Methane in the atmosphere
1.1 Methane in the atmosphere
Methane (CH4) is a trace gas in the atmosphere, 40% of which is emitted into the atmosphere through natural sources (such as wetlands), and about 60% of emissions come from human activities, such as animal husbandry, rice cultivation, fossil fuel utilization, landfills and biomass burning, etc.
Atmospheric methane goes up to a new record in 2018, about 1869 parts per billion (ppb), which is currently 259% of the pre-industrial level. For CH4, the growth from 2017 to 2018 is higher than the observed value from 2016 to 2017 and the average of the past ten years. The following figure shows the general content of methane in the atmosphere and the simulated data of the distribution of methane on the surface of China in 2014.
For the impact of atmospheric methane on detection, the following premises can be determined:
1. Atmospheric methane is unevenly distributed over a wide area.
2. Atmospheric methane is evenly distributed in the detection area (a distance of several hundred meters or less).
From this, the following conclusions can be drawn:
1. An device that calibrates the zero point in a methane-free environment can detect the concentration of methane in the atmosphere. With a detection distance of 15 meters and the concentration of methane in the atmosphere of 2ppm, the instrument reading should be 30ppm.m.
2. Because ppm.m is used as the concentration unit, the greater the detection distance, the higher the superimposed atmospheric methane concentration, for example, at a distance of 100 meters, the superimposed atmospheric methane concentration can reach 300 ppm.m.
Because of the actual presence of methane in the atmosphere and the stability of the line in the detection area, but it is related to the distance, the instrument manufacturer generally provides the background (the concentration of methane present in the atmosphere) filtering function. It may be a button or internal algorithm processing to help users filter the concentration information that does not need to be concerned and pay attention to the fact that the leakage area is higher than the surrounding concentration. Therefore, some device display zero when no leak is detected.
But it should be noted that if an instrument is aimed at a reflecting surface and does not move, the indication brought by methane in the atmosphere should be stable, and the fluctuating values are often the noise of the instrument.
1.2 Detection noise
In the real world, noise is ubiquitous. A laser detection device can obtain a trace amount of methane concentration from 100 meters away. This in itself is a process of extracting a weak signal from a massive amount of noise.
The sources of these noises are diverse, including inherent thermal noise, 1/f noise, interference noise caused by optical path multipath effects caused by multi-interface insertion and reflection, etc., which are not described here. But one thing is clear, the farther the detection distance, the more complex the detection environment, and the greater the noise.
Take the detection of high-rise residential buildings as an example: due to the penetration glass detection, the detection target is generally located 100 meters away, and the detection noise level of the equipment is generally around hundreds of ppm.m. It can be roughly considered that the detection below this concentration is uncertain and unreliable. Note that this is almost comparable to the superimposed atmospheric methane content, so atmospheric methane is a fact, but it is not a key factor in the inspection environment of the telemeter.
An actual testing instrument is in the actual testing environment. When the detection environment is close and the environment is gentle, try to find a smaller concentration of leakages with a lower alarm limit; while at a long distance, or the detection conditions are harsh, raise the alarm limit to avoid noise and false alarms as much as possible.
