Lab 2: Relative Humidity Measurement PDF

**Second: Relative humidity** Relative humidity is the ratio of the partial pressure of water vapor (in a gaseous mixture of air and water vapor) to the saturated vapor pressure of water at a given temperature. Relative humidity is an important metric used in weather forecasts and reports, as it is...

**Second: Relative humidity** Relative humidity is the ratio of the partial pressure of water vapor (in a gaseous mixture of air and water vapor) to the saturated vapor pressure of water at a given temperature. Relative humidity is an important metric used in weather forecasts and reports, as it is an indicator of the likelihood of precipitation, dew, or fog. In hot summer weather, a rise in relative humidity also increases the temperature of the humans (and other animals) by hindering the evaporation of perspiration from the skin as the relative humidity rises. Relative humidity is often determined by using the following **a. Hair hygrometers** A hygrometer is an instrument used for measuring the moisture content in the atmosphere. Humidity measurement instruments usually rely on measurements of some other quantity such as temperature, pressure, mass or a mechanical or electrical change in a substance as moisture is absorbed. By calibration and calculation, these measured quantities can lead to a measurement of humidity. Modern electronic devices use temperature of condensation (the dew point), or changes in electrical resistance to measure humidity differences**.** Hair hygrometer works on the fact that hair changes its length when humidity varies. This device usually consists of a number of human or horse hairs connected to a mechanical lever system. When humidity increases the length of the hairs becomes longer. This change in length is then transmitted and magnified by the lever system into measurement of relative humidity. **b. Psychrometer** A psychrometer or sling psychrometer is a device used to measure relative humidity. It has two thermometers. One thermometer is called a wet-bulb thermometer. The bulb of the wet-bulb thermometer is covered with a damp cloth. The sling psychrometer is whirled around, using the handle. As the instrument is whirled, water evaporates from the cloth on the wet-bulb thermometer and cools the thermometer. This value is known as the wet-bulb temperature. The drier the air the more quickly the thermometer cools and hence, the lower the wet-bulb temperature. If the humidity is high, only a small amount of water will evaporate and there will be little change in temperature. The other thermometer is a dry-bulb thermometer and measures air temperature. The difference in temperature reading between the wet-bulb and dry-bulb thermometers indicates the amount of water vapor in the air. A table like the one below is used to determine the relative humidity. Step 1: Determine the temperature of the dry-bulb thermometer. Step 2: Determine the temperature of the wet-bulb thermometer. Step 3: Determine the difference in the wet-bulb and dry-bulb thermometer. Step 4: Use the chart to determine the relative humidity. What is the relative humidity? **%** **c. Thermo-Hygrograph** **A Thermo-Hygrograph** is a chart recorder that measures and records both temperature and humidity (or dew point). Similar devices that record only one parameter are a **thermograph** for temperature and **hygrograph** for humidity. A thermograph usually configured with a pen that records temperature on a revolving cylinder. The pen located at the end of a lever which controlled by a bi-metal strip composed from temperature-sensitive metal which bends as the temperature changes. C:\\Users\\drgham alsultany\\Downloads\\504606599568\_m\_600x600.jpg  C:\\Users\\drgham alsultany\\Downloads\\images.jpg **d. Thermo-hygrometer** The thermo hygrometer measures both humidity of the air and temperature of the air. The thermo hygrometer measures different ranges of humidity and temperature depending on the model. In modern thermo hygrometer is versatile in that it can take measurements, store them to memory and transfer data to a computer for further detailed analysis.  C:\\Users\\drgham alsultany\\Downloads\\Termoigrografo\_01.jpg **Other instrument to measure indirect humidity** **I. Dew point apparatus** The Dew Point Apparatus consists of a closed stainless steel dew point chamber containing a highly polished stainless steel "target mirror" and sample inlet and outlet control valves. The chamber is chilled by refrigerant following through the outer cooling jacket, preventing any refrigerant contact with the test sample. The thermometer is inserted into the mirror support structure, providing the temperature of the "target mirror." As the sample flows in the chamber and is deflected across the surface of the mirror, the temperature at which condensation collects on the mirror is recorded as the dew point of the sample. II\. **Rain gauge** A rain gauge (also known as a udometer or a pluviometer \[Pluviograph \] is a type of instrument used by meteorologists and hydrologists to gather and measure the amount of liquid precipitation over a set period of time. The standard rain gauge, developed around the start of the 20th century, consists of a funnel attached to a graduated cylinder (2 cm in diameter) that fits inside a larger outside container (20 cm in diameter and 50 cm tall). If the water overflows the inside graduated cylinder, the outside larger container will catch it. When measurements are taken, the height of the water in the small graduated cylinder is measured and the excess overflow in the large container is carefully poured into another graduated cylinder and measured to give the total rainfall.