Methane Gas detection sensors: THERMAL vs LASER ?

Which is better for UAV based methane gas detection: Thermal or Laser sensors?

How does methane gas detection with a laser detector work?

Laser technology enables gas leaks to be located by pointing the laser beam towards the suspected leak, or along the survey line. The laser beam directed at targets such as gas piping, the ground, etc. A diffused beam is reflected back from the target. The device receives the reflected beam and measures the absorptivity of the beam, which is then calculated into methane column density (ppm-m).

How does methane gas detection with a thermal camera work?

If a thermal camera reads a scene with a gas leak, objects in the angle of view will radiate and reflect infra-red radiation that will react through the lens and the filter on the detector.

The filter only lets some wave lengths of radiation enter the detector from which the camera generates an image of the radiation.

If there is a cloud of gas between the lens and thermal camera, this gas will selectively absorb radiation to the extent of its spectral absorption. The amount of radiation passing through the gas which affects the detector will be lower.

A brief comparison

The laser detector allows gas hot-spots to be detected only at locations in a narrow field of view of the “one point”. The thermal camera, on the other hand, makes it possible to detect and measure hot-spots in the entire image which it is able to observe with its lens.

For example, a 320×240 resolution thermal camera allows visualisation and therefore detection of gas leaks on large areas due to the relatively large field of view and the large number of detectors.

This is not only one detector (as with the laser detector), but with its resolution there are 76,800. This makes it is possible to assess even very large areas. See comparison below:

During a one flight, you are able to control about 150m x 150m (at an altitude 80 m) area or about a pipe line up to 1000 m in length.

The following table provides more details of the differences between the two types of sensors against a broad array of dimensions:

Infrared vs Laser sensor comparison

	Infrared	Laser
Spectrum	the infrared sensor achieves rather weak resolution. spectral resolution a factor 1’000 weaker overlapping gas absorption bands a serious problem to  IR detectors – particularly with water absorption bands	the high resolution approach of the laser detectors makes resolution a non-issue, resulting in Zero Cross-Sensitivity.
Long term Stability	And light source of infrared sensors is just similar to that of light bulbs, as time going on, it will become weaker	Laser sensor modules adopt the most modern and stable semiconductor photoelectric components and have a long service life.
Single Channel stability	infrared sensor detectors need a second measurement channel as reference to avoid deterioration over the long term	no need for a reference channel or frequent calibration routines
Power consumption	NDIR detector uses electrical power to generate a broad spectrum of intensities therefore uses only a tiny fraction of the emitted light, leaving the main portion of the used electrical power wasted	laser detector emits 100% of its light at exactly the wavelength of interest
Principle technology	infrared sensor use infrared light source and filter technology.	uses near-infrared laser technology, laser type methane sensors response quicker and with anti-wet and anti-vibration capacity and more stable during working operations
Response time	light source of infrared sensors is infrared radiation which is from the heating special ligament. Only little effective light in infrared radiation could be used. Therefore it takes a long time to integrate to test the effective signal and makes a slower response speed (more than 30s).	response time of laser type methane sensors is a 1000ms
Humidity	water molecular has strong absorption characteristics in the infrared range (therefor inferior) infrared sensors are weak in light source signal, usually a difference needs to be recorded between standard gas chamber and detection gas chamber to find out the effective signal, gas chamber should be with complex structure, weak anti-vibration capacity	laser sensors trigger no response towards vapour and have excellent anti vibration capacity
Detectable gases:	100+ different gases	single gas (e.g. methane)
Type of measurement:	image stream	spot measurement
Detection:	gas propagation and leakage source	presence of gas
Typical application:	whole area scanning	spot measurement or liner inspection

In summary.

• Infrared sensors detect a wider range of gases over a wider area. The operator still has to CONSTANTLY MONITOR the image stream.
• Laser sensors, which are much sharper with a quicker response, automatically detect the relevant gas. Then, in conjunction with infrared and optical cameras, take both a visual and a thermal snapshot of the identified leak.

Remote Laser Gas Detector

The Remote Laser Gas Detector UAV can efficiently and rapidly detect methane gas from 30 meters away. This allows for accurate remote sensing on a UAV platform. With the fast detection rate, a UAV can rapidly cover a large area allowing for quick and efficient pipeline inspections.