How do you find that gas leak?
You stand there gazing at the plant... many hundreds of metres of tubing and dozens of valves and pressurised reaction vessels all over the place. You know that there is sporadic toxic gas leak somewhere, as when the wind is blowing in a certain direction, and when certain processes are under way, some perimeter gas sensors are indicating occasionally unacceptably high levels.
As you consider the cost of calling in a team of portable-sensor-wielding technicians clothed in protective suits complete with breathing equipment, and have them clambering around the plant sniffing around for goodness knows how long... You think of how simple it would be if you could just see the gas. The good news is that now you can.
The imaging spectrometer
Atis, a French company, has developed a scanning infrared spectrometer/camera hybrid that literally enables its user to see clouds of gas. The system superimposes coloured dots on the monochrome infrared image. The density of the coloured dots varies in proportion to the quantity of gas, and the colour of the dots indicates the individual gas that has been detected. So, with this device, the user can observe gas clouds in very close to realtime - and instantly identify the gases that are leaking.
The working principle
All objects emit infrared light in accordance to their temperature and surface characteristics. This emitted infrared light has to pass through the air - and any other gases between an observer and the scene being observed. As different gases exhibit different infrared absorption characteristics, establishing which spectral components from the scene are attenuated will reveal the identity of any gas between the observer and the scene.
The more gas that the infrared light has to travel through, and the denser the gas cloud, the greater will be the attenuation of the spectral components that correspond to the absorption characteristics (spectral signature) of any gas present. The trick is to be able to determine the existence of any dips in the spectral content for each pixel of the scanned image - and compare these dips with the stored signatures of known gases.
The simplest way is not so simple
The most logical way to determine the spectral content would appear to be splitting the incoming light into a spectrum and have a mask with a narrow slit (nanometres wide) scan across the spectrum. As the position (and thus corresponding wavelength of the light passing through) is known, it would seem a simple matter to log the detected level as the opening sweeps across the spectrum (stepping the slit position each time a complete image frame is scanned).
After the mask, the spectrum would be recombined and the tiny span of wavelengths that get through the slit are focused onto an image sensor. The problem is that such a narrow slit in the mask would allow very little infrared light energy through. The light level is so low, in fact, that the noise present in the sensor's elements (due to their own temperature) essentially swamps any input signal.
Lowering the temperature of the detector lowers the noise present in the detector and allows smaller signals to be detected. This same principle has been used in low noise amplifiers (LNAs) used in radio telescopes, for example. Naturally the inclusion of a suitable cooling system would raise the cost of the instrument significantly. This would make the system more complex to use, and it would cost considerably more. Atis found a way whereby a cheaper, industry-standard, ambient temperature-rated infrared image sensor could be used.
Atis's cost effective solution
Inside the Atis instrument, on its way to the infrared image sensor, the incoming light is first passed through a spectral selector. Here the light is split into the partial 'infrared rainbow' that covers the wavelengths from 8 to 12 µm. Instead of a mask with a slit, a very narrow 'needle' mask (100 nanometres wide) traverses across the spectrum, blocking the spectral components with wavelengths corresponding to the position of the mask. The light that gets past is then recombined and focused on the infrared image sensor. As only a small fraction of the light is blocked, a far stronger light signal reaches the image sensor - which means that a standard (and far cheaper) ambient temperature image sensor arrangement can be used.
A computer application takes the data from the output of the infrared image sensor and correlates this with the position of the moving mask, and over successive scanning cycles is able to calculate the spectral content of each pixel in the image. By comparing any dips in the spectral curve (to a resolution of 40 nanometres) with its library of gas fingerprints, the software is able to identify the gas. The degree of attenuation of these components (and similar spectral dips in adjacent pixels) works to reinforce the displayed 'density' of the gas cloud.
Identifiable gases
At the time of the launch, the demonstration package had a library of 26 gas fingerprints. As other gases with spectral absorption signatures within the 8 to 12 micron band are identified, these can easily be added to the software database.
Accuracy and limitations
Two important dynamics need to be borne in mind when using the instrument:
* The degree of spectral attenuation contributed by the gas cloud will depend on how far the light travels through the gas and how dense the cloud is.
* A large gas cloud at a considerable distance can appear the same in the picture as a small gas cloud that is much closer.
Clearly, as there are quantities that can vary considerably, it is not possible to specify the measurement accuracy as simply as is possible with other types of instruments. (Its makers report that observing gas concentrations as low as 10 ppm is quite feasible, as long as the cloud is big enough, or close enough.) Obviously, by correlating the displayed image with actual measurements taken from sampling the cloud, it is possible to set up a system that will give a dependable indication of real values.
As the instrument includes an infrared image sensor, the system can also be used to monitor the temperatures at different points in the scene, and these can be linked to alarms. This combination of monitoring ability is quite extraordinary.
Future developments
An instrument that can work over the 3 to 5 nanometre band is in development - this will permit the detection and observation of many more gases. Versions especially set up for use on airborne platforms are also on the cards.
Tailored solutions
The instrument will be available for purchase or hiring, and the local Spero Group is prepared to tailor a system set-up - and financial arrangement - that will suit the needs of the user.
| Tel: | +27 12 665 0317 |
| Email: | info@spero.co.za |
| www: | www.spero.co.za |
| Articles: | More information and articles about Spero Sensors & Instrumentation |
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