Laser diode-based monitoring systems for air quality monitoring
As an alternative to the DOAS systems, OPSIS also offers gas analysis based on laser diode technology, TDL. TDL technology is a good choice if only single or a few types of air pollutants are to be monitored, or if a certain measurement performance for a specific type of pollutant is to be achieved.
THE ANALYSER - THE CORE OF THE Monitoring SYSTEM
The analyser used when applying the TDL technique is called LD500. Unlike DOAS systems, the light source is located inside the analyser. The source consists of a narrow-band laser diode which wavelength can be adjusted within a narrow wavelength range. The range is chosen to match a specific absorption line for a particular type of gas. One laser (laser assembly, "laser head") is therefore usually needed for each type of gas to be monitored. In some cases, one laser can monitor two types of gases. An LD500 can hold up to four laser heads.
In addition to the light source, the LD500 also contains electronics and computing power to analyse the signal from the receiver and calculate the level of pollutants.
THE MONITORING PATH - EMITTER AND RECEIVER
The levels of the air pollutants of interest are measured along a measurement path just like in a DOAS system, in this case with a combined emitter and receiver (ER120L) and a reflector (RR090L). However, the details are different. The light emitted is delivered from the analyser via an optical fibre. The received light is measured by a photosensitive detector located inside the emitter/receiver. The signal from the detector is digitised and sent to the analyser via optical fibres. The design of the retroreflector is also slightly different from that used with the ER120 of the DOAS system.
OPTICAL FIBRES
As with DOAS systems, emitters/receivers and analysers are linked by optical fibres. This makes it possible to have the emitter/receiver outdoors in harsh environments, while the more sensitive analyser can be indoors. Two types of fibres are used: OF005/010 which carries the laser light from the analyser to the emitter, and CF120 which carries the detector signals in digitised form back to the analyser.
The fibres can be very long, several hundred metres. This gives great flexibility in the choice of measurement site and location of the analyser.
COMPLETE AND FLEXIBLE SYSTEMs
A basic LD500 system for monitoring gas concentrations in ambient air consists of an analyser (LD500), a combined emitter/receiver (ER120L) and reflector (RR090L), and optical fibre cables between the analyser and the emitter/receiver. A single analyser can monitor concentrations along several different monitoring paths using multiplexers and demultiplexers. This provides a complete monitoring system that is both versatile and cost-effective.
It is straightforward to supplement an existing system with the monitoring of additional substances and on more monitoring paths.
BLOGS ABOUT OUR PRODUCTS
On our blog you can read more about our products and what they are used for, for example:
Inside the Gas Analyser – the Tuneable Diode Laser
