3. Sensor design

Measurements are only possible if the sensor is able to inject sufficient acoustic power into the gas. The zirconium tube is a medium with a high acoustic impedance, 30.9 Mrayl, compared with around 300 rayl for the gas. As with a Fabry-Perot interferometer, multiple reflections from the tube walls can be used to inject waves into the gas. The wave can be transmitted if the cladding thickness is a multiple of the half-wavelength. For a thickness of e = 570 μm, the first frequency at which this phenomenon occurs is: f = 4.14 MHz. At this frequency, both the thickness of the piezoelectric ceramic (PZT) and that of the cladding must be equal to the half-wavelength.