|Open-path monitoring uses an optical telescope to transmit the infrared beam through the air monitoring compounds present in the air mass. The telescope based instrument can operate: in two modes 1) monostatic mode in which a single telescope is used at one end of the path and a passive retro reflecting mirror is at the other end or 2) bistatic in which transmitting and receiving telescopes are used at opposite ends of the path. Imacc offers both types of systems. In bistatic mode the gas detection limits are somewhat better but the systems are harder to align and maintain. Bistatic mode allows for a single transceiver (transmitter/receiver) to scan multiple paths using several retro mirrors.
Typical applications of open-path monitoring include fence-line monitoring of industrial sites, urban air monitoring in metropolitan areas, accidental release detection/identification, and homeland security applications.
The system at right is an Imacc monostatic system built for the US EPA. It is on a motorized azimuth-elevation mount that can automatically scan to multiple retro mirrors placed vertically or horizontally. This allows for area monitoring and use in SMART LDAR programs. The far right picture is a one of several 60 element retro arrays installed at a chemical plant in the US.
For industrial applications, the scanning FTIR is usually placed in a small shelter, as seen on the right. This shelter contains the FTIR, its control computer, and an auto-fill system for liquid nitrogen which is required for the detector. In the far ight picture the auto-fill system can be seen on top of the telescope. If one does not want to use liquid nitrogen refrigeration units are also available.