Chemical humidity sensors

The physical and chemical properties of MgO films prepared by the sol-gel technique were the area interest of the examinations presented by Shukla75. The aim of mentioned work was to produce films with nano size particles so as to employ them for the sensor applications, as adsorption in such films increases many folds due to the increase of surface area. Infra-red spectroscopic studies indicated the presence of solvent in the precursor, which helped in decomposition to nano-particles during nucleation of the film. The MgO sol-gel films were deposited on the glass rod bend in U-shape for humidity sensor. 

Humidity sensors are often technologically closely linked to chemical sensors and can be used e.g. to determine the drying cycle length in tumble dryers and dishwashers, by determining the remaining air humidity caused by wet laundry or dishes. 

Most microhotplate-based chemical sensors have been realized as multi-chip solutions with separate transducer and electronics chips. One example includes a gas sensor based on a thin metal film [16]. Another example is a hybrid sensor system comprising a tin-oxide-coated microhotplate, an alcohol sensor, a humidity sensor and a corresponding ASIC chip (Application Specific Integrated Circuit) [17]. More recent developments include an interface-circuit chip for metal oxide gas sensors and the conccept for an on-chip driving circuitry architecture of a gas sensor array [18,19]. 

L. Sabbatini, and P. G. Zambonin, NTCDA organic thin-film transistor as humidity sensor weaknesses and strengths , Sensors And Actuators B -Chemical 77, 7 (2001). 

The humidity of the room where the product is exposed should be considered a critical parameter when a humidity-sensitive product is being manufactured. The humidity sensors and the humidity monitoring system should, therefore, be qualified. The heat transfer system, chemical drier or steam humidifier, which is prodiicing the humidity controlled air, is further removed from the product and may not require operational qualification. 

Zeolite membranes and films have been employed to modify the surface of conventional chemical electrodes, or to conform different types of zeolite-based physical sensors [49]. In quartz crystal microbalances, zeolites are used to sense ethanol, NO, SO2 and water. Cantilever-based sensors can also be fabricated with zeolites as humidity sensors. The modification of the dielectric constant of zeolites by gas adsorption is also used in zeolite-coated interdigitaled capacitors for sensing n-butane, NH3, NO and CO. Finally, zeolite films can be used as barriers (for ethanol, alkanes,...) for increasing the selectivity of both semiconductor gas sensors (e.g. to CO, NO2, H2) and optical chemical sensors. 

Ceramic materials have an advantage in terms of mechanical strength and thermal and chemical stability. A number of investigations have been carried out on humidity sensors utilizing porous ceramic elements. There have been a number of reviews of ceramic humidity sensors [1, 5], and some examples of recent developments are introduced in this chapter. 

The impedance-type humidity sensor has the advantages of simple structure and rapid response. Moreover, long term operation becomes possible when the element has been stabilized by coating with a resin film. This type of sensor is also easily compatible, depends on the chemical structure of the polymers, and, in principle, enhancement of the water sensitivity can be achieved by cross-linking or copolymerisation. 

Q. Kuang, C. Lao, Z. L.Wang, Z. Xie, L. Zheng, High-Sensitivity humidity sensor based on a single Sn02 nanowire . Journal of American Chemical Society, 129(19), 6070-6071, (2007). 

The range of sample characteristics and manner of their detection, is much larger than can be realistically addressed in the space of a single chapter. We will confine this chapter mainly to the chemical sensor research areas discussed in other chapters in this volume, dividing them into electrical, optical, and mass and thermal measurements. Our focus will furthermore be on the generic chemical and physical phenomena upon which such measurements can be based, as opposed to the alternative organization that would address chemical sensors in the context of their application (i.e, auto exhaust sensor, clinical diagnostic sensor, environmental sensor) or of the kinds of samples detected (i.e, CO sensors, humidity sensor, biosensor, etc.), as used in a previous ACS Symposium Series volume on Chemical Sensors (D. Schuetzle, R. Hammerle, Eds., ACS Sympos. Ser. 309, 1986). 

Z. Ankara, T. Kammerer, A. Gramm, etal., Low power virtual sensor array based on a microma-chined gas sensor for fast discrimination between H2, CO and relative humidity, Sensors and Actuators B Chemical, vol. 100, no. 1, pp. 240-245, 2004. 

Humidity sensors The measurement of humidity is essential for various industrial process operations, such as in the control of drying plant, ovens, and effluent gases from metal refining furnaces, mineral processing kilns, power plants, chemical plants, and incinerators. Adequate expression of the measured gas pollutant concentrations (on a dry basis) relies on a reliable measurement of the humidity in the process environment. 

Among the various categories of smart textiles and flexible materials, which include optically, mechanically, chemically, electrically, and thermally activated substances/ stmctures (Tao, 2001), several have shown large opportunities of applications in PPE. This includes wearable electronics, for example, physiological condition, temperature, and humidity sensors, power and data transmitters, and end-of-life indicators, which are the subject of the next chapter of this book. Various types of smart flexible materials have also found their way into PPE, for instance, as responsive barriers, self-decontaminating membranes, thermoregulating layers, and shock-absorbing patches. 

McGovern, S.T., Spinks, G.M., Wallace, G.G., 2005. Micro-humidity sensors based on a pro-cessable polyaniline blend. Sensors and Actuators B Chemical 107 (2), 657—665. 

Nohria, R., et al., 2006. Humidity sensor based on ultrathin polyaniline film deposited using layer-by-layer nano-assembly. Sensors and Actuators B Chemical 114 (1), 218—222. 

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