Achieving accurate and repeatable lowlevel moisture analysis , while minimizing instrument maintenance , has long been a challenge .

Several different technologies have been employed over the years , but Tunable Diode Laser ( TDL ) Spectroscopy has emerged as the clear technology leader , and continues to develop , with instruments using several versions of this technique available to the market .

The latest laboratory moisture analyzers are based on a technique known as Advanced-TDL , and can operate for years in non-ideal environments without requiring re-calibration . They are capable of yielding moisture measurements with ppb sensitivity in a wide variety of single and multi-component gas mixtures .

As with all TDL techniques , highperformance measurements are enabled by virtue of the ultra-narrow diode laser bandwidth , which in turn enables selection of a single fine-structure water absorption line .

The Advanced-TDL method uses additional information buried within the shape of the water infrared ( IR ) absorption line which , after correction for absolute pressure , is subtly dependent on the chemical composition of the host gas or gas matrix being investigated .

Using this additional data , Advanced-TDL analyzers can measure moisture levels across a diverse range of background gases without needing re-calibration . These analyzers remain highly tolerant of “ real world ” environmental effects such as temperature , pressure , vibration and contamination of the optical surfaces .

This unique feature of Advanced-TDL can be better understood by looking at how it determines the shape factor and area under the H 2O absorption line to improve the stability and sensitivity of the moisture measurement .

Absorption line-shape factors are analogous to the unique “ idents ” used by air traffic controllers to mark an individual aircraft on their monitoring screens .

Key information about the composition of the background gas or mixture is derived by determining the shape factor of the absorption line . Moisture concentration is derived by measuring the area under the absorption line .

DERIVATION OF MOISTURE CONCENTRATION FROM MOISTURE ABSORPTION LINE USING ADVANCED-TDL

H 2 O Absorption Derivation of Ident

Background Gas Type Ident = H / B

Note the characteristic “ valley , hill , valley ” shape of the fine structure water line in the 1854nm region of the IR spectrum . This comes about through the second harmonic detection technique used in Advanced-TDL analyzers .

The ident triangle is created by joining the turning points of this spectral absorption curve . The area of this triangle relates to the moisture concentration . Lower concentrations result in a smaller triangle . Yet , for a constant background gas , in all cases the height-to-base ratio remains the same ( i . e . a continuous set of similar triangles ).

The ratio of the base to the peak of this triangle yields the background gas ident shape factor .

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