Practical instructions
1.1 Introduction
Laser methane detector is a gas detection device for remote measurement of methane gas. The device emits two laser beams simultaneously that a green indicator laser points to the detection target and the accompanying invisible infrared laser can measure the concentration of methane gas present remotely in the unit ppm.m.
1.2 Measurement principle
“Methane molecules absorb light at specific wavelengths.”
—— Molecular absorption spectroscopy principle
“There is a linear relationship between the concentration and the absorbance of the solution, which enables the concentration of a solution to be calculated by measuring its absorbance.”
—— Lambert-Beer law
The basic principle behind the TDLAS technique is simple. The focus here is on a single absorption line in the absorption spectrum of a particular species of interest.
To start, the wavelength of a diode laser is tuned over a particular absorption line of interest and the intensity of the transmitted radiation is measured. The transmitted intensity can be related to the concentration of the species present by the Beer-Lambert law, which states that when a radiation of wavenumber passes through an absorbing medium, the intensity variation along the path of the beam.
As to methane, the absorption line in the spectrum is distributed as this chart and we are using laser with the wavelength in 1653 nanometer. This absorption line is only effective for methane gas which is not affected by other species like water vapor, carbon dioxide or any other gas components in the atmosphere.
1.3 Unit（ppm.m）
“The meaning of 100 ppm.m : to uniformly distribute 100 ppm of methane gas on a gas mass of 1 m thickness.”
It can be approximated explained that the unit concentration is the concentration after the methane molecules on the straight line (laser detection line) between the telemeter and the reflection target are uniformly "compressed" or "expanded" into a region of 1 m thickness.
When the device has a measured value of 100 ppm.m for a methane agglomerate with a thickness of 5 m and a concentration of 20 ppm on the test line, which corresponds to the air mass being "compressed" to a thickness of 1 m and a concentration of 100 ppm.
For an air mass with a thickness of 0.5 meters and a concentration of 200 ppm, the measurement value of the instrument is also 100 ppm.m, which is equivalent to the air mass being “expanded” to a thickness of 1 meter and a concentration of 100 ppm.
1.4 Choose the appropriate reflective surface
The ability of the target to reflect the laser directly affects the measured distance and effect. If the target surface is of dark, tilted at a large angle, porous, mirrored or strongly reflective, please adjust the detective angle and position to achieve the best detection results.
1.5 Highly reflective surface
When device is facing the surface with high reflectivity (such as glass, smooth tiles, water surface or smooth stainless steel). Direct reflection of the reflected light from the mirror into the detector will overload the detector instantaneously which results in false alarms. In addition, the highly reflective surface may also cause the detection beam to be reflected directionally without reflecting light and an error of too weak light intensity.
1.6 Penetrating transparent objects
The laser can detect methane through transparent objects, but the penetration may cause the measured value to be relatively lower than the actual value. The more transparent objects penetrated, the greater the loss.
When the detector is aimed at a transparent container with a good reflection, the measured value will be lower than the actual unit concentration in the container. Because the laser will first reflect on the first interface when the reflected light does not pass through the gas. The stronger the reflective ability of the surface, the greater the deviation between the measured value and the actual value.
1.7 The long-distance detection cases
The beam emitted by laser collimation is not a strict straight line which will continue to diverge and become weaker and forms a conical divergent beam when the distance increases. The spot size of the detection beam is about a circle with a diameter of 1 meter at a distance of 100 meters. So, when the distance increases, more insufficient reflected detective light will be collected by device resulting in the measured value lower than the actual value, even that would be not able to detect effectively.
When detecting high-rise residential buildings, if the measurement elevation angle is too large there will be a situation where the elliptical spot covers multiple layers. So that the spot will not fully cover the target resulting in the measured value lower than the actual value.
In the case of long-distance detection, the laser itself will scatter which will be worse in windy, rainy and foggy weather, resulting in the insufficient reflected light intensity which cause a lower detection value.
