GAS DETECTION 41
Then there are complex diatomic molecules such as carbon dioxide (CO2
(C6 H5
), methane (CH4 CH=CH2
), sulphur hexafluoride (SF6 ) (these are just a few examples). ), or styrene VISUALIZING THE GAS STREAM
If the camera is directed at a scene without a gas leak, objects in the field of view will emit and reflect infrared radiation through the lens and filter of the camera. The filter will allow only certain wavelengths of radiation through to the detector and from this the camera will generate an uncompensated image of radiation intensity. If a gas cloud exists between the objects and the camera and that gas absorbs radiation in the band pass range of the filter, the amount of radiation passing through the cloud to the detector will be reduced (Figure 11).
Figure 7. Carbon dioxide – 3 atoms per molecule Figure 3B. Infrared absorption characteristics for methane
Most hydrocarbons absorb energy near 3.3 µm, so the sample filter in Figure 3 can be used to detect a wide variety of gases. Response factors (RF) for more than 400 additional compounds are available at the following site:
http://rfcalc.providencephotonics.com.
Ethylene has two strong absorption bands, but a longwave sensor will detect this gas with greater sensitivity than a midwave sensor based on the transmittance curve shown below.
Figure 11. Effect of a gas cloud
In order to see the cloud in relation to the background, there must be a radiant contrast between the cloud and the background. That is to say, the amount of radiation leaving the cloud must not be the same as the amount of radiation entering it (Figure 12). If the blue arrow in Figure 12 is the same size as the red arrow, the cloud will be invisible.
Figure 8. Methane – 5 atoms per molecule This assumption is also valid for multi-atomic molecules.
Figure 4. Infrared absorption characteristics for ethylene
Selecting a filter that restricts the camera to operating only in a wavelength where a gas has a very high absorption spike (or transmission trough) will enhance the visibility of the gas. The gas will effectively ‘block’ more of the radiation coming from the objects behind the plume in the background.
WHY DO SOME GASES ABSORB
INFRARED RADIATION? From a mechanical point of view, molecules in a gas could be compared to weights (the balls in Figure 5 below), connected together via springs. Depending on the number of atoms, their respective size and mass, and the elastic constant of the springs, molecules may move in given directions, vibrate along an axis, rotate, twist, stretch, rock, wag, etc. The simplest gas molecules are single atoms, such as helium (He), neon (Ne), or krypton (Kr). They have no way to vibrate or rotate, so they can only move by translation in one direction at a time.
Their increased degrees of mechanical freedom allow multiple rotational and vibrational transitions. Since they are built from multiple atoms they can absorb and emit heat more effectively than simple molecules. Depending on the frequency of the transitions, some of them fall into energy ranges that are located in the infrared region where the infrared camera is sensitive.
Table 1. Frequency and wavelength ranges of molecular movements
TRANSITION TYPE FREQUENCY SPECTRAL RANGE 109
Rotation of heavy molecules
Rotation of light molecules & vibration of heavy molecules
Figure 5. Single atom
The next most complex category of molecules is homonuclear, made of two atoms such as hydrogen (H2
), nitrogen (N2 oxygen (O2 addition to translational motion.
In order for a molecule to absorb a photon (of infrared energy) via a transition from one state to another, the molecule must have a dipole moment capable of briefly oscillating at the same frequency as the incident photon. This quantum mechanical interaction allows the electromagnetic field energy of the photon to be “transferred to” or absorbed by the molecule.
Figure 6. Two atoms
OGI cameras take advantage of the absorbing nature of certain molecules to visualize them in their native environments. The camera FPAs and optical systems are specifically tuned to very narrow spectral ranges, in the order of hundreds of nanometers, and are therefore ultra-selective. Only gases absorbent in the infrared region that is delimited by a narrow band pass filter can be detected (Figures 3, 4).
), and ). They have the ability to tumble around their axes in
Vibration of light molecules. Rotation and vibration of the structure
1011 to 1011 to 1013 Hz Hz 1013 to 1014 Hz
Microwaves, above 3 mm
Far infrared, between 30 µm and 3 mm
Infrared,
between 3 µm and 30 µm
Electronic transitions 1014 to 1016 Hz UV - Visible KEY CONCEPTS FOR MAKING GAS CLOUDS VISIBLE:
• Gas must absorb infrared radiation in the waveband that the camera sees
• Gas cloud must have radiant contrast with the background
• Apparent temperature of the cloud must be different than the background
• Motion makes the cloud easier to see
– Ensuring your OGI equipment is calibrated to measure temperature will offer critical value in being able to assess the Delta T (apparent temperature between gas and background).
Figure 14. Difference in apparent temperature
Figure 9. Sulfur hexafluoride 6 – 7 atoms per molecule
Figure 10. Styrene – 16 atoms per molecule
Figure 12. Radiant contrast of cloud
In reality, the amount of radiation reflected from the molecules in the cloud is very small and can be ignored. So the key to making the cloud visible is a difference in apparent temperature between the cloud and the background (Figure 13).
Author Contact Details Steve Beynon, Sales Manager North Europe, FLIR Systems Ltd • Tel: +44 1604 600005 • Email:
steve.beynon@
flir.com • Web:
www.flir.co.uk
WWW.ENVIROTECH-ONLINE.COM IET Annual Buyers’ Guide 2020/21
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