ABSTRACT
Gravimetric or mass-sensitive humidity sensors include a large group of devices, such as quartz crystal microbalance (QCM) or bulk acoustic wave (BAW) devices, surface acoustic wave (SAW) sensors, and either microbeam or membrane resonators. All gravimetric sensors differ constructively, but they have a similar humidity-sensitive mechanism. Mass-sensitive sensors rely on disturbances and changes to the mass of the sensor surface during interaction with water vapor. In other words, mass-sensitive devices transform the mass change at a specially modified surface into a change in some property of the support material. In this chapter, the reader will find a detailed description of QCM and SAW-based humidity sensors, developed both on the basis of conventional structures and thin-film resonators, thin-film bulk acoustic resonators, and solidly mounted resonator structures. Various humidity-sensitive materials used in gravimetric humidity sensors are also subjected to detailed consideration. It is shown that SAW and QCM devices are small, sensitive, inexpensive, and easy to fabricate, and can work without a local power source. This means that SAW and QCM-based humidity sensors are devices that are promising for various applications, including development of dew-point hygrometers. Advantages and disadvantages of sensors based on various materials are also analyzed.
