Capacitance hygrometer

On the air side calibrated thermocouples to measure temperature at different locations in the cell two capacitive hygrometers on the evaporator a humidity controller designed to deliver a maximum vapor flow rate of lOkg/h have been installed. A liquid flowmeter to measure the volumetric flow rate of the aqueous desiccant solution at the membrane contactor outlet was used. All instruments are connected to a data logger. 

The capacitance hygrometer detects the change of capacitance, which is a function of plate area, plate spacing, and the dielectric constant of the material between the plates. The dielectric constant of a material has a unique value for each substance. Water, having a dielectric constant of 80, is a good candidate for this measurement. When used to measure the moisture content of solids, compensation is required for particle size, packing, and material density because they all affect capacitance. The same principle can also... 

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. 

Moisture analyzers include a large variety of designs listed here. The list includes their inaccuracies (1) electrolytic hygrometer (2-5% FS), (2) capacitance (3% FS), (3) impedance (3% FS), (4) piezoelectric (10% AR or 2 ppm by volume), (5) heat of adsorption, (6) infrared (0.5-1% FS), (7) microwave (for a 1-15% moisture range, error is within 0.5%, less if corrected for density), (8) Karl Fischer titrator (0.5-1% FS), (9) drying oven (0.5-1% FS), (10) dipole, (11) cavity ring down, (12) fast neutron (0.2% in solid s density corrected), and (13) radio-frequency absorption (5 ppm). 

Electric hygrometers have been the fastest-growing form of humidity measurement in recent years. They measure the electrical resistance, capacitance, or impedance of a film of moistureabsorbing materials exposed to the gas. A wide variety of sensing... 

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. 

Electric hygrometer - utilizing a substrate, such as a thin film of polymer or aluminum oxide. Water from a sample is absorbed by the film that causes changes in the electrical properties of the film, such as capacitance or resistance. The change of the electrical quantity is related to the amount of water vapor contained in the gas sample and the amount of water is calculated by use of a calibration curve derived from standards. 

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... 

---