Hygrometers capacitive sensors

The more sophisticated injection-humidity cabinet permits a wide variation in temperature and humidity to be created with a few simple settings of the controls. The humidity is measured by a suitable moisture sensor, such as a wet and dry bulb hygrometer or capacitive sensor, and this is used to control the injection of moisture into the chamber. Through the use of suitable control circuits it is also possible to cause such a chamber to cycle in temperature and/or humidity so that varying ambient conditions over wide extremes may be simulated for assessing such effects on the environmental resistance of polymer products. 

Capacitance Hygrometer. This method measures water vapor pressure by the use of an Aluminum Oxide (AI2O3) sensor. The conductance of the sensor varies with the amount of absorption of water vapor. 

There are many types of devices such as mirror-chilled hygrometers, psychrometers, optical sensors, and solid-state sensors which can be used for humidity control (Nitta 1981 Rittersma 2002). Mirror-based dew point sensors have better accuracy, but these devices are more costly to make and use (Chen and Lu 2005). Solid-state humidity sensors can be fabricated at low cost and used for moisture control more conveniently. Solid-state humidity sensors are broadly classified into three types gravimetric (QCM and SAW), resistive, and capacitive, according to the output form of the sensor... 

Mathews DA (1963) Review of the lithium chloride radiosonde hygrometer. In Proceedings of the conference on humidity and moisture, vol VI, Washington, DC, pp 219-227 Matsuguchi M, Sadaoka Y, Sakai Y, Kuroiwa T, Ito A (1991) A capacitive-type humidity sensor using cross-linked poly(methylmethacrylate) thin films. J Electrochem Soc 138 1862-1865 Matsuguchi M, Sadaoka Y, Nosaka K, Ishibashi M, Sakai Y, Kuroiwa T, Ito A (1993) Effect of sorbed water on the dielectric properties of acetylene-terminated polyimide resins and their application to a humidity sensor. J Electrochem Soc 140 825-829... 

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