New differential mode excitation photoacoustic scheme for near-infrared water vapour sensing

A new gas sensor based on the differential mode excitation photoacoustic (DME-PA) technique is presented. The DME-PA method utilizes the selective excitation of two different modes in a resonant photoacoustic cell and the gas concentration is derived from the amplitude ratio of these acoustic modes. The presented device has only one cell that acts as both sample and photoacoustic cell and uses a current-modulated near-infrared light emitting diode as excitation source. No power-meter is required for PA signal normalization. The new DME-PA sensor was tested with water vapour yielding a ±250 ppm uncertainty for the water vapour content of ambient air.