Fence line monitoring around chemical plants is usually done with either open-path FTIR systems or with extractive FTIR systems. Both are described below.

Open-path FTIR
Open-path FTIR systems in fence line monitoring work by passing a beam of infrared light from one end of the fence to the other. Different configurations can be found, some of which are: 1) an IR source is on one end and the FTIR is on the other; 2) the FTIR and IR sources are near each other and a mirror (cornercube array, typically) is placed at some distance to reflect the light from the source back to the FTIR; or 3) the FTIR is the light source and the modulated beam is sent some distance and again reflected back to the detector. This last configuration suffers from a lower signal, but tends to provide the most accurate quantitative results.

So, when process or fugitive emissions are released from the plant and drift toward the fence line, they are measured by reducing the infrared signal at the FTIR detector. This technology, although very sensitive and able to observe gases leaving the fence line of a facility, tends to be expensive and prone to analysis errors if the analyzer is not operated by someone skilled in the technology.

Click here to read "Why FTIR for Air Monitoring?"
 
Extractive FTIR
Fence line monitoring can also be done using extractive FTIR analysis (sample drawn into the analyzer gas cell). With this technology, the FTIR is moved along the fence line, a sample probe is moved, or a sample multiplexer is switched to provide sampling at multiple points along the fence. Generally, in this mode a sample pump is continuously drawing samples into the gas cell to produce real-time results. Extractive FTIR along a fence line can provide a very accurate point measurement; however, if you do not move along the fence you may not be able to produce a good quantitative amount of chemicals leaving the facility. This is due to the fact that obstructions and wind currents can quickly move around the points of high and low concentrations.

We can assist you with your fence line application and help ensure accurate measurements.
To talk with an FTIR or Stack Testing Specialist, contact us by email or call us at 989-772-5088