ptical gas imaging (OGI) is increasingly being used in the oil and gas industry to

detect leaks. It provides a straightforward and

affordable method to visualise plumes of gas that would otherwise be invisible, allowing companies to trace and repair the source of the leaks. This imaging technique, which relies on thermal

cameras that measure changes in infrared (IR) radiation as it passes through gas, is still relatively new and no official standard exists to characterise how good a job any particular camera does at detecting leaks, or to say how small a leak a given camera can find. A standard that would make sense is Noise Equivalent Concentration Length (NECL), which is a measurement of how much gas over a path of a certain length can be detected above the intrinsic noise of the camera. The NECL is easy to measure, is similar to another familiar measurement, and provides a simple way to compare the performance of different cameras.

SPECTRAL SIGNATURES OGI works by measuring the IR radiation passing through a volume of gas. Any gas has its own spectral absorption characteristics. When there is a temperature difference between the cooler gas and the warmer background, light at certain wavelengths passing through the gas is absorbed. For example, natural gas, or methane, absorbs light at 3.3 ± 0.1 μm and 7.7 ± 0.1 μm. Other hydrocarbons common in the oil and gas

industry – propane or butane, for instance – have their own individual absorption spectra. If instead the gas is hotter and the background is cooler, the gas then emits rather than absorbs at certain wavelengths. If you are looking for natural gas leaks, it is a simple matter to use the IR camera to measure how much of the radiation is being absorbed by gas in a given volume of air. That tells you the concentration of the methane in that area. At the same time, the camera can visualise the gas by noting the temperature difference between the gas and the background, thus providing an image of the gas plume. Concentration length is the average amount of

gas in the air over a given distance, measured in parts per million per meter (ppm x m). If you had a 1-m long tube filled with 100 percent methane, the concentration length would be one million ppm x m. If the gas were diluted to 50 per cent, the concentration length would be 500,000 ppm x m. OGI is a line-of-sight measurement, so it entails measuring the average absorption of the target wavelength over a given distance and using that to calculate concentration. An older technique for leak detection, Method

21, also provides a concentration measurement in parts per million (ppm) using so-called “sniffers”. Method 21 measurements are made by sucking a small volume of air, with the gas


mixed in, into an instrument that uses a chemical sensor or IR detector to check for gas. The limitation of this technique is that it is a spot measurement and can only provide the concentration in a particular place. It does not tell the user anything about the way the gas is flowing or the direction it is coming from, which is important information for finding the source of the gas. The visual nature of OGI provides that information, making it easier to find and fix leaks. It also allows inspection of a wider area at a time, because it can look at a whole scene rather than measure in just one spot.

A FAMILIAR CONCEPT NECL is similar to another way of characterising IR cameras, Noise Equivalent Differential Temperature (NEDT). NEDT is a figure of merit that describes the temperature difference that could produce a signal equal to the camera’s temporal noise. When the NEDT is one, the signal- to-noise ratio is one, and any temperature difference the camera might detect would be no greater than the temperature difference attributable to the camera itself. In the same way, NECL is the signal from the gas that is equivalent to the noise in the camera. In other words, at an NECL of one, the signal being measured would be no greater than the electronic noise intrinsic to the instrument and therefore would be undetectable. So the NECL of a camera gives the concentration

of gas over a given path length that is undetectable above the intrinsic noise of the camera.

A MEASURE OF SENSITIVITY If sensitivity is the aim, a lower NECL is better. But there can be trade-offs. As noted, different gases that might be of interest to users have different spectral absorption signatures, so even with the same concentration length and temperature difference, they produce different NECL values. A broadband camera could visualise several gases, but the NECL would not be as good as in a camera optimised for, say, methane. Lowering the noise lowers the NECL and makes

the camera more sensitive. But lower-noise, higher-sensitivity detectors, usually cooled detectors, are more expensive than uncooled detectors with higher noise. With NECL, a user can compare cameras at different price points to see which is suitable for his or her application. When a user knows what NECL is needed, a manufacturer will be able to design cameras to specific needs. Another benefit of using NECL as a standard is

that it provides a way to be sure you are checking for small enough amounts of gas. For a gas like methane, there is a known lower explosive limit, the concentration of gas at which there is a danger of ignition. NECL will tell you whether the camera in question can detect the presence of methane

down to that limit. In a similar vein, some gases are toxic above certain concentrations. For instance, the extraction of methane can sometimes also capture fluoronium, a compound of hydrogen and fluorine that is extremely corrosive. It can be dangerous at concentrations as low as 10ppm, so detecting it would require a camera with a very low NECL.

SIMPLE SETUP The measurement setup for determining the NECL of a camera is fairly simple and consists of three main pieces of equipment. This setup requires a blackbody radiator that has high uniformity and can produce a stable background temperature of approximately 30° Celsius. The second piece is a gas cell filled with a calibration gas and a sensor to measure the temperature of the cell wall. A third instrument can measure the air

temperature and relative humidity. The user can surround the setup with simple

foam screens to reduce reflections from any warm objects or people in the room, then let everything settle into a thermal steady state. They then record at least 150 images over 10 seconds. Excluding the noise from the camera, the NECL is a function of the temperature difference between the cell, the gas, and the blackbody radiator, the concentration of gas in the cell, and the absorption properties of the gas. Evaluation programs in Matlab can handle the data produced. With such a setup, users can compare the performance of different cameras and perform tests often to confirm repeatability of their measurements.

AN OBJECTIVE MEASURE Another potential standard for measuring the performance of OGI cameras is the minimum detectable leak rate. That depends on the operator of the camera determining the smallest leak they believe can be seen, and it depends on the temperature contrast setting in the camera, the quality of the display, and the operator’s visual acuity, making it rather subjective. On the other hand, NECL is an objective measure.

It measures the average concentration of a gas over a given path length and tells users whether the absorption signal that gas produces is greater than the noise intrinsic to the camera, a useful way to characterise a camera’s performance. It is easily obtained with a simple setup, and it allows users to compare the capabilities of different cameras. It also allows them to select the requirements for their camera based on their application, including which gases they need to detect and at what concentration. As a standard, NECL can be very attractive to those who need to perform optical gas imaging.

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