This article provides information on Open Path Infra Red Gas Detectors Principle, Calibration, Advantages & Disadvantages.
Open Path (Line of Sight) Infra Red Gas Detectors
Principle of Operation
Infra-red (IR) gas detection is based on the absorption of energy by hydrocarbons.
The bond between hydrogen and carbon absorbs proton energy at a wavelength of 3.3 mm. The wavelength is carefully selected to accurately measure the concentration of methane and other hydrocarbon gases in air, ensuring that the measurement is not effected by water vapour or other gases.
Infra-Red (IR) Line of Sight (LOS) Open Path Gas Detectors measure the amount of hydrocarbon gas in an Infrared beam by determining the amount of Infrared Absorption of the air or air-gas mixture in the line of sight (LOS) path.
LOS IR Gas detectors utilise an IR source and a detector which measures the intensity of the IR of the received IR radiation at the appropriate wavelength. A separate Transmitter/Receiver configuration provides the most reliable basis for open path gas detection of up to 200meters.
There are also point type IR gas detectors.
A beam of IR energy is emitted between a source and detector and any attenuation caused by hydrocarbons in the beam being electronically processed to give a reading in LEL metres.
In order to ensure that dirty lenses, mirrors or other related issues do not cause errors two detectors are generally utilised and their measurements compared.
In order to reduce or eliminate the effects of other interfering light sources, particularly direct and reflected sun light, but also flames and welding arcs, higher-specification instruments employ strong pulsed light sources, and the times at which measurements are taken are synchronised with the timing of the light pulses.
Transmitter and Receiver units may incorporate heated optics designed to minimise the build-up of humidity, condensation, snow or ice on the glass windows, which could obscure the optics in extreme conditions. The sophisticated open-path technology provides immunity to sunlight and minimises the effects of environmental factors such as rain, fog, ice, snow and condensation.
The sample detector input is filtered at wavelengths where strong infrared absorption is exhibited by the target gas. The reference detector input is filtered at nearby wavelengths, where strong infrared absorption is not exhibited by the target gas.
By calculating the ratio of the sample to reference signal it is possible to measure the quantity of gas in the beam, whilst compensating for the effects of rain, fog, dirt etc.
Gases Measured by Open Path Infra Red Gas Detectors
Gases measured include Methane, Ethane, Propane, Butane, Pentane, Hexane, Ethylene, Propylene, Butadiene, Methanol and Ethanol.
Calibration of Open Path Infra Red Gas Detectors
Calibration for a gas concentration is expressed in LEL - metres. For various gases low and high test filters are used for calibration eg.,methane employs polypropylene sheets of specific thicknesses.
For more complex situations, gas tubes that can be inserted in the beam path and filled with specific gas mixtures may be employed to calibrate Open Path Gas Detectors.
OPGD beam paths need very careful design to ensure an uninterrupted Line of sight, this should occur at the detailed design stage. It is recommended that if a 3D model is available this tool is used to verify this.
Advantages and Disadvantages of Open Path Infra Red Gas Detectors
- Unit is ideal for open areas without obstructions as the beam must have a clear path.
- Covers large areas, thus minimising the number of units required.
- Very fast response.
- Limited maintenance required.
- Unsuitable for hydrocarbons which have various compositions ie., if one day the leak could be Methane and the next Propane the readings will change and will not be representative.
- Cannot measure non hydrocarbons eg., hydrogen.
References: Honeywell Analytics Searchline Excel Infrared Gas Detector Technical Handbook.