Highly sensitive Laser Photo-Acoustic SF6 Gas Analyzer with 10 decades dynamic range of concentration measurement

Abstract

A highly sensitive Laser Photo-Acoustic (PA) SF6 Gas Analyzer with dynamic range of ∼10 decades (∼0.1 ppb–100% SF6) has been developed and tested. Gas analyzer is built on the base of RF-excited waveguide CO2 laser, resonant differential photo-acoustic detector (PAD), and sealed-off SF6-gas-filled Ref-cell. The resonant frequency of differential PAD is measured automatically in real time (∼1750–1780 Hz, error ± 0.1 Hz). While measuring SF6 concentration (up to 1000 ppm), normalization of PAD’s absorption signals according by Ref-cell absorption signals significantly reduce the error in PA gas analyzer readings caused by spontaneous tuning of waveguide CO2 laser wavelength near ∼10.6 μm. PA gas analyzer’s background readings was ∼0.1 ppb SF6, the standard dispersion of background signal was ∼10 ppt SF6. Minimum average power of waveguide CO2 laser radiation should be no less than ∼150 mW. Dependence of speed of sound vs gas mixture composition was used to measure SF6 concentration in 0.1–100% range. Resonant frequency of differential PAD varied from ∼1780 Hz (pure N2) to ∼690 Hz (pure SF6), which, after appropriate calibration, makes it possible to measure SF6 concentration up to 100%.