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.

P06