Hygrometer

     Hygrometer is used to measure the water vapors in the atmosphere. Hygrometer measures and shows the humidity in the atmosphere at any given time.

     The temperature at which dew or frost forms is a measure of the absolute humidity—the weight of water vapor per unit volume of air or other gas at the temperature before cooling. Knowing absolute humidity and air temperature, the observer can calculate relative humidity.

     The simplest form of a hygrometer consists of two thermometers, one of which has its bulb constantly kept wet. Evaporation from the bulb lowers the temperature so that this thermometer shows a lower temperature. Humidity is computed from the ambient temperature as shown by the "dry bulb thermometer" and the difference in temperature shown by the "wet bulb" and dry bulb thermometers. One device that uses the wet/dry bulb method is the sling psychrometer, where the thermometers are attached to a handle or length of rope and spun around in the air for a few minutes.

      The most accurate way to measure humidity is with an electric how long does weed stay in your system hygrometer, in which a known volume of gas passes over a hygroscopic, or moisture-absorbing, material such as phosphorus pentoxide. It is weighed before and after to determine how much water it took out of the gas. The older types of mechanical hygrometer use human hair, which stretches as it absorbs moisture, with a linkage connecting the center of a bundle of hairs under tension to a pointer. Another uses goldbeater's skin, a membrane from the intestines of oxen. Some materials can determine relative changes in humidity because they change color as they absorb moisture.