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TEMPERATURE OF AN AIR-VAPOR MIXTURE.
Three different temperatures are used when referring to an air-vapor mixture, namely dry-bulb, wet-bulb, and dew-point temperatures. The dry-bulb temperature is usually measured with a common thermometer or thermocouple. The wet-bulb temperature is measured with a common thermometer or thermocouple having the bulb or junction completely covered by a water-moistened cloth or wick. Moisture evaporates from the wick into the surrounding air, and before equilibrium is reached, a portion of the heat used is taken from the thermometer or bulb or thermocouple wires, as the case may be, thus lowering the temperature indicated below the indicated dry-bulb temperature. The drier the surrounding air, the greater the rate of evaporation and the lower the wet-bulb temperature indicated.
As moisture from the wet bulb evaporates, the air surrounding the bulb becomes more humid, causing the rate of evaporation to decrease and the indicated wet-bulb temperature to rise. Therefore, in order to measure the wet-bulb temperature of air in a given space, a continuous sample of this air must pass around the wet bulb. Tests have been conducted to determine the effect of varying air velocities over thermocouple and thermometer wet bulbs. These indicate that velocities from 500 to 1000 fpm give a minimum error for the conventional-size thermometer bulb at temperatures from 20 to 60°F. The temperatures of near-by surfaces can affect the wet bulb reading due to radiation, and varying air velocities also influence this effect. Furthermore, the size of the wet bulb is significant in that the smaller the diameter, the lower the air velocity required. Thus obtaining an accurate wet-bulb reading can become rather complicated. The application of the reading determines the accuracy required. Soft fine-mesh cotton tubing is recommended for a wick; it should cover the bulb plus about an inch of the stem. The wick should be watched and replaced before it becomes dirty or crusty. Using distilled water is recommended to give greater accuracy longer.
The dew-point temperature is the temperature of an air-vapor mixture at which moisture will start to condense out of the air as the air is cooled; or it is the temperature of a surface just as condensate starts to collect on the surface when the surface is being cooled. In other words, it is the saturation temperature of water at the partial pressure of the water vapor in the air-vapor mixture. Therefore, if the partial pressure of the vapor is known, the dew-point can be determined directly from steam tables. If the dry-bulb and wet-bulb temperatures are known, the partial pressure can be found from suitable equation. Also, the dew-point temperature can be found on a psychrometric chart by reading the temperature where a constant moisture line from the dry bulb and wet-bulb point intersects the saturation curve.
The terms "wet-bulb depression" and "dew-point depression" are often used. The first refers to the difference in degrees Fahrenheit between the dry-bulb temperature and the wet-bulb temperature. Similarly, the second refers to the difference between the dry-bulb and dew-point temperatures.
Answer the questions:
1. What temperatures are used when referring to an air-vapor mixture?
2. How is the dry-bulb temperature measured?
3. In what way is the wet-bulb temperature measured?
4. How can the dew-point temperature be determined?
5. What do the terms "wet-bulb depression" and "dew-point depression" mean?
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