will be deactivated until the reset command is activated.

  2. If the gas detector is in calibration mode. Beam block alarm will be deactivated until the reset command is activated.

  3. If the detector is in the inhibit status. None of the other signals will be executed until the reset command is activated.

  4. If the High gas alarm (1 LEL.m) detected. High gas exection trip will be activated until the reset command is activated.

  5. If the High-high gas alarm (2.5 LEL.m) detected. High-high gas exection trip will be activated until the reset command is activated