Humidity, relative readings

Psychrometry has to do with the properties of the air-water vapor mixtures found in the atmosphere. Psychrometry tables, published by the US Weather Bureau, give detailed data about vapor pressure, relative humidity and dew point at the sea-level barometer of 30 in Hg, and at certain other barometric pressures. These tables are based on relative readings of dry bulb and wet bulb atmospheric temperatures as determined simultaneously by a sling psychrometer. The dry bulb reads ambient temperature while the wet bulb reads a lower temperature influenced by evaporation from a wetted wick surrounding the bulb of a parallel thermometer. 

The corresponding relative humidities and wet bulb temperatures and corresponding humidities are read off a psychrometric chart. The equilibrium moisture is found from the relative humidity by Eq. (2). The various corrections to the rate are applied in Eq. (3). The results are tabulated, and the time is found by integration of the rate data over the range 0.1 [Pg.238]

The dew point is the temperature at which a vapor condenses when cooled at constant pressure. If air of the condition denoted by point A in Fig. 4 is cooled, the relative humidity increases until the mixture is fully saturated. This condition is given by point B the temperature coordinator is the dew point, which can be measured rapidly by evaporating ether in a silvered bulb. The temperature at which dew deposits from the surrounding air is noted and the humidity is read directly from a psychrometric chart. 

Table 11.4 Solutions for Maintaining Constant Humidity Table 11.5 Concentration of Solutions of H2SO4, NaOH, and CaCi2 Giving Specified Vapor Pressures and Percent Humidities at 25°C Table 11.6 Relative Humidity from Wet and Dry Bulb Thermometer Readings Table 11.7 Relative Humidity from Dew Point Readings...

TABLE 11.6 Relative Humidity from Wet and Dry Bulb Thermometer Readings... 

TABLE 11.7 Relative Humidity from Dew Point Readings... 

A comparison of wet and dry bulb readings allows the relative humidity to be determined from a psychrometric chart. The wet bulb temperature is always lower than the dry bulb value except when the air is already saturated with water - 100% relative humidity. This is when the wet and dry bulb temperatures are the same. Tlie air will no longer accept water and the lack of evaporation does not allow the wetted bulb to reject heat into the air by evaporation. This situation would be... 

In addition, there are hand-held psychrometers for measuring air temperature and relative humidity available, and also hand-held wind meters, which measure wind speed. When using such hand-held equipment, readings should be taken each hour during the course of the day when worker exposure volunteers or re-entry worker volunteers are performing the test. 

One instrument for determining the relative humidity of the atmosphere. It consists of a pair of thermometers, the bulb of one of which is surrounded by a fabric wick kept moist from a small water reservoir. Due to evaporation the temperature indicated by the wet bulb thermometer is lower than that of the other. The difference in the readings compared to the dry bulb reading enables the relative humidity to be obtained from tables. 

At the given relative humidity, therefore, the air pressure read on the barometer is 1024 mbar. ... 

Wet- and dry-bulb temperatures are measured by exposing two temperature-sensitive elements to the atmosphere whose moisture level is to be measured. The wet bulb is wrapped with a wick soaked in water the other element, the dry bulb, is left bare. Water evaporating from the wick lowers its temperature, which is read as the wet-bulb temperature, whereas the other reads the dry-bulb temperature. The relative humidity can be read from a psychometric chart such as the one shown in Figure 3.25. 

Environmental Conditions. The last area of discussion concerns those studies that emphasize environmental factors indoors and their interrelationship with clothing. Fanger s multivariate equation for predicting thermal comfort indoors, which he defines as thermal neutrality, is based on statistical analysis of 1,300 Danish and American subjects and consists of six parameters metabolic activity of occupants, clothing insulative value (clo), air temperature, mean radiant temperature, relative humidity, and air velocity ( 8, TjO An instrument based m these parameters and the statistical analysis is available (Figure 2) a reading for the parameters is integrated and the percent of occupants satisfied with the thermal environment is displayed. 

Calculated from L,a,b values on a Hunter color difference meter (average of six readings on each sample) exposure in Weatherometer is listed in kilojoules per square meter (ambient dry bulb temperature, 85°F 3° wet bulb temperature, 70°F 4°, black panel temperature 100°F % relative humidity, 50 8%). Each 110 kJ/m2 corresponds to about 20 h exposure to noon sunlight conditions. 

The performance of the sensors tested here can be snmmarized as follows Sensor A (TCD) was not sensitive enongh to detect H2 anywhere, even up to 7000 pL/L in the calibration cell. It was however sensitive to condensing water vapor, reading the eqnivalent of 3000 pL/L H2 at 25 °C and 100 % relative humidity. 

Room air is charged into the hygrometer sample chamber on a day when the temperature is 22 C, barometric pressure is 1.00 atm, and the relative humidity is 40%. The resulting meter reading is // = 5.0. A second measurement is then made by heating water to 50 C in a sealed flask containing air. The system is allowed to equilibrate at a pressure of 839 mm Hg with liquid still present in the flask, and a sample of the air above the liquid is withdrawn and injected into the sample chamber (which is heated to prevent condensation). The meter reading in this case is // = 48. 

For example, suppose you wish to determine the wet-bulb temperature of air at 30°C (dry bulb) with a relative humidity of 30%. Locate the point on Figure 8.4-1 at the intersection of the vertical line corresponding to T = 30°C and the curve corresponding to h, = 30%. The diagonal line through the point is the constant wet-bulb temperature line for air at the given condition. Follow that line upward to the left until you reach the saturation curve. The temperature value you read on the curve (or vertically down from it on the abscissa) is the wet-bulb temperature of the air. You should get a value of 18°C. This means that if you wrap a wet wick around a thermometer bulb and blow air with T = 30°C and h, = 30% past the bulb, the thermometer reading will drop and eventually stabilize at 18 C. 

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