Combined Humidity- and Temperature Sensor

Abstract

Temperature and humidity are very important parameters by measuring gas concentration. Most of the measuring procedures are showing a high temperature dependency; hence, the exact knowledge of the medium temperature is necessary in order to carry out high accurate measurements. While temperature compensation can be found in most gas measuring systems, the humidity of the medium is neglected in many cases. For rough concentration measurement this limitation can be accepted, but for highly accurate measurements humidity acts like an interfering gas.

The measurement of both these parameters can happen via different procedures. For the temperature measurement, an often implemented system is the use of characteristics of the gas measuring procedures (e.g., diode structures at silicon-based semiconductor gas sensors) or present resources in the application (e.g., resistance temperature sensor in processing units). The measurement of the moisture is clearly more difficult to integrate into the systems. Most frequently, type of moisture detection is realized with capacitive humidity sensors. These sensors allow the measurement of the relative humidity by evaluation of a capacitive signal obtained by a humidity-sensitive polymer. The knowledge of the temperature in these sensors is also indispensable for the determination of the absolute humidity respectively of the dew point.

The relative humidity has a strong temperature dependency (especially in the high humidity range). Therefore, it is an advantage to measure the temperature close to the humidity sensor. Thereby measurement mistakes could be inhibited effectively.

In this chapter, the measurement principle of thin film temperature sensors and capacitive humidity sensors, and examples for integration of both these technologies are described.