Polymeric Sensors and Actuators

Chapter 4

Humidity Sensors

Humidity is a universal parameter of common environments and its control is recognized to be very important in a variety of fields, such as industries related to precision manufacturing and the fiber, food, and electronics industries (1).

Humidity sensors based on polymer thin films have been reviewed (2). Special methods of polymerization and functionalization can improve performance.

Sensor materials can be characterized by the amount of water absorbed under comparable conditions. The amount of absorbed water can be measured by a microbalance, or spectroscopic techniques. Sorption isotherms have been presented for a variety of polymers (3). In addition, spectroscopic techniques can elucidate the mechanisms of sorption.

Humidity sensors based on polymer thin films with electric response fall into two categories (3,4):

• Resistance type humidity sensors
• Capacitance type humidity sensors

A resistance type humidity sensor detects the relative humidity (RH) by measuring the change in the resistance of a sensor element. In comparison, a capacitance type humidity sensor detects the humidity by measuring the change in the electrostatic capacity of a sensor element (4).

Capacitance type humidity sensors typically do not exhibit a satisfactory linear relationship between the capacitance and the humidity, so an external circuit is required to overcome this disadvantage. However, this drawback increases the manufacturing cost of capacitance change type of humidity ...