What is a Hygrometer?
"A hygrometer is an instrument that can range from something that is really basic in design, such as a psychrometer or the hair hygrometer, on up to more detailed instruments, like a cooled mirror dew point hygrometer, which actually measures humidity using the condensation temperature. There are also more modern devices which track humidity from differences in both the electrical capacity and electrical resistance. If you are asking “what is a hygrometer?”, then you probably don’t work in an industrial, outdoor or military capacity, as these industries rely on them for their job. Hygrometers aren’t just used in industrial areas, greenhouses and other larger spaces, however, as they are also used in a variety of situations where controlling humidity is needed. For example, hygrometers are extremely helpful to those who work with surface coatings for industrial buildings, as these coatings are often very sensitive to changes in humidity and dew point and require careful monitoring of conditions in order to set correctly. A hygrometer makes these types of jobs much easier."
Source: Kestrel Meters
How do Hygrometers work?
"Mechanical hygrometers make use of the principle that organic substances (particularly finer substances such as goldbeater’s skin [ox gut] and human hair) contract and expand in response to the humidity. Contraction and expansion of the hair element in a mechanical hygrometer causes the spring to move the needle on the dial.
Electrical hygrometers measure the change in electrical resistance of a thin layer of lithium chloride, or of a semiconductor device, as the humidity changes. Other hygrometers sense changes in weight, volume, or transparency of various substances that react to humidity.
Dew-point hygrometers typically consist of a polished metal mirror that is cooled at a constant pressure and constant vapour content until moisture just starts to condense on it. The temperature of the metal at which condensation begins is the dew point.
The psychrometer (q.v.) is a hygrometer that utilizes two thermometers—one wet-bulb and one dry-bulb—to determine humidity through evaporation. A wetted cloth wraps the wet-bulb thermometerat its enlarged end. By rapidly rotating both thermometers, or by blowing air over the bulbs, the temperature of the wet-bulb thermometer is cooler than that of the dry-bulb thermometer. The difference in temperature between the wet- and dry-bulb thermometers can be used to compute the amount of water vapour in the air."