Silicon thermal anemometry: Developments and applications

In this paper we present a comprehensive discussion of silicon thermal anemometry. Its possibilities and the current state of the art are discussed in detail. Based on forced convective heat transfer, it is seen how silicon technology can be used to make thermal flow sensors. The mechanism is based on development of a thermal hydrodynamic boundary layer across a heated silicon chip due to a forced flow. It is shown how this mechanism is converted into an electrical signal. Important parameters such as possible sensing elements and geometries are discussed. Different biasing modes to operate thermal sensors are presented. One of the very basic issues in silicon thermal anemometry is drift. The performance can be expressed in terms of a quality factor Q_s = S / (D + N), where S is the desired signal, D is the drift and N is the contribution of noise. The control of this drift term plays a crucial role in the accuracy of silicon thermal flow sensors. A novel method to eliminate drift for thermal anemometry, namely the alternating direction method, is presented. In particular this method plays a crucial role when extremely low and long-term volume measurements must be performed. Examples of measurements are given. With regard to its applications, the type of packaging is a major issue. Dependent on its application, also the frequency behaviour is important. It is discussed briefly. Finally, applications and future developments of silicon thermal anemometry are discussed.