Robust optical measurement cell for sensitive <i>in-situ</i> characterization of reactive gas flows

We present an optical measurement cell that is capable of measuring <i>in-situ</i> gas-phase mole fractions, temperature, and pressure at high measurement frequencies of up to 10 kHz. The cell is designed for operating under harsh conditions, including high temperatures, reactive flows, soot pollution, and ensures reliable performance in challenging environments. Based on the principle of a White multi-pass cell, it features an inner diameter of just 70 mm and achieves absorption path lengths of up to roughly 5 m. As presented here, this enables <i>in-situ</i> detection of nitrogen monoxide at concentrations as low as single-digit ppm levels under engine exhaust flow conditions. The unique design accommodates both free-beam and fiber-coupled lasers, providing exceptional flexibility. Furthermore, its variable absorption path length enhances robustness, selectivity, and sensitivity, making it highly adaptable to demanding measurement scenarios.