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LOSLELm1

Technical Article: Open Path Hydrocarbon IR as Detectors

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As opposed to point detectors, the length of the measuring path of an open path detector is different from installation to installation. Normally, the measuring path is too long to be filled with a homogeneous gas concentration. In principle, an open path detector measures the total amount of gas present in the measuring path. Therefore, an open path detector will not distinguish between a small gas cloud with high concentration and a large gas cloud with a low level of concentration.

If the sensitivity of an open path detector is to be measured in %LEL, the length of the measuring path must be fixed and the gas concentration must be homogeneous over the entire measuring path. As open path detectors from practical reasons seldom are installed with fixed length measuring paths and gas concentrations are not normally homogeneous over the whole measuring path, gas sensitivity is not given in the same way as for point detectors.

The gas response of open path detectors is given in LEL x metres (abbreviated LELm), which is the product of the concentration and extension of the gas cloud. Thus, gas sensitivity becomes independent of the length of the measuring path, which makes it easy to compare the sensitivity of open path detectors. As an example, a gas cloud with a concentration of 1LEL and an extension of 5m will give a reading of 5LELm. Correspondingly, a gas cloud with a concentration of 5LELm and an extension of 1m will give a response of 5LELm.

Most Point flammable gas point IR detectors are calibrated for methane gas and monitor the region around 3.3 microns. Line of sight (LOS) open path detectors typically monitor the region around 2.3 microns. The 3.3 and 2.3 micron regions are known as fundamental and overtone wavelengths.

LOSLELm

Gas leaks in the form of gas clouds

If there is a gas leak, the highest gas concentration of the gas cloud will always be at the source of the leak. The gas concentration will get more and more diluted with increasing distance from the source of the leak. There are no typical gas clouds. The extension and concentration of gas clouds are dependent on the volume of the leak, wind direction and force and whether the gas is lighter or heavier than air. A frequently used definition of a hazardous gas cloud is: A gas cloud with a concentration of 100 % LEL or more and an extension of at least 4 to 5 m.

 

Open path detectors versus point detectors

On basis of the above definition, point detectors are normally installed 4 to 5m apart and positioned in relation to potential leak spots, ignition sources and surrounding environment. In order to compare the detection capability of open path and point detectors at different types of gas leak, open path and point detectors with the following gas sensitivity are used:

- Open path detector   5LELm =Full scale reading (FS)

- Point detector           100%LEL = Full scale reading (FS)

 

Gas leak generating a gas cloud with a concentration of 100 % LEL and an extension of 5m

A point detector located within the gas cloud will give a reading of 100%LEL=100%FS

An LOS detector monitoring the entire gas cloud will give a reading of 5LELm=100%FS.

The sensitivity of the two detectors is identical.

A gas cloud will, however, be heavily diluted with increasing distance from the leak source. Provided that the gas cloud has a concentration of 100 % LEL at a distance of 5 m from the leak, the concentration could be as low as 12 % LEL at a distance of 10 m.

 

Gas leak generating a gas cloud with a concentration of 12%LEL with an extension of 8m

A point detector located within the gas cloud will give a reading of 12%LEL = 12%FS.

An open path detector  monitoring the entire gas cloud will give a reading of 1 LELm =20%FS.

In this case, the sensitivity of the open path detector is factor 1.7 higher than the sensitivity of the point detector.

 

In order to maintain good sensitivity when there are gas leaks, open path gas detectors should be located near potential sources of gas leaks. Open path detectors are only able to measure the part of the gas cloud that is inside the measuring path. To enable detection of large gas clouds with low concentration and to avoid reduced detection capability, it is crucial to locate the detectors in such a way that the whole gas cloud comes within the measuring path.

 

 


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