Abstract
A compact laser photo-acoustic (PA) methane sensor based on a quantum-cascade laser (∼7.7 μm; 1750 Hz; 25 mW), a resonant differential photo-acoustic detector (PAD), and a sealed-off gas-filled PA Ref-cell has been developed. Normalization of the absorption signals in the PAD is carried out according to the absorption signals in the gas-filled PA Ref-cell, which significantly reduces the measurement errors of the methane concentration in the case of instability of the laser emission wavelength. The minimum measured background signal of the PA sensor (using high purity nitrogen) is nmin≈(26.6 ± 8.4) ppb CH4 (at a bandwidth of 20 Hz), the value of normalized noise equivalent absorption (NNEA) = 8.22 × 10–10 cm−1·W/Hz1/2. A comparison of various research groups results with mid-IR PA gas analyzers is carried out. The developed PA methane sensor is adapted for placement on the UAV’s board. Device has dimensions of 315 × 165 × 110 mm, weight ∼3.1 kg, power supply from an external source (9…60 VDC), power consumption ∼20 VA.
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