Hygrometers psychrometers

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... 

The simplest way to calibrate a hygrometer is to use ambient air. The humidity of ambient air is measured using both the calibrated meter and the reference meter. A simple psychrometer or an Assmann hygrometer can be used as a reference. Provided the air conditions are stable, this method is used to check hygrometers in field measurements and other not too demanding purposes. 

In reverse, by measuring the wet bulb temperature the moisture content XA of the humid air can be calculated using the two equations. The wet bulb hygrometer, sometimes called Assmann s Aspiration Psychrometer works by this method. It consists of two thermometers, one of which is covered by a porous textile, the lower part of which is dipped into a container full of water, so that the bulb of the thermometer is always wet. Air is blown over the thermometer and after some time it reaches the wet bulb temperature Meanwhile the second, so called dry thermometer, indicates the temperature of the air flowing around it. With these measured values the moisture content XA5 of the humid air can be found using equation (1.210) or (1.211). 

The actual amount of moisture in air is determined by paseing a known volume through tubes filled with calcium chloride whose increase in weight representa the amount of H,0 in the volume of air used. The traction of aaturatioD is detenmned by instruments called hygrometers, hygro-accpes or psychrometers. 

PSYCHROMETER OR WET BULB HYGROMETER - An instrument for measuring the relative humidity of atmospheric air. 

Psychrometer sI- kra-mo-t3r [ISV] (1838) n. A wet-and-dry bulb type of hygrometer. Used for the determination of relative humidity. 

The main types of instruments measuring humidity in gases are various types of hygrometers (gravimetric, mechanical, condensation, infrared absorbance detector, electric sensor, thermal conductivity, Al203/sili-con, P2O5) and psychrometers. 

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