Gas detectors
Gas detectors
What is the working principle of open path gas detectors. Also if someone can help me out in evaluating the number and location to be installed?
[ t o s i f ]
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ali.abbas
- Core Member
- Posts: 61
- Joined: 25 May 2010, 23:26
- Area of interest: Inst. & Control Engineering
Re: Gas detectors
Tosif,
Open Path Gas detectors use the principle of absorption of IR energy by gases to detect their presence.
The detector contains a set of Transmitter and Receiver (or in older designs, a combined Transmitter+ Receiver and a Reflector). The transmitter transmits 02 beams, each at a separate wavelength (within the IR range of course), one wavelength is tuned to lie in the range which is absorbed by the gas to be detected, and the other is a reference wavelength that remains unaffected by the gas. The receiver is placed in line of sight of the transmitted beam. It receives both the transmitted wavelengths and then compares them to detect if there has been any drop in the intensity of the former compared to the reference. A drop in energy/intensity level is an indication of the presence of the gas, the greater the drop, the greater the number of molecules present in the path.
Open Path Detectors work on a line of sight basis and therefore the transmitter and receiver (or reflector) must face each other. However, it will detect gas molecules any where in way of the beam. The measure of gas levels is actually in terms of the number of molecules encountered by the beam on the way and therefore the unit generally used by Open Path detectors is "LEL meters" or "ppm meters". The unit is not necessarily the concentration of the gas, but rather merely the quantity of molecules. In other words, this is saying that a small dense cloud of hydrocarbon can give the same output signal as a large dispersed cloud, provided that the product of concentration and path length are the same.
There seems to be no fixed rule as to the number and location of open path gas detectors. Drawing from the principle of operation, it is quite evident that they would be more preferable for use in open areas (i.e. no obstruction to line of sight) with multiple potential leakage points. Range of commercially available detectors generally varies between 100 to 150 meters. Therefore the number and location of detectors would generally be determined by location of potential leakage points. The more leakage points that can be made to lie in the line of sight of the detector, the fewer would be the number of detectors required.
This is of course a very brief explanation, though I hope I have cleared your confusion.
Open Path Gas detectors use the principle of absorption of IR energy by gases to detect their presence.
The detector contains a set of Transmitter and Receiver (or in older designs, a combined Transmitter+ Receiver and a Reflector). The transmitter transmits 02 beams, each at a separate wavelength (within the IR range of course), one wavelength is tuned to lie in the range which is absorbed by the gas to be detected, and the other is a reference wavelength that remains unaffected by the gas. The receiver is placed in line of sight of the transmitted beam. It receives both the transmitted wavelengths and then compares them to detect if there has been any drop in the intensity of the former compared to the reference. A drop in energy/intensity level is an indication of the presence of the gas, the greater the drop, the greater the number of molecules present in the path.
Open Path Detectors work on a line of sight basis and therefore the transmitter and receiver (or reflector) must face each other. However, it will detect gas molecules any where in way of the beam. The measure of gas levels is actually in terms of the number of molecules encountered by the beam on the way and therefore the unit generally used by Open Path detectors is "LEL meters" or "ppm meters". The unit is not necessarily the concentration of the gas, but rather merely the quantity of molecules. In other words, this is saying that a small dense cloud of hydrocarbon can give the same output signal as a large dispersed cloud, provided that the product of concentration and path length are the same.
There seems to be no fixed rule as to the number and location of open path gas detectors. Drawing from the principle of operation, it is quite evident that they would be more preferable for use in open areas (i.e. no obstruction to line of sight) with multiple potential leakage points. Range of commercially available detectors generally varies between 100 to 150 meters. Therefore the number and location of detectors would generally be determined by location of potential leakage points. The more leakage points that can be made to lie in the line of sight of the detector, the fewer would be the number of detectors required.
This is of course a very brief explanation, though I hope I have cleared your confusion.
Regards,
Ali Abbas
Ali Abbas