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A hygrometer is an instrument that measures relative humidity. An analog hygrometer measures continuous changes in relative humidity, but because it is analog, its resolution limits its accuracy. By comparison, a digital hygrometer measures discrete changes in relative humidity. A psychrometer is the most common type of analog hygrometer. It calculates the relative humidity by comparing the difference between the wet and dry temperatures.
A psychrometer consists of two thermometers, one with a wet bulb and one with a dry bulb. A sling psychrometer has a handle that allows the operator to spin it in the air before taking the readings. The water around the wet bulb evaporates when the temperature is above freezing, which generally lowers the temperature reading of that thermometer. The water forms an insulating layer of ice when the temperature is below freezing, which generally causes the wet bulb thermometer in an analog hydrometer to have a higher temperature reading than the dry bulb thermometer.
An analog hygrometer requires a psychrometric chart to provide the relative humidity. This type of chart generally consists of a row of dry bulb temperatures and a column of wet bulb temperatures. The relative humidity can be obtained by cross-referencing the wet and dry bulb temperatures.
An Assmann psychrometer is one of the most accurate types of analog hygrometers. Adolph Richard Assmann invented this device in the late 19th century. Each thermometer in an Assmann psychrometer is suspended inside a tube of polished metal, which is suspended inside another metal tube. This arrangement insulates the thermometers from radiant heat. A fan draws air over the thermometer bulbs to maximize the evaporation of water from the bulbs.
The accuracy of an analog hygrometer is highly dependent upon the isolation of the thermometers from radiant heat and adequate airflow over the thermometer bulbs. These factors are especially critical in conditions of low humidity, when the wet bulb temperature is close to the dry bulb temperature. The most accurate analog hygrometers that use wet bulb thermometers may have an error rate as high as 5 percent for low humidity.
An analog hygrometer may use an electric heater to improve its accuracy when the temperature is below freezing. The heater warms the air before it reaches the thermometers without affecting the moisture content. This feature eliminates the need to maintain a wet bulb in freezing conditions.