Humidity Sensing for free—advanced thermoacoustic signal models in miniaturized photoacoustic gas sensors

This study proposes a model-based method to extract relative humidity (RH) values from non-resonant photoacoustic gas spectroscopy (PAS) sensors without using an additional physical humidity sensor. PAS sensors detect gases by injecting optical power of a specific wavelength into a closed pressure chamber, where the target gas absorbs the energy and releases thermal energy through collision processes, leading to a pressure signal that is detected by a microphone. However, PAS sensors are prone to cross sensitivity, especially from humidity, which can alter relaxation paths and interfere with the measured sound signal. The proposed method combines an analytical model that calculates the thermal conductivity of the atmosphere inside the pressure chamber with measured values of the energy content of the transient microphone signal to determine the RH values. The study demonstrates that this method can be applied to complement the system with the capability to detect humidity as well, i.e., to establish a humidity sensor without adding additional components or sensors, but only by the smart evaluation of the PAS signal. The so-established RH sensor in PAS system exhibits an absolute mean error of smaller than 5 % RH and makes the PAS systems more reliable and robust.