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01 May 2004

Fixed-point infrared gas monitors

What every engineer should know

Detecting combustible gas leaks saves lives and property, but because the economic value of gas monitoring equipment is hard to quantify, engineers need to do more with less. Compliance officers require reliability and adherence to safety mandates. Infrared gas detectors are an answer in many situations.

Combustion requires three elements: oxygen (air), fuel (combustible gas or vapor), and a source of ignition. For combustion to occur, fuel needs to mix in proper proportion with oxygen. Combustible gas-air mixtures can burn or explode over a range of concentrations. An ignition source will cause an explosion or flame front propagation over a specific minimum concentration for each gas. This minimum concentration is the lower explosive limit (LEL) or lower flammable limit (LFL). Combustible gases also have an upper explosive limit (UEL). When the concentration of combustible gas goes above that limit, there is not enough oxygen to support an explosion or flame. The LEL and the UEL are different for each combustible gas. For most combustible hydrocarbons, this minimum concentration is between 0.5% and 15% by volume in air. For example, the LEL for methane is 5% gas by volume, and the UEL is 15% by volume. Pentane's LEL is 1.5% by volume, and the UEL is 7.8% by volume, so its explosive range is even narrower.

Detecting combustible gases

When you use infrared to detect combustible gases (selective absorption of infrared), you will find one of its primary benefits can also be one of its problems. A catalytic sensor can detect all combustible gases; however, most infrared devices can only detect the gas they were designed to detect. In most cases this is methane and a few other gases that happen to share the absorption wavelength band. The only ways to solve this problem are to use multiple detectors in a device or to use multiple devices—each intended to detect a particular gas. In addition, manufacturers choose a filter that works best with methane, but they often can't see other combustible gases, such as the aromatic compounds benzene and toluene.

Here is another problem with single-gas infrared sensors: once you set them to see one gas, they can significantly misinterpret other combustible gases. Unlike catalytic sensors, the response of infrared sensors is far from linear; the software must interpret the curve based on the gas it is expecting. If it sees another gas, the absorption curve of that other gas can be (and usually is) very different, leading to readings that can be more than 300% off from what they should be.

For fixed-point detectors, you can engineer the ideal path length into the product. For instance, with methane detectors designed to detect 100% LEL, the ideal path length would be between 4 and 7 inches. However, for open path products, the larger path lengths used often mean the product is flooded well before reaching 100% LEL. With a path of 5 feet, an infrared detector might not be able to detect more than 10% LEL of methane before flooding, assuming a constant methane/air concentration everywhere between the light source and the light receiver. The environment can compound this problem during hot, humid conditions when water vapor absorbs much of the light.

Open path detectors can see sources of infrared other than the one the manufacturer chooses. Solar radiation, hydrocarbon flames, even flashbulbs produce broad-spectrum infrared radiation, which could trick the detector, unless designers provide countermeasures in the design.

When choosing an infrared gas monitor, deal directly with a manufacturer rather than with a company relabeling another company's product. Infrared sensors are complex, and if there ever is a problem, it may be easier to solve directly with the manufacturer. Be sure the production and repair are in your country. Trying to ship a defective product across an international border can be expensive.

Choose a sensor approved by Factory Mutual (FM) Approvals and by Canadian Standards Association (CSA). Choosing manufacturers who have both certifications will assure the detectors are professionally made. And you will know independent sources have verified their performance and owner's manual claims. CSA and FM Approvals audit the manufacturers they approve at least once a year. 

Behind the byline

Gerald L. Anderson is chair of the ASTM Leak Testing Committee, a member of the U.S. Technical Advisory Group on NDT, and chief executive officer of Delphian Corp.