Tuneable Diode Laser (TDL) Gas Analysis

The LGA analysers use laser spectroscopy to generate a concentration signal for specific gas species.  The laser beam from the transmitter unit passes across the measurement path (process duct for in-situ type, measurement cell for extractive type) where some of the energy is absorbed by the measured gas.

This attenuated light is detected by a photoelectric sensor in the receiver unit, and a proportional concentration value is derived using the Beer-Lambert relationship.

Different compounds absorb light at different energy levels which relate to the way in which the molecules vibrate and rotate.  The absorption characteristics of different gases produce a unique pattern, sometimes referred to as the ‘fingerprint’.  Detailed examination reveals a hyper-fine structure consisting of a number of narrow absorption bands.
Conventional light sources emit energy over a wide spectral range, potentially correlating to a number of different absorption bands and capturing responses from a number of different molecules – cross-sensitivity.

The LGA TDL analysers use a tuneable diode laser as the light-source which is controlled to emit energy over a very narrow range.  With a spectral width as low as 0.01nm, the transmitted energy can correspond to a ‘single line’ in the fingerprint which is unique to the measurable compound, and eliminates uncertainty caused by cross-interfering molecules.

The laser sources can be tuned to correspond to different molecules, allowing the creation a range of analysers measuring different components with the same characteristic specificity.

Measurable gases include:

• O2
• CO
• CO2
• H2O
• H2S
• HCl
• HF
• NH3
• HCN
• CH4
• C2H2
• C2H4