A Novel Conductive Humidity Sensor

This content is excerpted from Sensor Technology Alert and Newsletter, a sensor intelligence service published by the Technical Insights unit of Frost & Sullivan.

Sensor Technology AlertKey characteristics and specifications for evaluating the performance of a humidity sensor are accuracy, repeatability, interchangeability, long-term stability, ability to recover from condensation, resistance to chemical and physical contaminants, size, packaging, and cost efficiency. In recent years, with advancements in materials technology, as well as in such technologies as thin film deposition, ion sputtering, and ceramic/silicon coatings, humidity sensors with high accuracy and resistance to chemicals and physical contaminants are more readily available at economical prices. Humidity sensors can be categorized into such technologies as capacitive, resistive, electro-optic, psychrometer (matched thermometers), optical, thermoelement, and so on.

At present, most humidity sensors are designed to detect humidity through changes of electrical properties, such as electrical resistance, adopt electrolytes, metal oxides, organic polymers, or porous semiconductors as sensitive materials. Capacitive polymer relative humidity sensors can offer such advantages as a relatively wide humidity and temperature range, very minimal hysteresis, good stability and repeatability, low temperature coefficient, and a relatively rapid response time (except after condensation on the upper electrode or in situations where air is not flowing across the sensor at high velocity). Resistive polymer relative humidity (RH) sensors have tended to cost less than capacitive RH sensors and can have a slower response time than capacitive RH sensors, but also can provide high accuracy over their humidity range. Thermal conductivity sensors can purportedly perform well in corrosive environments and at high temperatures.

Applications for capacitive and resistive RH sensors include building ventilation control, industrial drying, process monitoring, museums/archives, animal incubators, biomedical analysis, automotive/transportation (for example, truck engine management, windshield defogging). Moreover, there are opportunities for high-volume applications for lower-cost RH sensors in such areas as automotive, white goods, printers, and copiers.

The field emission properties of multiwalled nanotubes (MWNTs) have recently aroused a research interest, driven primarily by a wide range of potential commercial applications including bright electron sources and flat panel displays. Owing to the extremely sharp radii of nanotube tips, even a relatively moderate negative voltage at the nanotube tip is sufficient to cause field emission of electrons.

Recently, a group of researchers from the Center of Biomimetic Sensing and Control Research, Hefei Institute of Intelligent Machines, Chinese Academy of Sciences, has demonstrated a novel and high-efficiency humidity sensor. Details on this work will be published in an upcoming issue of the journal Sensors and Actuators A: Physical. In this sensor, the MWNTs grown on the silicon substrates served as anode, and indium tin oxide film glass plate acted as cathode. The cathode and anode were separated and insulated by a glass insulator. Application of a positive bias to the MWNTs generated high electric field sufficient to field-ionize passing gas-phase atoms especially for water vapor. The enhancement in the electric field at the sharp tips of MWNTs enables humidity sensors to work at a low voltage and micropower consumption. These sensors show promise for real-time monitoring of humidity.

The innovative humidity sensor presented by this group is expected to be widely used in a variety of measurement and control applications, including process control, meteorology, agriculture, and medical equipment. The commercialization of this product is expected in two years. This group may seek some companies for collaboration when this humidity sensor becomes more applicable. Subsequently, they plan to use the sensor to detect other gases, such as some organic vapors.