Hygrometer, wet and dry bulb

Both the heat and mass transfer coefficients are functions of air velocity. However, at air speeds greater than about 15 ft/s (4.5 m/s), the ratio h kgis approximately constant. The wet-bulb depression is directly proportional to the difference between the humidity at the surface and the humidity in the bulk of the air. In the wet- and dry-bulb hygrometer, the wet-bulb depression is measured by two thermometers, one of which is fitted with a fabric sleeve wetted with water. These thermometers are mounted side by side and shielded from radiation, an effect neglected in the derivation above. Air is drawn over the thermometers by means of a small fan. The derivation of the humidity from the wet-bulb depression and a psychrometric chart are discussed later. 

Many wet- and dry-bulb hygrometers operate without any form of induced air velocity at the wet bulb. This may be explained by examining another air-water system. If a limited quantity of air and water is allowed to equilibrate under conditions in which heat is neither gained nor lost by the system, the air becomes saturated and the latent heat required for evaporation is drawn from both fluids which cool to the same temperature. This temperature is the adiabatic saturation temperature, T. It is a peculiarity of the air-water system that the adiabatic saturation temperature and the wet-bulb temperature are the same. If water at this temperature is recycled in a system through which air is passing, the incoming air is cooled till it reaches the adiabatic saturation temperature at which point it is saturated. The temperature of the water, on the other hand, remains constant and all the latent heat required for evaporation is drawn from the sensible heat of the air. Equilibrium is expressed by Eq. (17),... 

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. 

The wet-and-dry bulb hygrometer consists of two thermometers that are adjacent to each other. The diy-bulb thermometer measures the temperature of the air. The bulb of the other thermometer is kept wet by a wick dipped in water. As the water evaporates due to the air flowing across, the wet bulb is cooled. 

Wet and Dry Bulb Hygrometer.—Again, the difference in temperature which is usually observed between the wet and dry bulb in the ordinary hygrometer depends on the difference between the partial pressure of the aqueous vapour in the air and the maximum pressure possible at the temperature of the dry bulb, that is to say, the vapour pressure of water at that temperature. 

Por details of other types of hygrometers, including the wet and dry bulb thermometer, the reader is referred to text-books of Physios and Meteorology. 

Figure 1 depicts the experimental apparatus used in the determination of heat and mass transfer coefficients.A compressor (A) feeds the air to a tank (B), to minimize pulse fluctuations in flowrate. The air is dried as it passes trough a bed of silica gel (C). Air flowrate is measured with a rotameter and in addition with a calibrated capillary meter(E). The inlet air moisture content is measured by means of a dry and wet bulb thermometer system (D) prior to its entrance to a coil submerged in a constant temperature bath (F). From here, the air enters the bottom of the fluidized bed (G) where its temperature is measured.The fluidized bed consisted of an insulated QVF glass tube 2 inches in diameter and 12 inches in length. A system for the collection of fines(I) was installed after the bed to evaluate entrainment, although at all experimental conditions used in this work entrainment was absent. A thermometer placed on top of the bed of solids was used to measure the temperature of the bed exit.Air moisture content was also determined at the outlet of the fluidized bed by means of a hygrometer and a wet and dry bulb temperature system(J). 

Relative humidity is difficult to measure reliably and instead it is determined indirectly from the wet- and dry-bulb thermometers of a hygrometer. The wet-bulb thermometer is kept moist with a fabric sleeve whose other end is in a reservoir of clean water. As air passes over the wet sleeve water is evaporated and cools the wet-bulb thermometer the drier the air the greater the cooling effect. The dry-bulb measures the air temperature there is no cooling effect on the dry-bulb thermometer. The difference between the dry-bulb and wet-bulb temperatures, the wet-bulb depression (AT), and the dry-bulb temperature are the parameters used to... 

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

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 occupier of a humid factory is required to inform the factory inspector in writing when artificial humidity is first produced (s. 68). Every room in which artificial humidity is produced should have two hygrometers and a humidity table. The readings of the hygrometers shall be taken at certain times of the day and entered on the prescribed form, F48. Artificial humidification is not permitted in a room in which the wet bulb thermometer exceeds 22.5 C (72.5 F) or for certain processes 26.5 C (80 F) or when the difference between the readings of the dry and wet bulb thermometers is less than that indicated in the humidity table. No water which is liable to cause injury to health shall be used for artificial humidification. 

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