Using thermal cameras to visualize gas leaks

Using thermal cameras to visualize gas leaks

Thermal cameras can efficiently detect (visualize) gas leaks that are hazardous for the environment and the health of people. The principles of detecting gas using thermal cameras is based on the fact that some gases in selected spectral zones behave as selective radiators with low throughput and reflectivity (and high emissivity) and under certain circumstances they can be easily observed by a thermal camera with sufficient temperature sensitivity and the correct spectral range. Special LWIR and MWIR thermal cameras were developed to detect (leaks) of gases, including, for example, the quite problematic gas SF6, which is 24 000 x more hazardous for the environment than the greenhouse gas CO2.  According to the spectral range of the thermal camera and the filter, gases that are selective radiators in the particular spectral range are detected.

Thermal cameras to detect gases differ from thermal measuring cameras. In addition to a lens, detector, cooling part (if the thermal camera detector is cooled) and electronics for processing the image, on the front part of the detector is a cooled optical band gate filter . This filter restricts the heating radiation wave lengths that the filter allows to act on the detector to narrow the band. This technology is known as spectral adaptation and the camera, in practice, is usually constructed (i.e. equipped with the particular filter) for the selected gas or group of gases (i.e., for example CO2, CO, SF6 etc.) and detecting other gases is considered as a certain type of “bonus”. The wave lengths of typical band gates for MWIR and LWIR cameras are given in the Tab. 1. Of course, in practice we also come across other types of spectral sensitivity depending on the expected purpose of the camera.

Camera model

Spectral range of the detector

Typical band gates

MWIR (medium waves)

3 – 5 μm (usually InSb)

3.2 – 3.4μm,

4.52 – 4.67µm

4.2 – 4.4 µm

8.0 – 8.6 µm

etc.

LWIR (long waves)

9 -11 μm (usually QWIP)

10.3 – 10.7 μm

Tab. 1: TSpectral response for gas detection cameras

Functioning principle

 

If a thermal camera reads a scene without 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 not compensated image of radiation intensity of the radiation (then we speak about the apparent temperature). 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 and the amount of radiation passing through the gas which affects the detector will be lower, see Fig. 1. and Fig. 2.
Fig. 1: Diagram of radiation flows when passing through a gas cloud. [5]
Fig. 1: Diagram of radiation flows when passing through a gas cloud.

To see the gas in against the background, there must be high contrast between the cloud and the background. i.e. the amount of radiation leaving the cloud must not be the same as the amount of radiation entering it. In fact, the amount of radiation reflecting off the molecules in the gas cloud is very small and can be ignored. The key for the gas to be visible is the difference of its temperature compared with the temperature of the background.

Fig. 2: To be able to detect a gas cloud, the amount of radiation leaving the cloud must not be the same as the amount of radiation entering it.

Therefore, in practice, it is important to know the spectral absorption of individual gases (absorption spectrum). To determine the absorption spectrum of infra-red radiation for a gas, a sample is placed in an infra-red spectrometer and the absorption (or transmission) of infra-red radiation is measured at various wave lengths. The result is then the graph of the spectral absorption line, e.g. in Fig. 3 or Fig. 4.

One of the verified sources of spectral characteristics data of individual gases is the American National Institute of Standards and Technology (NIST) and its web database  is a very useful source of this information.

Fig. 3: Spectral absorption of benzene. The gas is largely absorbed on a wave length of about 3.2µm. This can be used when detecting this gas by thermal camera.
Fig. 3: Spectral absorption of benzene. The gas is largely absorbed on a wave length of about 3.2µm. This can be used when detecting this gas by thermal camera.
Fig. 4: Spectral absorption of SF6 gas. The gas is very absorbent at a wave length of about 10.6 μm, which is used when designing thermal cameras for detecting this gas.
Fig. 4: Spectral absorption of SF6 gas. The gas is very absorbent at a wave length of about 10.6 μm, which is used when designing thermal cameras for detecting this gas.

The key conditions for a gas being visible to a thermal camera are as follows:

  • The gas must absorb infra-red radiation in the wave length visible to the thermal camera and optical filter
  • The gas cloud must have a high bright contrast compared with the background
  • The instantaneous gas temperature must be different from the background
  • The movement of the gas cloud helps its visibility

Those interested in a detailed physical explanation of the principles are referred to the publication

MWIR thermal cameras for detecting gas leaks

An example of an MWIR thermal camera is the Workswell GIS320, which is designated for unmanned aircraft (drones) [1], see Fig. 5. It is a thermal camera with a cooled InSb detector and resolution of 320×240 px and spectral range 3.2 – 3.4 μm. By its dimensions (201 x 150 x 105 mm) and weight (1. 6 kg) it is not classified as a small device, however a DJI M600 Pro drone can keep this device in the air for 20 minutes. This camera is displayed in this configuration in Fig. 5.  It can monitor hazardous gas leaks or monitor a wide and hard accessible technologies.

In this way and with the particular spectral range, for example, gases can also be detected (the list is in English): Benzene, Ethanol, Ethylbenzene, Heptane, Hexane, Isoprene, Methanol, MEK, MIBK, Octane, Pentane, 1-Pentane, Toluene, Xylene, Butane, Ethane, Methane, Propane, Ethylene and others.

Fig. 5: Workswell GIS320
Fig. 5: Workswell GIS320 is MWIR thermal camera with a cooled InSb detector, with resolution of 320x240 and a spectral range of 3.2 – 3.4 μm. This thermal camera is for drones and here it is shown on the DJI M600 Pro.

Conclusion

Emissions of leaking gases contribute to the worldwide growth of global heating. Industry pays billions of dollars for emissions in regulation penalties and damage and they are a fatal risk for people working in the surroundings of leaks, as well as for those living near damaged equipment. Detecting gases with a thermal camera enables leaks to be caught for tens of volatile organic compounds, including greenhouse gas sulphur hexafluoride (SF6), which effectively contributes to a better environment.

A specialized thermal camera for detecting gases is a fast, contact-less measuring tool which can be used in difficult to access places. It can detect small leaks at several metres, as well as larger leaks at hundreds of metres. It can also detect leaks on moving vehicles which significantly increases the safety of both involved subjects – the inspector and the inspected item. But a suitable spectral range (MWIR or LWIR) must be selected and in addition a band pass filter depending on which gas or gases are to be detected